[Bug target/37390] wrong-code on i486-linux-gnu with -O[12], -O0 works
--- Comment #4 from niklaus at gmail dot com 2008-09-06 17:42 --- On the below version of gcc on cygwin (winXP SP3) i don't have any problems with optimization on or off. They produce consistent correct result. Why is it a problem with linux ? or am i doing something wrong. I tried gcc -mfpmath=387 a.c -O2 and without -O2 , still the same correct results on both. Even with -ffloat-store . Is it glibc ? $ gcc -v Reading specs from /usr/lib/gcc/i686-pc-cygwin/3.4.4/specs Configured with: /usr/build/package/orig/test.respin/gcc-3.4.4-3/configure --verbose --prefix=/usr - -exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --enable-languages=c,ada,c++,d,f77,pascal,java,objc --enable-nls --without -included-gettext --enable-version-specific-runtime-libs --without-x --enable-libgcj --disable-java- awt --with-system-zlib --enable-interpreter --disable-libgcj-debug --enable-threads=posix --enable-j ava-gc=boehm --disable-win32-registry --enable-sjlj-exceptions --enable-hash-synchronization --enabl e-libstdcxx-debug Thread model: posix gcc version 3.4.4 (cygming special, gdc 0.12, using dmd 0.125) $ -- niklaus at gmail dot com changed: What|Removed |Added CC||niklaus at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37390
[Bug target/37390] wrong-code on i486-linux-gnu with -O[12], -O0 works
--- Comment #7 from niklaus at gmail dot com 2008-09-06 18:28 --- (In reply to comment #5) > Subject: Re: wrong-code on i486-linux-gnu with -O[12], -O0 works > > Because on x86 gnu/Linux, the precision is set to 80bits rather than > 64bit like it is on windows. > > Does increasing bits cause floating point errors. How could 64 bit precison give correct result where as 80 bit give incorrect one. I mean with changing the pow to powl and double to long double fixes the issue with optimization. Isn't long double precision 80 bits ? Also 1 more question . [EMAIL PROTECTED]:~/prog/tju$ gcc -O2 -mfpmath=sse bug_short.c -lm bug_short.c:1: warning: SSE instruction set disabled, using 387 arithmetics Shouldn't this be SSE instruction enabled and 387 disabled. 387 is causing the problem right ? I could be wrong , thats what i could figure out from the mail thread you marked as duplicate. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37390
[Bug target/37390] wrong-code on i486-linux-gnu with -O[12], -O0 works
--- Comment #10 from niklaus at gmail dot com 2008-09-06 21:23 --- (In reply to comment #8) > (In reply to comment #7) > > Does increasing bits cause floating point errors. How could 64 bit precison > > give correct result where as 80 bit give incorrect one. > > You can have rounding errors whether you increase the precision or not. In > particular, in practice, function pow() is not correctly rounded, and worse, > the error may be quite high. So, I'd say that whatever the choice made by > Linux, your code may be regarded as wrong (and I think this bug is just > invalid, as you could have similar problems with SSE2). The funny thing is that this only happens with -O2 or -O1 but not with -O0 ie no optimization it is all correct , when we optimize the results start varying. > > > [EMAIL PROTECTED]:~/prog/tju$ gcc -O2 -mfpmath=sse bug_short.c -lm > > bug_short.c:1: warning: SSE instruction set disabled, using 387 arithmetics > > You probably need another compilation flag, like -march=pentium4. > -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37390
