http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51938
Bug #: 51938
Summary: missed optimization: 2 comparisons
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: marc.gli...@normalesup.org
Hello,
this is possibly related to bug 28685, or to the bugs about the tracking of
flag register values.
void f();
enum Sign { NEG=-1, ZERO, POS };
enum Sign sign(double x){
if(x>0) return POS;
if(x<0) return NEG;
return ZERO;
}
void g(double x){
if(sign(x)==NEG) f();
}
Ideally, the compiler would realize that g is equivalent to:
if(x<0) f();
but on x86_64 I get with 4.6 (optimization -O3):
xorpd %xmm1, %xmm1
ucomisd %xmm1, %xmm0
ja .L8
ucomisd %xmm0, %xmm1
jbe .L8
xorl %eax, %eax
jmp f
.L8:
rep
ret
And with 4.7:
xorpd %xmm1, %xmm1
ucomisd %xmm1, %xmm0
jbe .L12
.L9:
rep
ret
.L12:
ucomisd %xmm0, %xmm1
jbe .L9
xorl %eax, %eax
jmp f
Other compilers: clang 2.9 does the optimization (it has only one ucomisd and
j*), but neither Intel 11.1 nor Oracle 5.12.
In my application, this optimization has a 5% impact on the total runtime.
