To get more information on what’s going on, you can set debugging flags on the context using gccjit;;context;;set_bool_option().
Setting GCC_JIT_BOOL_OPTION_DUMP_INITIAL_GIMPLE will dump a C-like representation to stderr when you compile (GCC’s “GIMPLE” representation):
ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_DUMP_INITIAL_GIMPLE, 1); result = ctxt.compile ();
square (signed int i)
{
signed int D.260;
entry:
D.260 = i * i;
return D.260;
}
We can see the generated machine code in assembler form (on stderr) by setting GCC_JIT_BOOL_OPTION_DUMP_GENERATED_CODE on the context before compiling:
ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_DUMP_GENERATED_CODE, 1); result = ctxt.compile ();
.file "fake.c"
.text
.globl square
.type square, @function
square:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl %edi, -4(%rbp)
.L14:
movl -4(%rbp), %eax
imull -4(%rbp), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size square, .-square
.ident "GCC: (GNU) 4.9.0 20131023 (Red Hat 0.2-0.5.1920c315ff984892399893b380305ab36e07b455.fc20)"
.section .note.GNU-stack,"",@progbits
By default, no optimizations are performed, the equivalent of GCC’s -O0 option. We can turn things up to e.g. -O3 by calling gccjit;;context;;set_int_option() with GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL:
ctxt.set_int_option (GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 3);
.file "fake.c"
.text
.p2align 4,,15
.globl square
.type square, @function
square:
.LFB7:
.cfi_startproc
.L16:
movl %edi, %eax
imull %edi, %eax
ret
.cfi_endproc
.LFE7:
.size square, .-square
.ident "GCC: (GNU) 4.9.0 20131023 (Red Hat 0.2-0.5.1920c315ff984892399893b380305ab36e07b455.fc20)"
.section .note.GNU-stack,"",@progbits
Naturally this has only a small effect on such a trivial function.