On Sun, 31 May 2020, H.J. Lu via Gcc-patches wrote:
> --- a/gcc/config/i386/i386-expand.c
> +++ b/gcc/config/i386/i386-expand.c
> @@ -7656,6 +7656,90 @@ ix86_expand_set_or_cpymem (rtx dst, rtx src, rtx
> count_exp, rtx val_exp,
> return true;
> }
>
> +/* Expand cmpstrn or memcmp. */
> +
> +bool
> +ix86_expand_cmpstrn_or_cmpmem (rtx result, rtx src1, rtx src2,
> + rtx length, rtx align, bool is_cmpstrn)
> +{
> + if (optimize_insn_for_size_p () && !TARGET_INLINE_ALL_STRINGOPS)
> + return false;
> +
> + /* Can't use this if the user has appropriated ecx, esi or edi. */
> + if (fixed_regs[CX_REG] || fixed_regs[SI_REG] || fixed_regs[DI_REG])
> + return false;
> +
> + if (is_cmpstrn)
> + {
> + /* For strncmp, length is the maximum length, which can be larger
> + than actual string lengths. We can expand the cmpstrn pattern
> + to "repz cmpsb" only if one of the strings is a constant so
> + that expand_builtin_strncmp() can write the length argument to
> + be the minimum of the const string length and the actual length
> + argument. Otherwise, "repz cmpsb" may pass the 0 byte. */
> + tree t1 = MEM_EXPR (src1);
> + tree t2 = MEM_EXPR (src2);
> + if (!((t1 && TREE_CODE (t1) == MEM_REF
> + && TREE_CODE (TREE_OPERAND (t1, 0)) == ADDR_EXPR
> + && (TREE_CODE (TREE_OPERAND (TREE_OPERAND (t1, 0), 0))
> + == STRING_CST))
> + || (t2 && TREE_CODE (t2) == MEM_REF
> + && TREE_CODE (TREE_OPERAND (t2, 0)) == ADDR_EXPR
> + && (TREE_CODE (TREE_OPERAND (TREE_OPERAND (t2, 0), 0))
> + == STRING_CST))))
> + return false;
> + }
> + else
> + {
> + /* Expand memcmp to "repz cmpsb" only for -minline-all-stringops
> + since "repz cmpsb" can be much slower than memcmp function
> + implemented with vector instructions, see
> +
> + https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052
> + */
> + if (!TARGET_INLINE_ALL_STRINGOPS)
> + return false;
> + }
This check seems to be misplaced, "rep cmps" is slower than either memcmp or
strcmp. The test for TARGET_INLINE_ALL_STRINGOPS should happen regardless of
is_cmpstrn, so it should go earlier in the function.
Alexander
