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From: Hans Horn <hannes AT 2horns DOT com>
Subject:  Re: status of gcc4 -ffast-math
Date:  Thu, 05 Nov 2009 15:00:57 -0800
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Hans Horn wrote:
> Dave Korn wrote:
>> Hans Horn wrote:
>>> Folks,
>>>
>>> what is the current status of -ffast-math for gcc4 under cygwin.
>>>
>>> I tried to use it for some numerical C code and get the following link
>>> errors:
>>>
>>> eval.o:eval.c:(.text+0x79c): undefined reference to `_f_pow'
>>> eval.o:eval.c:(.text+0x7d8): undefined reference to `_f_log'
>>> eval.o:eval.c:(.text+0x7f0): undefined reference to `_f_exp'
>>
>>   WJFFM:
>>
>>> $ cat math.c
>>>
>>> #include <math.h>
>>>
>>> int main (int argc, const char **argv)
>>> {
>>>
>>>   float f = atof (argv[1]);
>>>   double d = log (f);
>>>   return exp (f);
>>> }
>>>
>>>
>>> admin AT ubik /tmp/math
>>> $ gcc-4 math.c -o math
>>>
>>> admin AT ubik /tmp/math
>>> $ gcc-4 math.c -o math -ffast-math
>>>
>>> admin AT ubik /tmp/math
>>> $ gcc-4 math.c -o math -ffast-math -O2
>>>
>>> admin AT ubik /tmp/math
>>> $ gcc-4 math.c -o math -ffast-math -O3
>>>
>>> admin AT ubik /tmp/math
>>> $
>>
>>   Perhaps a few more details about what you're doing, a simple 
>> reproducible
>> testcase, what kind of command-lines you're using, etc. etc... might 
>> help.
>>
>>     cheers,
>>       DaveK
> 
> Dave,
> 
> I was just shooting into the dark expecting more like a yes/no wrt use 
> of -ffast-math. Thx for responding!
> 
> I'm linking with gfortran (v4.5 or v4.3.4) (because the C code calls a 
> lot of fortran under the hood).
> 
> This is how I compile & link:
> 
> gcc-4 -c -D_XOPEN_SOURCE_EXTENDED=1 -D_ALL_SOURCE -D_POSIX_SOURCE \ 
> -std=c99 -Wall -Wshadow -Wreturn-type -Wunused \
> -Wuninitialized -Wformat -Wunused-function -Wunused-macros 
> -Wunused-label -Wredundant-decls -fno-leading-underscore -O3 \
> -fno-strict-aliasing -Winline -fexpensive-optimizations \ 
> -finline-functions -finline-limit=500000 -fstrength-reduce -fgcse \
> -fgcse-lm -fgcse-sm -funroll-loops -fforce-addr -fomit-frame-pointer \
> -ftree-vectorize -mfpmath=sse -msse3 -ffast-math \
> -march=pentium4 -pipe -fbounds-check -Wextra -Winit-self  \
> -DYY_NO_UNISTD_H -o math.o math.c
> 
> gfortran -O3 -fno-strict-aliasing -Winline -fexpensive-optimizations \
> -finline-functions -finline-limit=500000 \
> -fstrength-reduce -fgcse -fgcse-lm -fgcse-sm -funroll-loops -fforce-addr 
> -fomit-frame-pointer -ftree-vectorize -mfpmath=sse \
> -msse3 -ffast-math -march=pentium4 -pipe -L. -o math.exe math.o -Xlinker 
> -\( -O3 -fno-strict-aliasing -Winline -fexpensive-optimizations \
>  -finline-functions -finline-limit=500000 -fstrength-reduce -fgcse 
> -fgcse-lm -fgcse-sm -funroll-loops -fforce-addr -fomit-frame-pointer \
>  -ftree-vectorize -mfpmath=sse -msse3 -ffast-math -march=pentium4 -pipe 
> -lm -lgsl -ly -lfl --enable-auto-import -Xlinker -\)
> 
> Compiling/linking your example gives:
> 
> math.c: In function ‘main’:
> math.c:4: warning: implicit declaration of function ‘atof’
> math.c:5: warning: unused variable ‘d’
> math.c:3: warning: unused parameter ‘argc’
> math.o:math.c:(.text+0x30): undefined reference to `_f_exp'
> collect2: ld returned 1 exit status
> 
> Do you where this gobble stuff ‘ comes from, btw?
> 
> H.
> 
> 
Dave,

I just found the culprit!
It's -fno-leading-underscore.
Have to see whether I can live without it!

Thx. again


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