llvm.org GIT mirror llvm / release_39 docs / tutorial / OCamlLangImpl3.rst
release_39

Tree @release_39 (Download .tar.gz)

OCamlLangImpl3.rst @release_39

ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
b04877c
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
 
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
ee47edf
022b109
ee47edf
 
 
 
 
022b109
ee47edf
 
 
022b109
ee47edf
 
022b109
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
022b109
ee47edf
 
022b109
ee47edf
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
========================================
Kaleidoscope: Code generation to LLVM IR
========================================

.. contents::
   :local:

Chapter 3 Introduction
======================

Welcome to Chapter 3 of the "`Implementing a language with
LLVM <index.html>`_" tutorial. This chapter shows you how to transform
the `Abstract Syntax Tree <OCamlLangImpl2.html>`_, built in Chapter 2,
into LLVM IR. This will teach you a little bit about how LLVM does
things, as well as demonstrate how easy it is to use. It's much more
work to build a lexer and parser than it is to generate LLVM IR code. :)

**Please note**: the code in this chapter and later require LLVM 2.3 or
LLVM SVN to work. LLVM 2.2 and before will not work with it.

Code Generation Setup
=====================

In order to generate LLVM IR, we want some simple setup to get started.
First we define virtual code generation (codegen) methods in each AST
class:

.. code-block:: ocaml

    let rec codegen_expr = function
      | Ast.Number n -> ...
      | Ast.Variable name -> ...

The ``Codegen.codegen_expr`` function says to emit IR for that AST node
along with all the things it depends on, and they all return an LLVM
Value object. "Value" is the class used to represent a "`Static Single
Assignment
(SSA) <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
register" or "SSA value" in LLVM. The most distinct aspect of SSA values
is that their value is computed as the related instruction executes, and
it does not get a new value until (and if) the instruction re-executes.
In other words, there is no way to "change" an SSA value. For more
information, please read up on `Static Single
Assignment <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
- the concepts are really quite natural once you grok them.

The second thing we want is an "Error" exception like we used for the
parser, which will be used to report errors found during code generation
(for example, use of an undeclared parameter):

.. code-block:: ocaml

    exception Error of string

    let context = global_context ()
    let the_module = create_module context "my cool jit"
    let builder = builder context
    let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
    let double_type = double_type context

The static variables will be used during code generation.
``Codgen.the_module`` is the LLVM construct that contains all of the
functions and global variables in a chunk of code. In many ways, it is
the top-level structure that the LLVM IR uses to contain code.

The ``Codegen.builder`` object is a helper object that makes it easy to
generate LLVM instructions. Instances of the
`IRBuilder <http://llvm.org/doxygen/IRBuilder_8h-source.html>`_
class keep track of the current place to insert instructions and has
methods to create new instructions.

The ``Codegen.named_values`` map keeps track of which values are defined
in the current scope and what their LLVM representation is. (In other
words, it is a symbol table for the code). In this form of Kaleidoscope,
the only things that can be referenced are function parameters. As such,
function parameters will be in this map when generating code for their
function body.

With these basics in place, we can start talking about how to generate
code for each expression. Note that this assumes that the
``Codgen.builder`` has been set up to generate code *into* something.
For now, we'll assume that this has already been done, and we'll just
use it to emit code.

Expression Code Generation
==========================

Generating LLVM code for expression nodes is very straightforward: less
than 30 lines of commented code for all four of our expression nodes.
First we'll do numeric literals:

.. code-block:: ocaml

      | Ast.Number n -> const_float double_type n

In the LLVM IR, numeric constants are represented with the
``ConstantFP`` class, which holds the numeric value in an ``APFloat``
internally (``APFloat`` has the capability of holding floating point
constants of Arbitrary Precision). This code basically just creates
and returns a ``ConstantFP``. Note that in the LLVM IR that constants
are all uniqued together and shared. For this reason, the API uses "the
foo::get(..)" idiom instead of "new foo(..)" or "foo::Create(..)".

