[llvm] [GlobalISel] replace right identity X * -1.0 with fneg(x) (PR #80526)

[Pierre van Houtryve via llvm-commits]

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@@ -493,6 +493,13 @@ def right_identity_one_fp: GICombineRule<
   (apply (GIReplaceReg $dst, $x))
 >;
 
+def right_identity_neg_one_fp: GICombineRule<
+  (defs root:$dst),
+  (match (G_FMUL $dst, $x, $y):$root,
+    [{ return Helper.matchConstantFPOp(${y}, -1.0); }]),
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Pierre-vh wrote:

Unfortunately not, it's also been on my list but I'm not sure how to handle them well. TableGen doesn't have float literals. I can see a few options:

- Add float literals to tablegen
- Parse them from strings using APFloat
- Only allow hex/integer immediates and bitconvert to FP

What do you prefer?

https://github.com/llvm/llvm-project/pull/80526