[llvm-branch-commits] [llvm] InstCombine: Handle rounding intrinsics in SimplifyDemandedFPClass (PR #174842)

[Yingwei Zheng via llvm-branch-commits]

================
@@ -2519,6 +2519,73 @@ Value *InstCombinerImpl::SimplifyDemandedUseFPClass(Instruction *I,
 
       return getFPClassConstant(VTy, ValidResults, /*IsCanonicalizing=*/true);
     }
+    case Intrinsic::trunc:
+    case Intrinsic::floor:
+    case Intrinsic::ceil:
+    case Intrinsic::rint:
+    case Intrinsic::nearbyint:
+    case Intrinsic::round:
+    case Intrinsic::roundeven: {
+      FPClassTest DemandedSrcMask = DemandedMask;
+
+      // Zero results imply valid subnormal sources.
+      if (DemandedMask & fcNegZero)
+        DemandedSrcMask |= fcNegSubnormal;
+
+      if (DemandedMask & fcPosZero)
+        DemandedSrcMask |= fcPosSubnormal;
+
+      KnownFPClass KnownSrc;
+      if (SimplifyDemandedFPClass(CI, 0, DemandedSrcMask, KnownSrc, Depth + 1))
+        return I;
+
+      // Note: Possibly dropping snan quiet.
+      if (KnownSrc.isKnownAlways(fcInf | fcNan | fcZero))
+        return CI->getArgOperand(0);
+
+      // Propagate nnan-ness to source to simplify source checks.
+      if ((DemandedMask & fcNan) == fcNone)
+        KnownSrc.knownNot(fcNan);
+
+      bool IsRoundNearest =
+          IID == Intrinsic::round || IID == Intrinsic::roundeven ||
+          IID == Intrinsic::nearbyint || IID == Intrinsic::rint;
+
+      // Ignore denormals-as-zero, as canonicalization is not mandated.
+      if ((IID == Intrinsic::trunc || IID == Intrinsic::floor ||
+           IsRoundNearest) &&
+          (KnownSrc.isKnownAlways(fcPosZero | fcPosSubnormal)))
+        return ConstantFP::getZero(VTy);
+
+      if ((IID == Intrinsic::trunc || IsRoundNearest) &&
----------------
dtcxzyw wrote:

This is still standing.

https://github.com/llvm/llvm-project/pull/174842