.. code-block:: ocaml

      | Ast.Variable name ->
          (try Hashtbl.find named_values name with
            | Not_found -> raise (Error "unknown variable name"))

References to variables are also quite simple using LLVM. In the simple
version of Kaleidoscope, we assume that the variable has already been
emitted somewhere and its value is available. In practice, the only
values that can be in the ``Codegen.named_values`` map are function
arguments. This code simply checks to see that the specified name is in
the map (if not, an unknown variable is being referenced) and returns
the value for it. In future chapters, we'll add support for `loop
induction variables <LangImpl5.html#for-loop-expression>`_ in the symbol table, and for
`local variables <LangImpl7.html#user-defined-local-variables>`_.

.. code-block:: ocaml

      | Ast.Binary (op, lhs, rhs) ->
          let lhs_val = codegen_expr lhs in
          let rhs_val = codegen_expr rhs in
          begin
            match op with
            | '+' -> build_fadd lhs_val rhs_val "addtmp" builder
            | '-' -> build_fsub lhs_val rhs_val "subtmp" builder
            | '*' -> build_fmul lhs_val rhs_val "multmp" builder
            | '<' ->
                (* Convert bool 0/1 to double 0.0 or 1.0 *)
                let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
                build_uitofp i double_type "booltmp" builder
            | _ -> raise (Error "invalid binary operator")
          end

Binary operators start to get more interesting. The basic idea here is
that we recursively emit code for the left-hand side of the expression,
then the right-hand side, then we compute the result of the binary
expression. In this code, we do a simple switch on the opcode to create
the right LLVM instruction.

In the example above, the LLVM builder class is starting to show its
value. IRBuilder knows where to insert the newly created instruction,
all you have to do is specify what instruction to create (e.g. with
``Llvm.create_add``), which operands to use (``lhs`` and ``rhs`` here)
and optionally provide a name for the generated instruction.

One nice thing about LLVM is that the name is just a hint. For instance,
if the code above emits multiple "addtmp" variables, LLVM will
automatically provide each one with an increasing, unique numeric
suffix. Local value names for instructions are purely optional, but it
makes it much easier to read the IR dumps.

`LLVM instructions <../LangRef.html#instruction-reference>`_ are constrained by strict
rules: for example, the Left and Right operators of an `add
instruction <../LangRef.html#add-instruction>`_ must have the same type, and the
result type of the add must match the operand types. Because all values
in Kaleidoscope are doubles, this makes for very simple code for add,
sub and mul.

On the other hand, LLVM specifies that the `fcmp
instruction <../LangRef.html#fcmp-instruction>`_ always returns an 'i1' value (a
one bit integer). The problem with this is that Kaleidoscope wants the
value to be a 0.0 or 1.0 value. In order to get these semantics, we
combine the fcmp instruction with a `uitofp
instruction <../LangRef.html#uitofp-to-instruction>`_. This instruction converts its
input integer into a floating point value by treating the input as an
unsigned value. In contrast, if we used the `sitofp
instruction <../LangRef.html#sitofp-to-instruction>`_, the Kaleidoscope '<' operator
would return 0.0 and -1.0, depending on the input value.

.. code-block:: ocaml

      | Ast.Call (callee, args) ->
          (* Look up the name in the module table. *)
          let callee =
            match lookup_function callee the_module with
            | Some callee -> callee
            | None -> raise (Error "unknown function referenced")
          in
          let params = params callee in

          (* If argument mismatch error. *)
          if Array.length params == Array.length args then () else
            raise (Error "incorrect # arguments passed");
          let args = Array.map codegen_expr args in
          build_call callee args "calltmp" builder

Code generation for function calls is quite straightforward with LLVM.
The code above initially does a function name lookup in the LLVM
Module's symbol table. Recall that the LLVM Module is the container that
holds all of the functions we are JIT'ing. By giving each function the
same name as what the user specifies, we can use the LLVM symbol table
to resolve function names for us.

