[Bug tree-optimization/43197] Endianness and Optimization

["pinskia at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #3 from pinskia at gmail dot com  2010-02-27 13:06 -------
Subject: Re:   New: Endianness and Optimization



Sent from my iPhone

On Feb 27, 2010, at 2:56 AM, "kai dot extern at googlemail dot com"
<gcc-bugzilla@gcc.gnu.org 
 > wrote:

> The attached code (which tries to generically load given-endianness  
> values of
> varying width from memory) shows some interesting optimization  
> quirks. It's
> especially pussling why optimization quality varies so greatly with  
> width, and
> is actually worst for the native word width.


You are violating c++ aliasing rules. You access a uint8_t via  
different types.


>
> For example:
>
> $ gcc -Wall -Wextra bad.cc -lstdc++ -O3 -###
> Using built-in specs.
> Target: x86_64-linux-gnu
> Configured with: ../src/configure -v --with-pkgversion='Ubuntu  
> 4.4.1-4ubuntu9'
> --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
> --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable- 
> shared
> --enable-multiarch --enable-linker-build-id --with-system-zlib
> --libexecdir=/usr/lib --without-included-gettext --enable- 
> threads=posix
> --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 -- 
> enable-nls
> --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc -- 
> disable-werror
> --with-arch-32=i486 --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 4.4.1 (Ubuntu 4.4.1-4ubuntu9)
> COLLECT_GCC_OPTIONS='-Wall' '-Wextra' '-O3' '-mtune=generic'
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/cc1plus" "-quiet" "-D_GNU_SOURCE"
> "bad.cc" "-D_FORTIFY_SOURCE=2" "-quiet" "-dumpbase" "bad.cc" "- 
> mtune=generic"
> "-auxbase" "bad" "-O3" "-Wall" "-Wextra" "-fstack-protector" "-o"
> "/tmp/ccnHgEb9.s"
> COLLECT_GCC_OPTIONS='-Wall' '-Wextra' '-O3' '-mtune=generic'
> "as" "-Qy" "-o" "/tmp/cceJWVC8.o" "/tmp/ccnHgEb9.s"
> COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/ 
> x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/ 
> x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/ 
> x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/x86_64-linux-gnu/
> LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/ 
> x86_64-linux-gnu/4.4.1/:/usr/lib/gcc/x86_64-linux-gnu/ 
> 4.4.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/ 
> x86_64-linux-gnu/4.4.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64- 
> linux-gnu/
> COLLECT_GCC_OPTIONS='-Wall' '-Wextra' '-O3' '-mtune=generic'
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/collect2" "--build-id" "--eh- 
> frame-hdr"
> "-m" "elf_x86_64" "--hash-style=both" "-dynamic-linker"
> "/lib64/ld-linux-x86-64.so.2" "-z" "relro"
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../../../lib/crt1.o"
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../../../lib/crti.o"
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/crtbegin.o"
> "-L/usr/lib/gcc/x86_64-linux-gnu/4.4.1" "-L/usr/lib/gcc/x86_64-linux- 
> gnu/4.4.1"
> "-L/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../../../lib" "-L/lib/../ 
> lib"
> "-L/usr/lib/../lib" "-L/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../.."
> "-L/usr/lib/x86_64-linux-gnu" "/tmp/cceJWVC8.o" "-lstdc++" "-lgcc"
> "--as-needed" "-lgcc_s" "--no-as-needed" "-lc" "-lgcc" "--as-needed"  
> "-lgcc_s"
> "--no-as-needed" "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/crtend.o"
> "/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../../../lib/crtn.o"
> $ objdump -d a.out | c++filt | sed -n '/Test_/,/constructors/ p' > out
>
> (See attachments for source and output.)
>
>
> -- 
>           Summary: Endianness and Optimization
>           Product: gcc
>           Version: 4.4.1
>            Status: UNCONFIRMED
>          Severity: normal
>          Priority: P3
>         Component: tree-optimization
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: kai dot extern at googlemail dot com
> GCC build triplet: x86_64-linux-gnu
>  GCC host triplet: x86_64-linux-gnu
> GCC target triplet: x86_64-linux-gnu
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43197
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43197