[Bug tree-optimization/50557] New: [4.7 Regression] Register pressure increase after reassociation (x86, 32 bits)

[izamyatin at gmail dot com]

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: unassig...@gcc.gnu.org
        ReportedBy: izamya...@gmail.com


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