Once we have the function to call, we recursively codegen each argument
that is to be passed in, and create an LLVM `call
instruction <../LangRef.html#call-instruction>`_. Note that LLVM uses the native C
calling conventions by default, allowing these calls to also call into
standard library functions like "sin" and "cos", with no additional
effort.

This wraps up our handling of the four basic expressions that we have so
far in Kaleidoscope. Feel free to go in and add some more. For example,
by browsing the `LLVM language reference <../LangRef.html>`_ you'll find
several other interesting instructions that are really easy to plug into
our basic framework.

Function Code Generation
========================

Code generation for prototypes and functions must handle a number of
details, which make their code less beautiful than expression code
generation, but allows us to illustrate some important points. First,
lets talk about code generation for prototypes: they are used both for
function bodies and external function declarations. The code starts
with:

.. code-block:: ocaml

    let codegen_proto = function
      | Ast.Prototype (name, args) ->
          (* Make the function type: double(double,double) etc. *)
          let doubles = Array.make (Array.length args) double_type in
          let ft = function_type double_type doubles in
          let f =
            match lookup_function name the_module with

This code packs a lot of power into a few lines. Note first that this
function returns a "Function\*" instead of a "Value\*" (although at the
moment they both are modeled by ``llvalue`` in ocaml). Because a
"prototype" really talks about the external interface for a function
(not the value computed by an expression), it makes sense for it to
return the LLVM Function it corresponds to when codegen'd.

The call to ``Llvm.function_type`` creates the ``Llvm.llvalue`` that
should be used for a given Prototype. Since all function arguments in
Kaleidoscope are of type double, the first line creates a vector of "N"
LLVM double types. It then uses the ``Llvm.function_type`` method to
create a function type that takes "N" doubles as arguments, returns one
double as a result, and that is not vararg (that uses the function
``Llvm.var_arg_function_type``). Note that Types in LLVM are uniqued
just like ``Constant``'s are, so you don't "new" a type, you "get" it.

The final line above checks if the function has already been defined in
``Codegen.the_module``. If not, we will create it.

.. code-block:: ocaml

            | None -> declare_function name ft the_module

This indicates the type and name to use, as well as which module to
insert into. By default we assume a function has
``Llvm.Linkage.ExternalLinkage``. "`external
linkage <../LangRef.html#linkage>`_" means that the function may be defined
outside the current module and/or that it is callable by functions
outside the module. The "``name``" passed in is the name the user
specified: this name is registered in "``Codegen.the_module``"s symbol
table, which is used by the function call code above.

In Kaleidoscope, I choose to allow redefinitions of functions in two
cases: first, we want to allow 'extern'ing a function more than once, as
long as the prototypes for the externs match (since all arguments have
the same type, we just have to check that the number of arguments
match). Second, we want to allow 'extern'ing a function and then
defining a body for it. This is useful when defining mutually recursive
functions.

.. code-block:: ocaml

            (* If 'f' conflicted, there was already something named 'name'. If it
             * has a body, don't allow redefinition or reextern. *)
            | Some f ->
                (* If 'f' already has a body, reject this. *)
                if Array.length (basic_blocks f) == 0 then () else
                  raise (Error "redefinition of function");

                (* If 'f' took a different number of arguments, reject. *)
                if Array.length (params f) == Array.length args then () else
                  raise (Error "redefinition of function with different # args");
                f
          in

In order to verify the logic above, we first check to see if the
pre-existing function is "empty". In this case, empty means that it has
no basic blocks in it, which means it has no body. If it has no body, it
is a forward declaration. Since we don't allow anything after a full
definition of the function, the code rejects this case. If the previous
reference to a function was an 'extern', we simply verify that the
number of arguments for that definition and this one match up. If not,
we emit an error.

