[PATCH] D146494: [X86] Combine constant vector inputs for FMA

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:54173
+
+  // If the inverted value also can be eliminated, we have to persistancy
+  // prefer one of the values. We prefer a constant with negative value on the
----------------
persistently


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:54178
+    if (auto *Cst = dyn_cast<ConstantFPSDNode>(op)) {
+      if (Cst->isNegative())
+        return SDValue();
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e-kud wrote:
> RKSimon wrote:
> > Couldn't both V and NV have negative values now that they aren't splats?
> It seems to me that they can't. If `V` is `<x0, x1, ..., xn>` then `NV` is `<-x0, -x1, ..., -xn>`. So, `V` and `NV` always have elements of different sign. The corner case is `V` consisting only of `undef`s. But original and negated versions of such vector are the same. So, everything should be handled well.
But couldn't both `V` and `NV` contain some negative and some positive values?


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https://reviews.llvm.org/D146494