http://llvm.org/bugs/show_bug.cgi?id=943
Summary: Too much spill code generated after loop unrolling
Product: libraries
Version: trunk
Platform: Macintosh
OS/Version: MacOS X
Status: NEW
Severity: normal
Priority: P2
Component: Backend: X86
AssignedTo: unassignedbugs at nondot.org
ReportedBy: isanbard at gmail.com
This code:
int value(uint64 b1, uint64 b2)
{
int i, j, k;
int value = 0;
for (k = 0; k < 2; k++)
for (i = 0; i < 6; i++)
for (j = 0; j < 2; j++)
if ((b2 & 0xf << (j + i * 6)) == 0xf << (j + i * 6))
value += 1000;
return value;
}
generates too many unnecessary spills/reloads when rerun through the optimizer (at -O2). See "***"
below:
LBB1_1: #bb17.preheader
movl $15, %eax
*** movb 39(%esp), %cl
movl %eax, %edx
shll %cl, %edx
*** movb 39(%esp), %cl
incb %cl
shll %cl, %eax
movl %edx, %ecx
The result is that the loop will run slower than if optimizations were turned on only once.
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