.. code-block:: ocaml

          (* Set names for all arguments. *)
          Array.iteri (fun i a ->
            let n = args.(i) in
            set_value_name n a;
            Hashtbl.add named_values n a;
          ) (params f);
          f

The last bit of code for prototypes loops over all of the arguments in
the function, setting the name of the LLVM Argument objects to match,
and registering the arguments in the ``Codegen.named_values`` map for
future use by the ``Ast.Variable`` variant. Once this is set up, it
returns the Function object to the caller. Note that we don't check for
conflicting argument names here (e.g. "extern foo(a b a)"). Doing so
would be very straight-forward with the mechanics we have already used
above.

.. code-block:: ocaml

    let codegen_func = function
      | Ast.Function (proto, body) ->
          Hashtbl.clear named_values;
          let the_function = codegen_proto proto in

Code generation for function definitions starts out simply enough: we
just codegen the prototype (Proto) and verify that it is ok. We then
clear out the ``Codegen.named_values`` map to make sure that there isn't
anything in it from the last function we compiled. Code generation of
the prototype ensures that there is an LLVM Function object that is
ready to go for us.

.. code-block:: ocaml

          (* Create a new basic block to start insertion into. *)
          let bb = append_block context "entry" the_function in
          position_at_end bb builder;

          try
            let ret_val = codegen_expr body in

Now we get to the point where the ``Codegen.builder`` is set up. The
first line creates a new `basic
block <http://en.wikipedia.org/wiki/Basic_block>`_ (named "entry"),
which is inserted into ``the_function``. The second line then tells the
builder that new instructions should be inserted into the end of the new
basic block. Basic blocks in LLVM are an important part of functions
that define the `Control Flow
Graph <http://en.wikipedia.org/wiki/Control_flow_graph>`_. Since we
don't have any control flow, our functions will only contain one block
at this point. We'll fix this in `Chapter 5 <OCamlLangImpl5.html>`_ :).

.. code-block:: ocaml

            let ret_val = codegen_expr body in

            (* Finish off the function. *)
            let _ = build_ret ret_val builder in

            (* Validate the generated code, checking for consistency. *)
            Llvm_analysis.assert_valid_function the_function;

            the_function

Once the insertion point is set up, we call the ``Codegen.codegen_func``
method for the root expression of the function. If no error happens,
this emits code to compute the expression into the entry block and
returns the value that was computed. Assuming no error, we then create
an LLVM `ret instruction <../LangRef.html#ret-instruction>`_, which completes the
function. Once the function is built, we call
``Llvm_analysis.assert_valid_function``, which is provided by LLVM. This
function does a variety of consistency checks on the generated code, to
determine if our compiler is doing everything right. Using this is
important: it can catch a lot of bugs. Once the function is finished and
validated, we return it.

.. code-block:: ocaml

          with e ->
            delete_function the_function;
            raise e

The only piece left here is handling of the error case. For simplicity,
we handle this by merely deleting the function we produced with the
``Llvm.delete_function`` method. This allows the user to redefine a
function that they incorrectly typed in before: if we didn't delete it,
it would live in the symbol table, with a body, preventing future
redefinition.

This code does have a bug, though. Since the ``Codegen.codegen_proto``
can return a previously defined forward declaration, our code can
actually delete a forward declaration. There are a number of ways to fix
this bug, see what you can come up with! Here is a testcase:

::

    extern foo(a b);     # ok, defines foo.
    def foo(a b) c;      # error, 'c' is invalid.
    def bar() foo(1, 2); # error, unknown function "foo"

Driver Changes and Closing Thoughts
===================================

For now, code generation to LLVM doesn't really get us much, except that
we can look at the pretty IR calls. The sample code inserts calls to
Codegen into the "``Toplevel.main_loop``", and then dumps out the LLVM
IR. This gives a nice way to look at the LLVM IR for simple functions.
For example:

::

    ready> 4+5;
    Read top-level expression:
    define double @""() {
    entry:
            %addtmp = fadd double 4.000000e+00, 5.000000e+00
            ret double %addtmp
    }

Note how the parser turns the top-level expression into anonymous
functions for us. This will be handy when we add `JIT
support <OCamlLangImpl4.html#adding-a-jit-compiler>`_ in the next chapter. Also note that
the code is very literally transcribed, no optimizations are being
performed. We will `add
optimizations <OCamlLangImpl4.html#trivial-constant-folding>`_ explicitly in the
next chapter.

