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From: "tim at klingt dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug other/37367] New: gcc-4.4 speed regression Date: Thu, 04 Sep 2008 10:03:00 -0000 [thread overview] Message-ID: <bug-37367-12873@http.gcc.gnu.org/bugzilla/> (raw) using a small piece of code of a digital filter, i was trying to benchmark several looping constructs. on x86_64 the following code was running 5% faster with g++-4.3 than with g++-4.4: float __attribute__ ((noinline)) bench_5(float * out_sample, int n) { float b1 = std::cos(0.01); float y1 = 0; float y2 = 1; do { float y0 = b1 * y1 - y2; *out_sample++ = y0; --n; } while (__builtin_expect(n!=0, 1)); } tim@thinkpad:~$ g++-4.3 -v Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.2-0ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.3.2 (Ubuntu 4.3.2-0ubuntu3) tim@thinkpad:~$ g++-4.4 -v Using built-in specs. Target: x86_64-linux-gnu Configured with: ../gcc-4.4-20080815/configure --enable-languages=c,c++,fortran,objc,obj-c++ --enable-shared --with-system-zlib --enable-mpfr --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/local/include/c++/4.4 --program-suffix=-4.4 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc Thread model: posix gcc version 4.4.0 20080815 (experimental) (GCC) the command line to compile the code was: g++ benchmarks/loop_benchmark.cpp -O3 -lrt -march=core2 the difference in the machine code is the order of two subl and leal instructions: *** 340,347 **** addq $16, %rax cmpl %r8d, %edx jb .L61 - subl %r9d, %esi leal 0(,%r9,4), %eax mov %eax, %eax addq %rax, %rcx cmpl %r9d, %r10d --- 340,347 ---- addq $16, %rax cmpl %r8d, %edx jb .L61 leal 0(,%r9,4), %eax + subl %r9d, %esi mov %eax, %eax addq %rax, %rcx cmpl %r9d, %r10d *************** since i read that gcc-4.4 is supposed to be aimed at code optimization, i thought it may be interesting to report it ... the complete code can be found at http://tinyurl.com/5socts -- Summary: gcc-4.4 speed regression Product: gcc Version: 4.4.0 Status: UNCONFIRMED Severity: minor Priority: P3 Component: other AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: tim at klingt dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37367
next reply other threads:[~2008-09-04 10:03 UTC|newest] Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top 2008-09-04 10:03 tim at klingt dot org [this message] 2008-09-06 21:45 ` [Bug target/37367] [4.4 Regression] " pinskia at gcc dot gnu dot org 2008-10-22 3:07 ` mmitchel at gcc dot gnu dot org 2009-01-31 14:19 ` bonzini at gnu dot org 2009-01-31 14:33 ` rguenth at gcc dot gnu dot org 2009-02-05 22:03 ` rguenth at gcc dot gnu dot org 2009-04-21 16:00 ` [Bug target/37367] [4.4/4.5 Regression] gcc-4.4/4.5 " jakub at gcc dot gnu dot org 2009-07-22 10:33 ` jakub at gcc dot gnu dot org 2009-10-15 12:55 ` jakub at gcc dot gnu dot org 2010-01-21 13:18 ` jakub at gcc dot gnu dot org 2010-03-21 12:20 ` steven at gcc dot gnu dot org 2010-03-21 14:44 ` howarth at nitro dot med dot uc dot edu 2010-03-21 16:20 ` hjl dot tools at gmail dot com 2010-03-22 10:01 ` rguenther at suse dot de 2010-03-22 10:47 ` jakub at gcc dot gnu dot org 2010-03-22 10:49 ` jakub at gcc dot gnu dot org
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