Re: [Patch, MIPS] Enable fp-contract on MIPS and update -mfused-madd

[Joseph Myers <joseph at codesourcery dot com>]

On Thu, 11 Jun 2015, Steve Ellcey  wrote:

> loongson and r8000 have the most changes,  they no longer generate msub
> instructions with -mfused-madd because that instruction does not generate
> the correct NAN in some cases (the sign may be wrong).  If HONOR_NANS
> is not set then they will generate msub instructions.

There's no such thing as a correct NaN sign for fused multiply-add (at the 
C / GIMPLE / RTL level, that is; there may be a correct sign at the level 
of semantics for processor instructions).  IEEE 754 only specifies signs 
of NaNs for a few operations (copy, negate, abs, copySign).  So while you 
need to avoid negate / abs instructions that don't work properly on NaNs, 
if signs of NaNs are the only concern then that's not a reason to avoid 
any other arithmetic operations.

> One thing I did not put in this patch was to add code to use the
> mips32r6/mips64r6 msubf instruction.  This instruction implements
> 'c - (a * b)', not '(a * b) - c' and since it not currently used by
> GCC I decided not to add it to this patch.

Fused c - (a * b) is exactly equivalent to fused (-a * b) + c or 
(a * -b) + c (I don't know which is canonical in RTL).  (It's *not* 
equivalent to fused -((a * b) - c), when the result is an exact zero.  
And moving negation inside multiplication like that is only valid for a 
fused operation, not unfused if -frounding-math.)

-- 
Joseph S. Myers
joseph@codesourcery.com