http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55342
--- Comment #2 from Yuri Rumyantsev <ysrumyan at gmail dot com> 2012-11-19
12:06:20 UTC ---
The patching compiler produces better binaries but we still have -6%
performance degradation on corei7. The main cause of it it that LRA compiler
generates spill of 'pure' byte 'g' whereas old compiler generates spill for 'm'
that is negation of 'g':
gcc wwithout LRA (assembly part the head of loop)
.L7:
movzbl 1(%edi), %edx
leal 3(%edi), %ebp
movzbl (%edi), %ebx
movl %ebp, %edi
notl %edx // perform negation on register
movb %dl, 3(%esp)
gcc with LRA
.L7:
movzbl (%edi), %ebx
leal 3(%edi), %ebp
movzbl 1(%edi), %ecx
movl %ebp, %edi
movzbl -1(%ebp), %edx
notl %ebx
notl %ecx
movb %dl, (%esp)
cmpb %cl, %bl
notb (%esp) // perform nagation in memory
i.e. wwe have redundant load and store form/to stack.
I assume that this should be fixed also.
