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Re: unfused fma question
- From: Andrew Haley <aph at redhat dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>, GCC Development <gcc at gcc dot gnu dot org>
- Cc: Jeff Law <law at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Steve Ellcey <sellcey at imgtec dot com>
- Date: Tue, 24 Feb 2015 13:56:23 +0000
- Subject: Re: unfused fma question
- Authentication-results: sourceware.org; auth=none
- References: <02d0fee7-2c86-4291-8405-ae250d3210d9 at BAMAIL02 dot ba dot imgtec dot org> <alpine dot DEB dot 2 dot 10 dot 1502231833070 dot 7064 at digraph dot polyomino dot org dot uk> <54EB73FD dot 1000003 at redhat dot com> <alpine dot DEB dot 2 dot 11 dot 1502231946100 dot 1758 at laptop-mg dot saclay dot inria dot fr> <CAFiYyc02RYcLTH4dbz2PtKaHXSs+TuBbAxek6aMvAXA25xyd4w at mail dot gmail dot com>
On 02/24/2015 12:14 PM, Richard Biener wrote:
> These concerns are correct. Btw, as an answer to Steve - within
> -funsafe-math-optimizations FMA_EXPR basically can be either
> fused or not fused (but yes, bad as to Josephs concern).
> So you could guard the pattern by flag_unsafe_math_optimizations.
The unfused fma is always safe to use: it's the same as a multiply
and an add, surely.