[LLVMdev] Clarifying FMA-related TargetOptions

[James Molloy]

On that, I'm afraid I have no clue. Hopefully someone more knowledgeable than me will chip in.

Cheers,

James
________________________________________
From: Owen Anderson [resistor at mac.com]
Sent: 08 February 2012 18:48
To: James Molloy
Cc: List
Subject: Re: [LLVMdev] Clarifying FMA-related TargetOptions

On Feb 8, 2012, at 10:44 AM, James Molloy wrote:

> Hi Owen,
>
> Having looked into this due to Clang failing PlumHall with it recently I can give an opinion...
>
> I think !NoExcessFPPrecision covers FMA completely. There are indeed some algorithms which give incorrect results when FMA is enabled, examples being those that do floating point comparisons such as: a * b + c - d. If c == d, it is still possible for that result not to equal a*b, as "+c " will have been fused with the multiply whereas "- d" won't.

I agree that !NoExcessFPPrecision seems like it should cover FMA, but if that that is the case, what does LessPreciseFPMADOption cover?

--Owen



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