[cfe-dev] question about fused multiply add and Clang GNU modes

[Hal Finkel via cfe-dev]

----- Original Message -----
> From: "Joerg Sonnenberger via cfe-dev" <cfe-dev at lists.llvm.org>
> To: cfe-dev at lists.llvm.org
> Sent: Monday, September 21, 2015 8:06:00 AM
> Subject: Re: [cfe-dev] question about fused multiply add and Clang GNU modes
> 
> On Mon, Sep 21, 2015 at 07:37:50AM -0500, Hal Finkel via cfe-dev
> wrote:
> > We don't track the C-level expressions in the IR, but Clang will
> > directly form @llvm.fmuladd intrinsics where allowed. CodeGen then
> > converts these into FMA nodes, or expands them into ADD + MUL
> > depending
> > on target hooks.
> 
> Wouldn't that be suboptimal from a CSE PoV? Consider something like:
> 
> r = a + b * c + b * c * d;
> 
> If we are greedy, the b * c would end up as (a + b * c) FMA
> instrinsic
> and the multiplication computed twice?

Yes, I think that it could. That's a good point.

 -Hal

> 
> Joerg
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory