Analysis: Make isSafeToSpeculativelyExecute fire less for divides

Divides and remainder operations do not behave like other operations
when they are given poison: they turn into undefined behavior.

It's really hard to know if the operands going into a div are or are not
poison.  Because of this, we should only choose to speculate if there
are constant operands which we can easily reason about.

This fixes PR21412.

llvm-svn: 221318

1 files changed, 23 insertions, 15 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 3e8cb93..0d93285 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2549,23 +2549,31 @@ bool llvm::isSafeToSpeculativelyExecute(const Value *V,
   default:
     return true;
   case Instruction::UDiv:
-  case Instruction::URem:
-    // x / y is undefined if y == 0, but calculations like x / 3 are safe.
-    return isKnownNonZero(Inst->getOperand(1), TD);
+  case Instruction::URem: {
+    // x / y is undefined if y == 0.
+    const APInt *V;
+    if (match(Inst->getOperand(1), m_APInt(V)))
+      return *V != 0;
+    return false;
+  }
   case Instruction::SDiv:
   case Instruction::SRem: {
-    Value *Op = Inst->getOperand(1);
-    // x / y is undefined if y == 0
-    if (!isKnownNonZero(Op, TD))
-      return false;
-    // x / y might be undefined if y == -1
-    unsigned BitWidth = getBitWidth(Op->getType(), TD);
-    if (BitWidth == 0)
-      return false;
-    APInt KnownZero(BitWidth, 0);
-    APInt KnownOne(BitWidth, 0);
-    computeKnownBits(Op, KnownZero, KnownOne, TD);
-    return !!KnownZero;
+    // x / y is undefined if y == 0 or x == INT_MIN and y == -1
+    const APInt *X, *Y;
+    if (match(Inst->getOperand(1), m_APInt(Y))) {
+      if (*Y != 0) {
+        if (*Y == -1) {
+          // The numerator can't be MinSignedValue if the denominator is -1.
+          if (match(Inst->getOperand(0), m_APInt(X)))
+            return !Y->isMinSignedValue();
+          // The numerator *might* be MinSignedValue.
+          return false;
+        }
+        // The denominator is not 0 or -1, it's safe to proceed.
+        return true;
+      }
+    }
+    return false;
   }
   case Instruction::Load: {
     const LoadInst *LI = cast<LoadInst>(Inst);