::

    ready> def foo(a b) a*a + 2*a*b + b*b;
    Read function definition:
    define double @foo(double %a, double %b) {
    entry:
            %multmp = fmul double %a, %a
            %multmp1 = fmul double 2.000000e+00, %a
            %multmp2 = fmul double %multmp1, %b
            %addtmp = fadd double %multmp, %multmp2
            %multmp3 = fmul double %b, %b
            %addtmp4 = fadd double %addtmp, %multmp3
            ret double %addtmp4
    }

This shows some simple arithmetic. Notice the striking similarity to the
LLVM builder calls that we use to create the instructions.

::

    ready> def bar(a) foo(a, 4.0) + bar(31337);
    Read function definition:
    define double @bar(double %a) {
    entry:
            %calltmp = call double @foo(double %a, double 4.000000e+00)
            %calltmp1 = call double @bar(double 3.133700e+04)
            %addtmp = fadd double %calltmp, %calltmp1
            ret double %addtmp
    }

This shows some function calls. Note that this function will take a long
time to execute if you call it. In the future we'll add conditional
control flow to actually make recursion useful :).

::

    ready> extern cos(x);
    Read extern:
    declare double @cos(double)

    ready> cos(1.234);
    Read top-level expression:
    define double @""() {
    entry:
            %calltmp = call double @cos(double 1.234000e+00)
            ret double %calltmp
    }

This shows an extern for the libm "cos" function, and a call to it.

::

    ready> ^D
    ; ModuleID = 'my cool jit'

    define double @""() {
    entry:
            %addtmp = fadd double 4.000000e+00, 5.000000e+00
            ret double %addtmp
    }

    define double @foo(double %a, double %b) {
    entry:
            %multmp = fmul double %a, %a
            %multmp1 = fmul double 2.000000e+00, %a
            %multmp2 = fmul double %multmp1, %b
            %addtmp = fadd double %multmp, %multmp2
            %multmp3 = fmul double %b, %b
            %addtmp4 = fadd double %addtmp, %multmp3
            ret double %addtmp4
    }

    define double @bar(double %a) {
    entry:
            %calltmp = call double @foo(double %a, double 4.000000e+00)
            %calltmp1 = call double @bar(double 3.133700e+04)
            %addtmp = fadd double %calltmp, %calltmp1
            ret double %addtmp
    }

    declare double @cos(double)

    define double @""() {
    entry:
            %calltmp = call double @cos(double 1.234000e+00)
            ret double %calltmp
    }

When you quit the current demo, it dumps out the IR for the entire
module generated. Here you can see the big picture with all the
functions referencing each other.

This wraps up the third chapter of the Kaleidoscope tutorial. Up next,
we'll describe how to `add JIT codegen and optimizer
support <OCamlLangImpl4.html>`_ to this so we can actually start running
code!

Full Code Listing
=================

Here is the complete code listing for our running example, enhanced with
the LLVM code generator. Because this uses the LLVM libraries, we need
to link them in. To do this, we use the
`llvm-config <http://llvm.org/cmds/llvm-config.html>`_ tool to inform
our makefile/command line about which options to use:

.. code-block:: bash

    # Compile
    ocamlbuild toy.byte
    # Run
    ./toy.byte

Here is the code:

\_tags:
    ::

