I was playing around with GCC disassembler on gcc.godbolt.org and I noticed that GCC starting version 4.6 compiles multiplication differently. I have the following two functions:
unsigned m126(unsigned i)
{
return i * 126;
}
unsigned m131(unsigned i)
{
return i * 131;
}
m126
compiles into:
mov eax, edi
mov edx, 126
imul eax, edx
ret
And m131
compiles into:
imul eax, edi, 131
ret
Why is the difference? GCC 4.5 generates the same opcode in both cases.
Found this in gcc/config/i386/i386.md
(see the comment on the top):
;; imul $8/16bit_imm, regmem, reg is vector decoded.
;; Convert it into imul reg, reg
;; It would be better to force assembler to encode instruction using long
;; immediate, but there is apparently no way to do so.
(define_peephole2
[(parallel [(set (match_operand:SWI248 0 "register_operand")
(mult:SWI248
(match_operand:SWI248 1 "nonimmediate_operand")
(match_operand:SWI248 2 "const_int_operand")))
(clobber (reg:CC FLAGS_REG))])
(match_scratch:SWI248 3 "r")]
"TARGET_SLOW_IMUL_IMM8 && optimize_insn_for_speed_p ()
&& satisfies_constraint_K (operands[2])"
[(set (match_dup 3) (match_dup 2))
(parallel [(set (match_dup 0) (mult:SWI248 (match_dup 0) (match_dup 3)))
(clobber (reg:CC FLAGS_REG))])]
{
if (!rtx_equal_p (operands[0], operands[1]))
emit_move_insn (operands[0], operands[1]);
})
Seems like it has something to do with instruction decoding (sorry I'm not an expert)