[clang] [clang] Set correct FPOptions if attribute 'optnone' presents (PR #85605)

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dyung wrote:

> Hi @dyung.
> 
> The observed difference is due to the FP contraction turned off if optnone is specified. In O0 this optimization is still applied. As a result, the function with optnone contains separate fadd and fmul, while without this attribute the function contains combined operatin fmuladd.
> 
> This optimization is turned off in `FPOption.setDisallowOptimizations` (introduced by this patch) by the call `setDisallowFPContract()`. If it is removed, I think, the output would be identical.

Thanks @spavloff. I can confirm that if I remove the call you mentioned the assembly generated is identical.

I am slightly confused why we are running FP contraction at O0. I would have thought no optimization at all be done at O0?

https://github.com/llvm/llvm-project/pull/85605