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[Bug c/77975] New: [6 / 7 Regression] Missed optimization for some small constants
- From: "quiath at go2 dot pl" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Thu, 13 Oct 2016 19:49:01 +0000
- Subject: [Bug c/77975] New: [6 / 7 Regression] Missed optimization for some small constants
- Auto-submitted: auto-generated
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77975
Bug ID: 77975
Summary: [6 / 7 Regression] Missed optimization for some small
constants
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: quiath at go2 dot pl
Target Milestone: ---
When compiling with GCC 6.1, 6.2 or 7.0 the code generated with -O1 -O2 and -O3
does not fold depending on some small constants e.g. 3 used in the body of the
function. GCC 5.4 was better.
Example code:
// missed optimization, compiled to a loop
unsigned int f3() {
unsigned int a = 3;
while (a) {
a >>= 1;
}
return a;
}
// expected optimization, compiled to a single instruction
unsigned int f7() {
unsigned int a = 7;
while (a) {
a >>= 1;
}
return a;
}
x86-64 assembly from gcc 7 (identical for 6) according to godbolt.org
Note that f7 is folded while f3 is not.
f3():
mov eax, 3
.L2:
shr eax
jne .L2
mov eax, 0
ret
f7():
mov eax, 0
ret
See also example here: https://godbolt.org/g/ircKQ0