[ValueTracking] enhance isKnownNeverInfinity to understand sitofp

As discussed in D87877, instcombine already has this fold,
but it was missing from the more general ValueTracking logic.

https://alive2.llvm.org/ce/z/PumYZP

1 files changed, 14 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index ac83869..2b8b1ef 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3523,11 +3523,20 @@ bool llvm::isKnownNeverInfinity(const Value *V, const TargetLibraryInfo *TLI,
       return isKnownNeverInfinity(Inst->getOperand(1), TLI, Depth + 1) &&
              isKnownNeverInfinity(Inst->getOperand(2), TLI, Depth + 1);
     }
-    case Instruction::UIToFP:
-      // If the input type fits into the floating type the result is finite.
-      return ilogb(APFloat::getLargest(
-                 Inst->getType()->getScalarType()->getFltSemantics())) >=
-             (int)Inst->getOperand(0)->getType()->getScalarSizeInBits();
+    case Instruction::SIToFP:
+    case Instruction::UIToFP: {
+      // Get width of largest magnitude integer (remove a bit if signed).
+      // This still works for a signed minimum value because the largest FP
+      // value is scaled by some fraction close to 2.0 (1.0 + 0.xxxx).
+      int IntSize = Inst->getOperand(0)->getType()->getScalarSizeInBits();
+      if (Inst->getOpcode() == Instruction::SIToFP)
+        --IntSize;
+
+      // If the exponent of the largest finite FP value can hold the largest
+      // integer, the result of the cast must be finite.
+      Type *FPTy = Inst->getType()->getScalarType();
+      return ilogb(APFloat::getLargest(FPTy->getFltSemantics())) >= IntSize;
+    }
     default:
       break;
     }