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Re: -mfpmath=387 flag does not prevent gcc from using SSE/SSE2 instructions

From: Jan Hubicka <jh at suse dot cz>
To: Andi Kleen <ak at suse dot de>
Cc: Andrew Derrick Balsa <andrebalsa at mailingaddress dot org>,gcc-bugs at gcc dot gnu dot org, Andreas Jaeger <aj at suse dot de>
Date: Fri, 2 May 2003 06:54:23 +0200
Subject: Re: -mfpmath=387 flag does not prevent gcc from using SSE/SSE2 instructions
References: <200305020714.50606.andrebalsa@mailingaddress.org> <20030502041259.GA30009@oldwotan.suse.de>

> > I thought the behaviour would be not to generate any SSE/SSE2 instruction with 
> > this flag. Am I wrong, or could this be a (small) bug?
> 
> The x86-64 ABI requires passing floating pointer arguments
> in SSE registers. Disabling it is not possible, you would
> be incompatible to all libraries.

Additionally -mfpmath=i387 will still use SSE for scalar floating point
operations where the results are known to be equivalent to the 387
counterparts (ie the extra precisity of 80/bit 387 registers does not
matter, such as for negation, moves, some conversions and comparsions).

Honza
> 
> -Andi

References:
- -mfpmath=387 flag does not prevent gcc from using SSE/SSE2 instructions
  - From: Andrew Derrick Balsa
- Re: -mfpmath=387 flag does not prevent gcc from using SSE/SSE2 instructions
  - From: Andi Kleen

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