https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67089
--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Mike from comment #0)
> Starting from 4.8.3, gcc does not optimize integer overflow and underflow
> checks inserting a redundant cmp instruction in both 32-bit and 64-bit
> modes. Having git-bisected the revisions, I found out r204088 (by pr58779)
> had added such behavior.
It was removed for a reason, as explained in the referred PR.
> ------------ gcc-4.8.3 -O1 ------------
> sub(unsigned int, unsigned int):
> movl %edi, %eax
> subl %esi, %eax
> cmpl %eax, %edi /* absolutely redundant */
> jnb .L4
> subq $8, %rsp
> call underflow()
> .L4:
> rep ret
> ---------------------------------------
The sequence is actually:
(insn 7 4 8 2 (parallel [
(set (reg/v:SI 87 [ r ])
(minus:SI (reg/v:SI 89 [ a ])
(reg/v:SI 90 [ b ])))
(clobber (reg:CC 17 flags))
]) pr67089.c:4 259 {*subsi_1}
(expr_list:REG_DEAD (reg/v:SI 90 [ b ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 8 7 9 2 (set (reg:CC 17 flags)
(compare:CC (reg/v:SI 89 [ a ])
(reg/v:SI 87 [ r ]))) pr67089.c:5 7 {*cmpsi_1}
(expr_list:REG_DEAD (reg/v:SI 89 [ a ])
(nil)))
(jump_insn 9 8 10 2 (set (pc)
(if_then_else (geu (reg:CC 17 flags)
(const_int 0 [0]))
(label_ref 13)
(pc))) pr67089.c:5 609 {*jcc_1}
(expr_list:REG_DEAD (reg:CC 17 flags)
(int_list:REG_BR_PROB 9996 (nil)))
-> 13)
and combine tries to simplify this sequence with:
Trying 7 -> 8:
Failed to match this instruction:
(parallel [
(set (reg:CC 17 flags)
(compare:CC (minus:SI (reg/v:SI 89 [ a ])
(reg/v:SI 90 [ b ]))
(reg/v:SI 89 [ a ])))
(set (reg/v:SI 87 [ r ])
(minus:SI (reg/v:SI 89 [ a ])
(reg/v:SI 90 [ b ])))
])
and further to:
Trying 7, 8 -> 9:
Failed to match this instruction:
(parallel [
(set (pc)
(if_then_else (leu (minus:SI (reg/v:SI 89 [ a ])
(reg/v:SI 90 [ b ]))
(reg/v:SI 89 [ a ]))
(label_ref 13)
(pc)))
(set (reg/v:SI 87 [ r ])
(minus:SI (reg/v:SI 89 [ a ])
(reg/v:SI 90 [ b ])))
])
LEU condition was created from GEU as the operands in (insn 8) were swapped. As
can be seen from config/i386.c, ix86_cc_mode, LEU can't be implemented using
only Carry flag:
case GTU: /* CF=0 & ZF=0 */
case LEU: /* CF=1 | ZF=1 */
return CCmode;
The removed code compensated this with *reversion* of the flag check, e.g. the
same condition was emitted for LEU with CCCmode as for GEU (and in a similar
for GTU in CCCmode vs. LTU, as was the case in PR58779).
We shouldn't do this, and it reflected in PR58779.
