http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50557
Bug #: 50557
Summary: [4.7 Regression] Register pressure increase after
reassociation (x86, 32 bits)
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
After the fix for http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49749
(http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=176984) I observed a
degradation for the attached test. (~9% on Core)
Before the fix RA managed to use registers for code related to line #30. After
the fix an order of operations has been changed and this apparently led to
change in live ranges and hence to increased register pressure.
Asm snippet for fast case
# 4long.c:30
.loc 1 30 0
movl 8(%ecx), %esi
xorl %edi, %edi
addl %eax, %esi
movl 52(%esp), %eax
adcl %edx, %edi
mull 8(%ebp)
addl %eax, %esi
adcl %edx, %edi
Asm snippet for slow case
# 4long.c:30
.loc 1 30 0
movl 52(%esp), %eax
mull 8(%ebp)
movl %eax, (%esp)
movl 8(%ecx), %eax
movl %edx, 4(%esp)
xorl %edx, %edx
addl %eax, (%esp)
adcl %edx, 4(%esp)
addl %esi, (%esp)
adcl %edi, 4(%esp)
gcc is:
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --disable-bootstrap --enable-languages=c,c++
--prefix=/export/users/izamyati/build/
Thread model: posix
gcc version 4.7.0 20110731 (experimental) (GCC)
Compilation flags:
-O2 -mssse3 -mfpmath=sse -ffast-math -m32
