http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57204
Bug #: 57204
Summary: Auto-vectorization in nested loops with non-varying
indexed array access results in very poor performance
(worse than no auto-vectorization)
Classification: Unclassified
Product: gcc
Version: 4.7.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: snagaval...@outlook.com
In the good case below, auto-vectorization improves performance by a factor of
3. In the bad case, it actually decreases performance compared to no
auto-vectorization.
Good:
void foo(float * d, int n)
{
int i, j, k;
for (k=0; k<n; ++k) {
for (i=0; i<n; ++i) {
float d_ik = d[i*n+k];
for (j=0; j<n; ++j) {
float t = d_ik + d[k*n+j];
d[i*n+j] = (d[i*n+j] < t) ? d[i*n+j] : t;
}
}
}
}
Bad:
void foo(float * d, int n)
{
int i, j, k;
for (k=0; k<n; ++k) {
for (i=0; i<n; ++i) {
for (j=0; j<n; ++j) {
float t = d[i*n+k] + d[k*n+j];
d[i*n+j] = (d[i*n+j] < t) ? d[i*n+j] : t;
}
}
}
}
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.3-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--with-system-zlib --enable-objc-gc --with-cloog --enable-cloog-backend=ppl
--disable-cloog-version-check --disable-ppl-version-check --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)
$ gcc -march=native -Q --help=target
The following options are target specific:
-m128bit-long-double [disabled]
-m32 [disabled]
-m3dnow [disabled]
-m3dnowa [disabled]
-m64 [enabled]
-m80387 [enabled]
-m8bit-idiv [disabled]
-m96bit-long-double [enabled]
-mabi= sysv
-mabm [disabled]
-maccumulate-outgoing-args [disabled]
-maddress-mode= short
-maes [disabled]
-malign-double [disabled]
-malign-functions= 0
-malign-jumps= 0
-malign-loops= 0
-malign-stringops [enabled]
-mandroid [disabled]
-march= corei7
-masm= att
-mavx [disabled]
-mavx2 [disabled]
-mavx256-split-unaligned-load [disabled]
-mavx256-split-unaligned-store [disabled]
-mbionic [disabled]
-mbmi [disabled]
-mbmi2 [disabled]
-mbranch-cost= 0
-mcld [disabled]
-mcmodel= 32
-mcpu=
-mcrc32 [disabled]
-mcx16 [enabled]
-mdispatch-scheduler [disabled]
-mf16c [disabled]
-mfancy-math-387 [enabled]
-mfentry [enabled]
-mfma [disabled]
-mfma4 [disabled]
-mforce-drap [disabled]
-mfp-ret-in-387 [enabled]
-mfpmath= 387
-mfsgsbase [disabled]
-mfused-madd
-mglibc [enabled]
-mhard-float [enabled]
-mieee-fp [enabled]
-mincoming-stack-boundary= 0
-minline-all-stringops [disabled]
-minline-stringops-dynamically [disabled]
-mintel-syntax
-mlarge-data-threshold= 0x10000
-mlwp [disabled]
-mlzcnt [disabled]
-mmmx [disabled]
-mmovbe [disabled]
-mms-bitfields [disabled]
-mno-align-stringops [disabled]
-mno-fancy-math-387 [disabled]
-mno-push-args [disabled]
-mno-red-zone [disabled]
-mno-sse4 [disabled]
-momit-leaf-frame-pointer [disabled]
-mpc32 [disabled]
-mpc64 [disabled]
-mpc80 [disabled]
-mpclmul [disabled]
-mpopcnt [enabled]
-mprefer-avx128 [disabled]
-mpreferred-stack-boundary= 0
-mpush-args [enabled]
-mrdrnd [disabled]
-mrecip [disabled]
-mrecip=
-mred-zone [enabled]
-mregparm= 0
-mrtd [disabled]
-msahf [enabled]
-msoft-float [disabled]
-msse [enabled]
-msse2 [enabled]
-msse2avx [disabled]
-msse3 [enabled]
-msse4 [enabled]
-msse4.1 [enabled]
-msse4.2 [enabled]
-msse4a [disabled]
-msse5
-msseregparm [disabled]
-mssse3 [enabled]
-mstack-arg-probe [disabled]
-mstackrealign [enabled]
-mstringop-strategy= [default]
-mtbm [disabled]
-mtls-dialect= gnu
-mtls-direct-seg-refs [enabled]
-mtune= corei7
-muclibc [disabled]
-mveclibabi= [default]
-mvect8-ret-in-mem [disabled]
-mvzeroupper [disabled]
-mx32 [disabled]
-mxop [disabled]
Known assembler dialects (for use with the -masm-dialect= option):
att intel
Known ABIs (for use with the -mabi= option):
ms sysv
Known code models (for use with the -mcmodel= option):
32 kernel large medium small
Valid arguments to -mfpmath=:
387 387+sse 387,sse both sse sse+387 sse,387
Known vectorization library ABIs (for use with the -mveclibabi= option):
acml svml
Known address mode (for use with the -maddress-mode= option):
long short
Valid arguments to -mstringop-strategy=:
byte_loop libcall loop rep_4byte rep_8byte rep_byte unrolled_loop
Known TLS dialects (for use with the -mtls-dialect= option):
gnu gnu2