        <{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
        <*.{byte,native}>: g++, use_llvm, use_llvm_analysis

myocamlbuild.ml:
    .. code-block:: ocaml

        open Ocamlbuild_plugin;;

        ocaml_lib ~extern:true "llvm";;
        ocaml_lib ~extern:true "llvm_analysis";;

        flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;

token.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Lexer Tokens
         *===----------------------------------------------------------------------===*)

        (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
         * these others for known things. *)
        type token =
          (* commands *)
          | Def | Extern

          (* primary *)
          | Ident of string | Number of float

          (* unknown *)
          | Kwd of char

lexer.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Lexer
         *===----------------------------------------------------------------------===*)

        let rec lex = parser
          (* Skip any whitespace. *)
          | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream

          (* identifier: [a-zA-Z][a-zA-Z0-9] *)
          | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
              let buffer = Buffer.create 1 in
              Buffer.add_char buffer c;
              lex_ident buffer stream

          (* number: [0-9.]+ *)
          | [< ' ('0' .. '9' as c); stream >] ->
              let buffer = Buffer.create 1 in
              Buffer.add_char buffer c;
              lex_number buffer stream

          (* Comment until end of line. *)
          | [< ' ('#'); stream >] ->
              lex_comment stream

          (* Otherwise, just return the character as its ascii value. *)
          | [< 'c; stream >] ->
              [< 'Token.Kwd c; lex stream >]

          (* end of stream. *)
          | [< >] -> [< >]

        and lex_number buffer = parser
          | [< ' ('0' .. '9' | '.' as c); stream >] ->
              Buffer.add_char buffer c;
              lex_number buffer stream
          | [< stream=lex >] ->
              [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]

        and lex_ident buffer = parser
          | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
              Buffer.add_char buffer c;
              lex_ident buffer stream
          | [< stream=lex >] ->
              match Buffer.contents buffer with
              | "def" -> [< 'Token.Def; stream >]
              | "extern" -> [< 'Token.Extern; stream >]
              | id -> [< 'Token.Ident id; stream >]

        and lex_comment = parser
          | [< ' ('\n'); stream=lex >] -> stream
          | [< 'c; e=lex_comment >] -> e
          | [< >] -> [< >]

ast.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Abstract Syntax Tree (aka Parse Tree)
         *===----------------------------------------------------------------------===*)

        (* expr - Base type for all expression nodes. *)
        type expr =
          (* variant for numeric literals like "1.0". *)
          | Number of float

          (* variant for referencing a variable, like "a". *)
          | Variable of string

          (* variant for a binary operator. *)
          | Binary of char * expr * expr

          (* variant for function calls. *)
          | Call of string * expr array

        (* proto - This type represents the "prototype" for a function, which captures
         * its name, and its argument names (thus implicitly the number of arguments the
         * function takes). *)
        type proto = Prototype of string * string array

        (* func - This type represents a function definition itself. *)
        type func = Function of proto * expr

parser.ml:
    .. code-block:: ocaml

        (*===---------------------------------------------------------------------===
         * Parser
         *===---------------------------------------------------------------------===*)

        (* binop_precedence - This holds the precedence for each binary operator that is
         * defined *)
        let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10

        (* precedence - Get the precedence of the pending binary operator token. *)
        let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1

        (* primary
         *   ::= identifier
         *   ::= numberexpr
         *   ::= parenexpr *)
        let rec parse_primary = parser
          (* numberexpr ::= number *)
          | [< 'Token.Number n >] -> Ast.Number n

          (* parenexpr ::= '(' expression ')' *)
          | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e

          (* identifierexpr
           *   ::= identifier
           *   ::= identifier '(' argumentexpr ')' *)
          | [< 'Token.Ident id; stream >] ->
              let rec parse_args accumulator = parser
                | [< e=parse_expr; stream >] ->
                    begin parser
                      | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
                      | [< >] -> e :: accumulator
                    end stream
                | [< >] -> accumulator
              in
              let rec parse_ident id = parser
                (* Call. *)
                | [< 'Token.Kwd '(';
                     args=parse_args [];
                     'Token.Kwd ')' ?? "expected ')'">] ->
                    Ast.Call (id, Array.of_list (List.rev args))

                (* Simple variable ref. *)
                | [< >] -> Ast.Variable id
              in
              parse_ident id stream

          | [< >] -> raise (Stream.Error "unknown token when expecting an expression.")

