http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57051
Bug #: 57051
Summary: Optimization regression in 4.8.0 from 4.7.2
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: tony.howard-co84u...@yopmail.com
The problem can be exhibited using the following snippet of code:
/* begin */
int total = 0;
inline void foo(int bar)
{
total += bar*10;
}
void hello()
{
for(int i = 0; i < 10; ++i)
{
foo(i);
}
}
/* end */
With the option -O3,
gcc 4.7.2 and previous versions produce the following assembly code:
hello():
addl $450, total(%rip)
ret
gcc 4.8.0 fails to optimize the code:
hello():
movdqa .LC0(%rip), %xmm0
movdqa %xmm0, %xmm2
psrldq $8, %xmm2
paddd %xmm2, %xmm0
movdqa %xmm0, %xmm3
psrldq $4, %xmm3
paddd %xmm3, %xmm0
movdqa %xmm0, %xmm4
movd %xmm4, -12(%rsp)
movl -12(%rsp), %eax
addl total(%rip), %eax
addl $170, %eax
movl %eax, total(%rip)
ret
The results can be easily reproduced using http://gcc.godbolt.org/
The 4.7.2's configure options are:
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.2-11precise2' --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
--with-system-zlib --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 --enable-objc-gc --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
The 4.8.0's configure options are:
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.8.0-3ubuntu3~12.04' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --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 --enable-multiarch --disable-werror
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
The problem is not linked to Linaro or Ubuntu patches as it can be reproduced
as well on a stock version of gcc 4.8.0
