[Fwd: [Fwd: Re: cygwin gcc compatibility with MSVC numerics]]

[Fwd: [Fwd: Re: cygwin gcc compatibility with MSVC numerics]]

[Tim Prince]

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Leena.Padgaonkar@patni.com wrote:
 > My basic problem is that the cygwin floating addition is giving 
different results  than VS 2008 for certain float values .Both the 
environments are on the same PC. So I was wondering about the floating 
point format used in cygwin.
 >
 > Btw, the makefile which I am using are having following options 
OPTFLAGS    = -I . -O3 -funroll-loops -mtune=pentium3 -ffast-math 
-mfancy-math-387
 >
 >
Only 64-bit Windows passes the same settings of x87 precision mode 
(53-bit) and SSE abrupt underflow mode to both gcc and MSVC built .exe.
-ffast-math would not be recommended for similarity to MSVC build, where 
none of the aggressive options would normally be in use.  Only the 
abrupt underflow setting matches MSVC.
If /Ox is set for MSVC, similar optimization should be obtained with gcc 
-O3.
If you are looking for full performance, and don't need compatibility 
with 10-year-old CPUs, you would normally set /fp:fast /arch:SSE2 in 
MSVC, and corresponding -march=pentium-m -mfpmath=sse (or newer -march) 
in gcc.  If you don't set /arch:SSE2 /fp:fast in MSVC, you imply K&R 
style promotion of certain float expressions to double, such as you get 
with 387 math in gcc.

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Leena.Padgaonkar@patni.com wrote:
> My basic problem is that the cygwin floating addition is giving 
> different results  than VS 2008 for certain float values .Both the 
> environments are on the same PC. So I was wondering about the floating 
> point format used in cygwin.
>
> Btw, the makefile which I am using are having following options 
> OPTFLAGS    = -I . -O3 -funroll-loops -mtune=pentium3 -ffast-math 
> -mfancy-math-387
>
>   
Only 64-bit Windows passes the same settings of x87 precision mode 
(53-bit) and SSE abrupt underflow mode to both gcc and MSVC built .exe.
-ffast-math would not be recommended for similarity to MSVC build, where 
none of the aggressive options would normally be in use.  Only the 
abrupt underflow setting matches MSVC.
If /Ox is set for MSVC, similar optimization should be obtained with gcc 
-O3.
If you are looking for full performance, and don't need compatibility 
with 10-year-old CPUs, you would normally set /fp:fast /arch:SSE2 in 
MSVC, and corresponding -march=pentium-m -mfpmath=sse (or newer -march) 
in gcc.  If you don't set /arch:SSE2 /fp:fast in MSVC, you imply K&R 
style promotion of certain float expressions to double, such as you get 
with 387 math in gcc.

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From: Tim Prince <n8tm@aol.com>
To: Leena.Padgaonkar@patni.com
Subject: Re: cygwin gcc compatibility with MSVC numerics
Date: Sun, 26 Jul 2009 12:18:03 -0700
Message-ID: <4A6CABEB.4000701@aol.com>




Leena.Padgaonkar@patni.com wrote:
> My basic problem is that the cygwin floating addition is giving different results  than VS 2008 for certain float values .Both the environments are on the same PC. So I was wondering about the floating point format used in cygwin.
>
> Btw, the makefile which I am using are having following options 
> OPTFLAGS    = -I . -O3 -funroll-loops -mtune=pentium3 -ffast-math -mfancy-math-387
>
>   
Only 64-bit Windows passes the same settings of x87 precision mode 
(53-bit) and SSE abrupt underflow mode to both gcc and MSVC built .exe.
-ffast-math would not be recommended for similarity to MSVC build, where 
none of the aggressive options would normally be in use.  Only the 
abrupt underflow setting matches MSVC.
If /Ox is set for MSVC, similar optimization should be obtained with gcc 
-O3.
If you are looking for full performance, and don't need compatibility 
with 10-year-old CPUs, you would normally set /fp:fast /arch:SSE2 in 
MSVC, and corresponding -march=pentium-m -mfpmath=sse (or newer -march) 
in gcc.  If you don't set /arch:SSE2 /fp:fast in MSVC, you imply K&R 
style promotion of certain float expressions to double, such as you get 
with 387 math in gcc.


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