        (* binoprhs
         *   ::= ('+' primary)* *)
        and parse_bin_rhs expr_prec lhs stream =
          match Stream.peek stream with
          (* If this is a binop, find its precedence. *)
          | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
              let token_prec = precedence c in

              (* If this is a binop that binds at least as tightly as the current binop,
               * consume it, otherwise we are done. *)
              if token_prec < expr_prec then lhs else begin
                (* Eat the binop. *)
                Stream.junk stream;

                (* Parse the primary expression after the binary operator. *)
                let rhs = parse_primary stream in

                (* Okay, we know this is a binop. *)
                let rhs =
                  match Stream.peek stream with
                  | Some (Token.Kwd c2) ->
                      (* If BinOp binds less tightly with rhs than the operator after
                       * rhs, let the pending operator take rhs as its lhs. *)
                      let next_prec = precedence c2 in
                      if token_prec < next_prec
                      then parse_bin_rhs (token_prec + 1) rhs stream
                      else rhs
                  | _ -> rhs
                in

                (* Merge lhs/rhs. *)
                let lhs = Ast.Binary (c, lhs, rhs) in
                parse_bin_rhs expr_prec lhs stream
              end
          | _ -> lhs

        (* expression
         *   ::= primary binoprhs *)
        and parse_expr = parser
          | [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream

        (* prototype
         *   ::= id '(' id* ')' *)
        let parse_prototype =
          let rec parse_args accumulator = parser
            | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
            | [< >] -> accumulator
          in

          parser
          | [< 'Token.Ident id;
               'Token.Kwd '(' ?? "expected '(' in prototype";
               args=parse_args [];
               'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
              (* success. *)
              Ast.Prototype (id, Array.of_list (List.rev args))

          | [< >] ->
              raise (Stream.Error "expected function name in prototype")

        (* definition ::= 'def' prototype expression *)
        let parse_definition = parser
          | [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
              Ast.Function (p, e)

        (* toplevelexpr ::= expression *)
        let parse_toplevel = parser
          | [< e=parse_expr >] ->
              (* Make an anonymous proto. *)
              Ast.Function (Ast.Prototype ("", [||]), e)

        (*  external ::= 'extern' prototype *)
        let parse_extern = parser
          | [< 'Token.Extern; e=parse_prototype >] -> e

codegen.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Code Generation
         *===----------------------------------------------------------------------===*)

        open Llvm

        exception Error of string

        let context = global_context ()
        let the_module = create_module context "my cool jit"
        let builder = builder context
        let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
        let double_type = double_type context

        let rec codegen_expr = function
          | Ast.Number n -> const_float double_type n
          | Ast.Variable name ->
              (try Hashtbl.find named_values name with
                | Not_found -> raise (Error "unknown variable name"))
          | Ast.Binary (op, lhs, rhs) ->
              let lhs_val = codegen_expr lhs in
              let rhs_val = codegen_expr rhs in
              begin
                match op with
                | '+' -> build_add lhs_val rhs_val "addtmp" builder
                | '-' -> build_sub lhs_val rhs_val "subtmp" builder
                | '*' -> build_mul lhs_val rhs_val "multmp" builder
                | '<' ->
                    (* Convert bool 0/1 to double 0.0 or 1.0 *)
                    let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
                    build_uitofp i double_type "booltmp" builder
                | _ -> raise (Error "invalid binary operator")
              end
          | Ast.Call (callee, args) ->
              (* Look up the name in the module table. *)
              let callee =
                match lookup_function callee the_module with
                | Some callee -> callee
                | None -> raise (Error "unknown function referenced")
              in
              let params = params callee in

