Re: New insns for the s390 backend (2)

[Bradley Lucier <lucier at math dot purdue dot edu>]

(This is the second attempt, sorry if you get it twice.)

Re:

> Due to the fact that between the two operations no rounding takes 
> place the
> result maybe different compared to the same calculation performed with
> separated muliply and add/sub instructions. That's why the patterns 
> match
> only if unsafe math optimization are allowed (e.g. if the --fast-math
> optimization flag is present).

The rs6000 port has the TARGET_FUSED_MADD macro that determines when 
the associated patterns match, triggered by the -mfused-madd and 
-mno-fused-madd flags.  It seems that -mfused-madd is the default, but 
for the life of me I can't find the code that would verify this.

I bring this up because some people (like me) would like to have fused 
madd without setting flag_unsafe_math_optimizations; the latter flag 
allows so much strange floating-point behavior in the name of 
performance that it does not meet my needs, and by comparison fmadd is 
a well-behaved and useful addition to my toolkit.

I suggest that the s390 port do something similar.

Brad