[KnownBits] Return `0` for poison {s,u}div inputs

It seems consistent to always return zero for known poison rather than
varying the value. We do the same elsewhere.

Differential Revision: https://reviews.llvm.org/D150922

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 8bc6562..ce2813e 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -758,6 +758,13 @@ KnownBits KnownBits::sdiv(const KnownBits &LHS, const KnownBits &RHS,
   assert(!LHS.hasConflict() && !RHS.hasConflict() && "Bad inputs");
   KnownBits Known(BitWidth);
 
+  if (LHS.isZero() || RHS.isZero()) {
+    // Result is either known Zero or UB. Return Zero either way.
+    // Checking this earlier saves us a lot of special cases later on.
+    Known.setAllZero();
+    return Known;
+  }
+
   std::optional<APInt> Res;
   if (LHS.isNegative() && RHS.isNegative()) {
     // Result non-negative.
@@ -819,6 +826,13 @@ KnownBits KnownBits::udiv(const KnownBits &LHS, const KnownBits &RHS,
   assert(!LHS.hasConflict() && !RHS.hasConflict());
   KnownBits Known(BitWidth);
 
+  if (LHS.isZero() || RHS.isZero()) {
+    // Result is either known Zero or UB. Return Zero either way.
+    // Checking this earlier saves us a lot of special cases later on.
+    Known.setAllZero();
+    return Known;
+  }
+
   // We can figure out the minimum number of upper zero bits by doing
   // MaxNumerator / MinDenominator. If the Numerator gets smaller or Denominator
   // gets larger, the number of upper zero bits increases.