http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47258
Summary: Extra instruction generated in 4.5.2
Product: gcc
Version: 4.5.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: b...@broadcom.com
I encounter a performance regression in 4.5.2 (4.6 as well) compared with
4.5.1.
The code is from Core Mark.
Compile the attached .i file.
~/work/install-x86-452/bin/gcc core_matrix.i -O2 -S -o x86-452.s
...
.L5:
movl %r8d, %r10d
.L3:
mov %r9d, %r8d
movswl (%rcx,%rax), %r11d
addq $2, %rax
movswl (%rdx,%r8,2), %r8d
addl $1, %r9d
imull %r11d, %r8d
addl %r10d, %r8d
cmpq %rbx, %rax
jne .L5
...
~/work/install-x86-451/bin/gcc core_matrix.i -O2 -S -o x86-451.s
...
.L3:
mov %r9d, %r8d
movswl (%rcx,%rax), %r11d
addq $2, %rax
movswl (%rdx,%r8,2), %r8d
addl $1, %r9d
imull %r11d, %r8d
addl %r8d, %r10d
cmpq %rbx, %rax
jne .L3
...
The performance hit is even worse on our architecture because zero-overhead
loop instruction cannot be used in such irregular loop produced by 4.5.2
The configuration used is:
../gcc-4.5.1/configure
--prefix=/projects/firepath/tools/work/bmei/install-x86-451
--with-mpfr=/projects/firepath/tools/work/bmei/packages/mpfr/2.4.1/x86-64
--with-gmp=/projects/firepath/tools/work/bmei/packages/gmp/4.3.0/x86-64
--with-mpc=/projects/firepath/tools/work/bmei/packages/mpc/0.8.1/x86-64
--with-elf=/projects/firepath/tools/work/bmei/packages/libelf/x86-64
--disable-bootstrap --enable-languages=c --no-create --no-recursion
The difference between 4.5.1 and 4.5.2 seems to occur in RTL expand pass.
