[llvm] [DAGCombiner] Fold freeze(fmul) + fadd/fsub into FMA combine (PR #142250)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

> By the looks of it SelectionDAG::canCreateUndefOrPoison doesn't handle fp ops at all - should we start with fixing those to more closely match ValueTracking llvm::canCreateUndefOrPoison?

Thanks for the pointer. After rereading the **LLVM Language Reference Manual** I agree that  
`SelectionDAG::canCreateUndefOrPoison` should return **false** for plain FP ops (`fadd`, `fmul`, …):  
by default they **propagate** poison but never **create** it.  
The only time they can generate new poison is when the instruction carries the `nnan` or `ninf`  
(or the umbrella `fast`) *fast-math* flags, because those flags require a NaN/Inf result to become poison.

The LangRef notes for every FP op include:

> “This instruction can also take any number of  
> [fast-math flags](https://llvm.org/docs/LangRef.html#fastmath), which are optimization hints to enable otherwise  
> unsafe floating-point optimizations.”

What do you think of this?

**References**

* [`fadd` instruction](https://llvm.org/docs/LangRef.html#fadd-instruction)  
* [Fast-Math Flags](https://llvm.org/docs/LangRef.html#fast-math-flags)



https://github.com/llvm/llvm-project/pull/142250