[ValueTracking] Compute knownbits from known fp classes (#86409)

This patch calculates knownbits from fp instructions/dominating fcmp
conditions. It will enable more optimizations with signbit idioms.

1 files changed, 36 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 375385a..2fdbb6e 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1118,6 +1118,42 @@ static void computeKnownBitsFromOperator(const Operator *I,
       break;
     }
 
+    const Value *V;
+    // Handle bitcast from floating point to integer.
+    if (match(I, m_ElementWiseBitCast(m_Value(V))) &&
+        V->getType()->isFPOrFPVectorTy()) {
+      Type *FPType = V->getType()->getScalarType();
+      KnownFPClass Result = computeKnownFPClass(V, fcAllFlags, Depth + 1, Q);
+      FPClassTest FPClasses = Result.KnownFPClasses;
+
+      if (Result.isKnownNever(fcNormal | fcSubnormal | fcNan)) {
+        Known.Zero.setAllBits();
+        Known.One.setAllBits();
+
+        if (FPClasses & fcInf)
+          Known = Known.intersectWith(KnownBits::makeConstant(
+              APFloat::getInf(FPType->getFltSemantics()).bitcastToAPInt()));
+
+        if (FPClasses & fcZero)
+          Known = Known.intersectWith(KnownBits::makeConstant(
+              APInt::getZero(FPType->getScalarSizeInBits())));
+      }
+
+      if (Result.SignBit) {
+        if (*Result.SignBit)
+          Known.makeNegative();
+        else
+          Known.makeNonNegative();
+      } else {
+        Known.Zero.clearSignBit();
+        Known.One.clearSignBit();
+      }
+
+      assert(!Known.hasConflict() && "Bits known to be one AND zero?");
+
+      break;
+    }
+
     // Handle cast from vector integer type to scalar or vector integer.
     auto *SrcVecTy = dyn_cast<FixedVectorType>(SrcTy);
     if (!SrcVecTy || !SrcVecTy->getElementType()->isIntegerTy() ||