Add instructions on how to generate GIMPLE format

Readme.md

@@ -227,6 +227,11 @@ https://rust-lang.zulipchat.com/#narrow/stream/301329-t-devtools/topic/subtree.2
 
 `rustc` needs to be built without `jemalloc` so that `mem-trace` can overload `malloc` since `jemalloc` is linked statically, so a `LD_PRELOAD`-ed library won't a chance to intercept the calls to `malloc`.
 
+### How to generate GIMPLE
+
+If you need to check what gccjit is generating (GIMPLE), then take a look at how to
+generate it in [gimple.md](./doc/gimple.md).
+
 ### How to build a cross-compiling libgccjit
 
 #### Building libgccjit

doc/gimple.md

@@ -0,0 +1,80 @@
+# GIMPLE
+
+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.
+
+First, we'll copy the content from `gcc/gcc/testsuite/jit.dg/test-const-attribute.c` into a
+file named `local.c` and remove the content we're not interested into:
+
+```diff
+- /* { dg-do compile { target x86_64-*-* } } */
+...
+- /* We don't want set_options() in harness.h to set -O3 to see that the const
+-    attribute affects the optimizations. */
+- #define TEST_ESCHEWS_SET_OPTIONS
+- static void set_options (gcc_jit_context *ctxt, const char *argv0)
+- {
+-   // Set "-O3".
+-   gcc_jit_context_set_int_option(ctxt, GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 3);
+- }
+-
+- #define TEST_COMPILING_TO_FILE
+- #define OUTPUT_KIND      GCC_JIT_OUTPUT_KIND_ASSEMBLER
+- #define OUTPUT_FILENAME  "output-of-test-const-attribute.c.s"
+- #include "harness.h"
+...
+- /* { dg-final { jit-verify-output-file-was-created "" } } */
+- /* Check that the loop was optimized away */
+- /* { dg-final { jit-verify-assembler-output-not "jne" } } */
+```
+
+Then we'll add a `main` function which will call the `create_code` function but
+also add the calls we need to generate the GIMPLE:
+
+```C
+int main() {
+    gcc_jit_context *ctxt = gcc_jit_context_acquire();
+    create_code(ctxt, NULL);
+    gcc_jit_context_compile_to_file(ctxt, GCC_JIT_OUTPUT_KIND_DYNAMIC_LIBRARY, "tmp");
+    return 0;
+}
+```
+
+Then we can compile it by using:
+
+```console
+gcc const.c -I `pwd`/gcc/gcc/jit/ -L `pwd`/gcc-build/gcc -lgccjit -o out
+```
+
+And finally when you run it:
+
+```console
+LD_LIBRARY_PATH=`pwd`/gcc-build/gcc ./out
+```
+
+It should display:
+
+```c
+__attribute__((const))
+int xxx ()
+{
+  int D.3394;
+  int sum;
+  int x;
+
+  <D.3377>:
+  x = 45;
+  sum = 0;
+  goto loop_cond;
+  loop_cond:
+  x = x >> 1;
+  if (x != 0) goto after_loop; else goto loop_body;
+  loop_body:
+  _1 = foo (x);
+  _2 = _1 * 2;
+  x = x + _2;
+  goto loop_cond;
+  after_loop:
+  D.3394 = sum;
+  return D.3394;
+}
+```