[llvm-dev] Should NaN payloads be preserved through compilation?

[Friedman, Eli via llvm-dev]

On 11/9/2018 2:04 PM, Thomas Lively via llvm-dev wrote:
> Hi everyone,
>
> The WebAssembly backend recently had Bug 39448 
> <https://bugs.llvm.org/show_bug.cgi?id=39448> filed against it because 
> NaN payloads in floating-point immediates are not preserved through 
> compilation on 32-bit builds. I took a look and the corruption takes 
> place when the immediates are converted from APFloats to be stored as 
> native doubles in MCOperand. I assume this bug only appears in 32-bit 
> builds because they are using x87 doubles that happen to not preserve 
> all possible NaN payloads.
>
> There are two things we could do here: Change MCOperand to not store 
> floating point immediates as native doubles, or explicitly accept that 
> NaN payloads in immediates will not necessarily be preserved through 
> compilation.
>
> The ability to have custom NaN payloads in immediates could be useful 
> to the WebAssembly community, but if the consensus is that LLVM should 
> not guarantee their preservation, that's fine too. What do you think?

Seems fine, as long as you don't increase the size of MCOperand. 
(MachineOperand already uses ConstantFP, so it should be a tiny patch.)

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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