On Mon, Jun 1, 2020 at 3:19 AM Alexander Monakov <amona...@ispras.ru> wrote:
>
> 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.
>
My patch doesn't change strncmp at all. I opened:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95458
--
H.J.
