[llvm] [KnownFPClass] Add neg_square and fma_neg_square (PR #227248)

[Matt Arsenault via llvm-commits]

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@@ -287,6 +287,27 @@ struct KnownFPClass {
     return Known;
   }
 
+  // Special cases of fmul -x, x and fmul x, -x.
+  static KnownFPClass
+  neg_square(const KnownFPClass &Src,
+             DenormalMode Mode = DenormalMode::getDynamic()) {
+    KnownFPClass Known = fmul(fneg(Src), Src, Mode);
+
+    // -X * X is always negative, zero, or a NaN.
+    Known.knownNot(fcPosSubnormal | fcPosNormal | fcPosInf);
+
+    // Zero results are -0 unless a denormal is flushed to +0.
+    if ((Mode.Input == DenormalMode::IEEE ||
+         Mode.Input == DenormalMode::PreserveSign) &&
+        (Mode.Output == DenormalMode::IEEE ||
+         Mode.Output == DenormalMode::PreserveSign)) {
+      Known.knownNot(fcPosZero);
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arsenm wrote:

I think you can't conclude this without a range check. A denormal input can still underflow to 0 

https://github.com/llvm/llvm-project/pull/227248