compiler/rustc_codegen_gcc/doc/gimple.md

   1 # GIMPLE
   2
   3 You can see the full documentation about what GIMPLE is [here](https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html). In this document we will explain how to generate it.
   4
   5 First, we'll copy the content from `gcc/gcc/testsuite/jit.dg/test-const-attribute.c` into a
   6 file named `local.c` and remove the content we're not interested into:
   7
   8 ```diff
   9 - /* { dg-do compile { target x86_64-*-* } } */
  10 ...
  11 - /* We don't want set_options() in harness.h to set -O3 to see that the const
  12 -    attribute affects the optimizations. */
  13 - #define TEST_ESCHEWS_SET_OPTIONS
  14 - static void set_options (gcc_jit_context *ctxt, const char *argv0)
  15 - {
  16 -   // Set "-O3".
  17 -   gcc_jit_context_set_int_option(ctxt, GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 3);
  18 - }
  19 -
  20 - #define TEST_COMPILING_TO_FILE
  21 - #define OUTPUT_KIND      GCC_JIT_OUTPUT_KIND_ASSEMBLER
  22 - #define OUTPUT_FILENAME  "output-of-test-const-attribute.c.s"
  23 - #include "harness.h"
  24 ...
  25 - /* { dg-final { jit-verify-output-file-was-created "" } } */
  26 - /* Check that the loop was optimized away */
  27 - /* { dg-final { jit-verify-assembler-output-not "jne" } } */
  28 ```
  29
  30 Then we'll add a `main` function which will call the `create_code` function but
  31 also add the calls we need to generate the GIMPLE:
  32
  33 ```C
  34 int main() {
  35     gcc_jit_context *ctxt = gcc_jit_context_acquire();
  36     // To set `-O3`, update it depending on your needs.
  37     gcc_jit_context_set_int_option(ctxt, GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 3);
  38     // Very important option to generate the gimple format.
  39     gcc_jit_context_set_bool_option(ctxt, GCC_JIT_BOOL_OPTION_DUMP_INITIAL_GIMPLE, 1);
  40     create_code(ctxt, NULL);
  41
  42     gcc_jit_context_compile(ctxt);
  43     // If you want to compile to assembly (or any other format) directly, you can
  44     // use the following call instead:
  45     // gcc_jit_context_compile_to_file(ctxt, GCC_JIT_OUTPUT_KIND_ASSEMBLER, "out.s");
  46
  47     return 0;
  48 }
  49 ```
  50
  51 Then we can compile it by using:
  52
  53 ```console
  54 gcc local.c -I `pwd`/gcc/gcc/jit/ -L `pwd`/gcc-build/gcc -lgccjit -o out
  55 ```
  56
  57 And finally when you run it:
  58
  59 ```console
  60 LD_LIBRARY_PATH=`pwd`/gcc-build/gcc LIBRARY_PATH=`pwd`/gcc-build/gcc ./out
  61 ```
  62
  63 It should display:
  64
  65 ```c
  66 __attribute__((const))
  67 int xxx ()
  68 {
  69   int D.3394;
  70   int sum;
  71   int x;
  72
  73   <D.3377>:
  74   x = 45;
  75   sum = 0;
  76   goto loop_cond;
  77   loop_cond:
  78   x = x >> 1;
  79   if (x != 0) goto after_loop; else goto loop_body;
  80   loop_body:
  81   _1 = foo (x);
  82   _2 = _1 * 2;
  83   x = x + _2;
  84   goto loop_cond;
  85   after_loop:
  86   D.3394 = sum;
  87   return D.3394;
  88 }
  89 ```
  90
  91 An alternative way to generate the GIMPLE is to replace:
  92
  93 ```c
  94     gcc_jit_context_set_bool_option(ctxt, GCC_JIT_BOOL_OPTION_DUMP_INITIAL_GIMPLE, 1);
  95 ```
  96
  97 with:
  98
  99 ```c
 100     gcc_jit_context_add_command_line_option(ctxt, "-fdump-tree-gimple");
 101 ```
 102
 103 (although you can have both at the same time too). Then you can compile it like previously. Only one difference: before executing it, I recommend to run:
 104
 105 ```console
 106 rm -rf /tmp/libgccjit-*
 107 ```
 108
 109 to make it easier for you to know which folder to look into.
 110
 111 Once the execution is done, you should now have a file with path looking like `/tmp/libgccjit-9OFqkD/fake.c.006t.gimple` which contains the GIMPLE format.