              (* If argument mismatch error. *)
              if Array.length params == Array.length args then () else
                raise (Error "incorrect # arguments passed");
              let args = Array.map codegen_expr args in
              build_call callee args "calltmp" builder

        let codegen_proto = function
          | Ast.Prototype (name, args) ->
              (* Make the function type: double(double,double) etc. *)
              let doubles = Array.make (Array.length args) double_type in
              let ft = function_type double_type doubles in
              let f =
                match lookup_function name the_module with
                | None -> declare_function name ft the_module

                (* If 'f' conflicted, there was already something named 'name'. If it
                 * has a body, don't allow redefinition or reextern. *)
                | Some f ->
                    (* If 'f' already has a body, reject this. *)
                    if block_begin f <> At_end f then
                      raise (Error "redefinition of function");

                    (* If 'f' took a different number of arguments, reject. *)
                    if element_type (type_of f) <> ft then
                      raise (Error "redefinition of function with different # args");
                    f
              in

              (* Set names for all arguments. *)
              Array.iteri (fun i a ->
                let n = args.(i) in
                set_value_name n a;
                Hashtbl.add named_values n a;
              ) (params f);
              f

        let codegen_func = function
          | Ast.Function (proto, body) ->
              Hashtbl.clear named_values;
              let the_function = codegen_proto proto in

              (* Create a new basic block to start insertion into. *)
              let bb = append_block context "entry" the_function in
              position_at_end bb builder;

              try
                let ret_val = codegen_expr body in

                (* Finish off the function. *)
                let _ = build_ret ret_val builder in

                (* Validate the generated code, checking for consistency. *)
                Llvm_analysis.assert_valid_function the_function;

                the_function
              with e ->
                delete_function the_function;
                raise e

toplevel.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Top-Level parsing and JIT Driver
         *===----------------------------------------------------------------------===*)

        open Llvm

        (* top ::= definition | external | expression | ';' *)
        let rec main_loop stream =
          match Stream.peek stream with
          | None -> ()

          (* ignore top-level semicolons. *)
          | Some (Token.Kwd ';') ->
              Stream.junk stream;
              main_loop stream

          | Some token ->
              begin
                try match token with
                | Token.Def ->
                    let e = Parser.parse_definition stream in
                    print_endline "parsed a function definition.";
                    dump_value (Codegen.codegen_func e);
                | Token.Extern ->
                    let e = Parser.parse_extern stream in
                    print_endline "parsed an extern.";
                    dump_value (Codegen.codegen_proto e);
                | _ ->
                    (* Evaluate a top-level expression into an anonymous function. *)
                    let e = Parser.parse_toplevel stream in
                    print_endline "parsed a top-level expr";
                    dump_value (Codegen.codegen_func e);
                with Stream.Error s | Codegen.Error s ->
                  (* Skip token for error recovery. *)
                  Stream.junk stream;
                  print_endline s;
              end;
              print_string "ready> "; flush stdout;
              main_loop stream

toy.ml:
    .. code-block:: ocaml

        (*===----------------------------------------------------------------------===
         * Main driver code.
         *===----------------------------------------------------------------------===*)

        open Llvm

        let main () =
          (* Install standard binary operators.
           * 1 is the lowest precedence. *)
          Hashtbl.add Parser.binop_precedence '<' 10;
          Hashtbl.add Parser.binop_precedence '+' 20;
          Hashtbl.add Parser.binop_precedence '-' 20;
          Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)

          (* Prime the first token. *)
          print_string "ready> "; flush stdout;
          let stream = Lexer.lex (Stream.of_channel stdin) in

          (* Run the main "interpreter loop" now. *)
          Toplevel.main_loop stream;

          (* Print out all the generated code. *)
          dump_module Codegen.the_module
        ;;

        main ()

`Next: Adding JIT and Optimizer Support <OCamlLangImpl4.html>`_