https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67089
Bug ID: 67089
Summary: [4.8/4.9/5/6 Regression] Integer overflow checks not
optimized on x86/x86_64
Product: gcc
Version: 5.2.1
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: m.mukovnikov at gmail dot com
Target Milestone: ---
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.
--------------- example ---------------
extern void underflow(void) __attribute__((noreturn));
unsigned sub(unsigned a, unsigned b)
{
unsigned r = a - b;
if (r > a) underflow();
return r;
}
------------ gcc-4.8.2 -O1 ------------
sub(unsigned int, unsigned int):
movl %edi, %eax
subl %esi, %eax /* CF is set on overflow */
jae .L4 /* jae = jnb = jnc = 73h */
subq $8, %rsp
call underflow()
.L4:
rep; ret
------------ 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
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