http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117
Bug #: 53117
Summary: missed-optimization: worse code for 'x <= 0' compared
to 'x < 0'
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: vermaelen.wou...@gmail.com
void f1(int* p) {
p[1] -= 5;
if (p[1] < 0) p[2] += 3;
}
void f2(int* p) {
p[1] -= 5;
if (p[1] <= 0) p[2] += 3;
}
The only difference between f1() and f2() is the comparison ('<' vs '<='). On
x86_64 (and x86) gcc revision trunk@186808 generates more efficient code for
f1() than for f2(). Here's the assembler output when compiled with -Os (but -O2
and -O3) show a similar difference:
0000000000000000 <_Z2f1Pi>:
0: 83 6f 04 05 subl $0x5,0x4(%rdi)
4: 79 04 jns a <_Z2f1Pi+0xa>
6: 83 47 08 03 addl $0x3,0x8(%rdi)
a: c3 retq
000000000000000b <_Z2f2Pi>:
b: 8b 47 04 mov 0x4(%rdi),%eax
e: 83 e8 05 sub $0x5,%eax
11: 85 c0 test %eax,%eax
13: 89 47 04 mov %eax,0x4(%rdi)
16: 7f 04 jg 1c <_Z2f2Pi+0x11>
18: 83 47 08 03 addl $0x3,0x8(%rdi)
1c: c3 retq
gcc-4.6.1 generates the less efficient variant for both functions.
