[llvm] [GlobalISel] An optimizing MIR builder (PR #132282)

[Amara Emerson via llvm-commits]

aemerson wrote:

> CSEMIRBuilder constant folds G_SITOFP and G_UITOFP. The parameters switch between integers and floats, but more importantly the bit width may change. For all cast operations, the input and output differ. I would prefer to the query the legalizer before folding.
> 
> mul(undef) combines into 0.
> 
> ```
>  if ((isUndef(SrcOps[1].getReg()) || isUndef(SrcOps[0].getReg())) &&
>       isConstantLegalOrBeforeLegalizer(DstTy))
>     return buildConstant(DstTy, 0);
> ```
> 
> My PR checks whether it is legal to write 0 into the destination.

Let me repeat my earlier question. Is this a real problem in practice?

https://github.com/llvm/llvm-project/pull/132282