[llvm-dev] how to simplify FP ops with an undef operand?

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 28 15:06:11 PST 2018

On 02/27/2018 06:29 PM, Sanjay Patel via llvm-dev wrote:
> %y = fadd float %x, undef
> Can we simplify this?
> Currently in IR, we do nothing for fadd/fsub/fmul. For fdiv/frem, we 
> propagate undef. The code comment for fdiv/frem says:
> "the undef could be a snan"
> If that's correct, then shouldn't it be the same for fadd/fsub/fmul? 
> But this can't be correct because we support targets that don't raise 
> exceptions...and even targets that raise exceptions do not trap by 
> default on snan?
Fair warning, the following is a tangent from the actual topic under 
discussion...  It's only vaguely connected to the example at hand and is 
mostly a reaction to a collection of comments in various previous 
floating point discussion threads.

The reasoning here worries me.  The semantics of LLVM IR is specified 
independently of what any particular target does. Referring to target 
behavior to decide on appropriate semantics for a particular 
optimization seems inherently problematic. We should instead use LLVM 
IR's stated semantics to decide legality.  We should only use target 
semantics to help shape proposals to change the IR semantics.

To be specific, something along the lines of the following seems 
entirely reasonable: "LLVM IR allows us to perform the following 
optimization here, but doing that would radically complicate the 
lowering on target X which doesn't support Y.  Should we change the IR 
specification to Z?"  On the other hand, reasoning like "Target X 
doesn't do Y, so this optimization can't be legal" are problematic.

If the code is more specific than the LangRef, we should propose a 
clarification to the LangRef.  The specification for floating point 
semantics is admittedly a weakness in the current version.  Rather than 
working around it, we should fix that.

End vaguely related rant, thanks for reading!


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