[llvm] [KnownFPClass] Fix fsub input denormal handling (PR #225605)

[Eric Ross via llvm-commits]

================
@@ -494,47 +494,59 @@ static KnownFPClass fadd_impl(const KnownFPClass &KnownLHS,
       Known.knownNot(fcZero | fcNegSubnormal);
   }
 
+  if (!IsFMA) {
+    // (fadd x, +0.0) is guaranteed to return +0.0, not -0.0.
+    if (KnownLHS.isKnownNeverNegZero() || KnownRHS.isKnownNeverNegZero())
+      Known.knownNot(fcNegZero);
+
+    Known.propagateNonSNaN(KnownLHS, KnownRHS);
+  }
+
   return Known;
 }
 
-KnownFPClass KnownFPClass::fadd(const KnownFPClass &KnownLHS,
-                                const KnownFPClass &KnownRHS,
+KnownFPClass KnownFPClass::fadd(const KnownFPClass &KnownLHS_,
+                                const KnownFPClass &KnownRHS_,
                                 DenormalMode Mode) {
-  KnownFPClass Known = fadd_impl(KnownLHS, KnownRHS, Mode);
-
-  // (fadd x, 0.0) is guaranteed to return +0.0, not -0.0.
-  if ((KnownLHS.isKnownNeverLogicalNegZero(Mode) ||
-       KnownRHS.isKnownNeverLogicalNegZero(Mode)) &&
-      // Make sure output negative denormal can't flush to -0
-      (Mode.Output == DenormalMode::IEEE ||
-       Mode.Output == DenormalMode::PositiveZero))
-    Known.knownNot(fcNegZero);
+  KnownFPClass KnownLHS = applyInputDenormalMode(KnownLHS_, Mode);
+  KnownFPClass KnownRHS = applyInputDenormalMode(KnownRHS_, Mode);
 
-  Known.propagateNonSNaN(KnownLHS, KnownRHS);
+  KnownFPClass Known = fadd_impl(KnownLHS, KnownRHS, /*IsFMA=*/false);
 
-  return Known;
+  return applyOutputDenormalMode(Known, Mode);
 }
 
-KnownFPClass KnownFPClass::fadd_self(const KnownFPClass &KnownSrc,
+KnownFPClass KnownFPClass::fadd_self(const KnownFPClass &KnownSrc_,
                                      DenormalMode Mode) {
-  KnownFPClass Known = fadd(KnownSrc, KnownSrc, Mode);
+  KnownFPClass KnownSrc = applyInputDenormalMode(KnownSrc_, Mode);
+
+  KnownFPClass Known = fadd_impl(KnownSrc, KnownSrc, /*IsFMA=*/false);
+
+  if (KnownSrc.isKnownNever(fcPosSubnormal))
+    Known.knownNot(fcPosSubnormal);
+  if (KnownSrc.isKnownNever(fcNegSubnormal))
+    Known.knownNot(fcNegSubnormal);
----------------
ZERICO2005 wrote:

`fadd_impl` is unable to rule out a subnormal result in some cases, so we have to manually rule out subnormal here so `applyOutputDenormalMode` knows that the result won't flush to zero.

https://github.com/llvm/llvm-project/pull/225605