[SCCP] Teach the pass how to handle `div` with overdefined operands.

This can prove that:

extern int f;
int g() {
    int x = 0;
    for (int i = 0; i < 365; ++i) {
        x /= f;
    }
    return x;
}

always returns zero. Thanks to Sanjoy for confirming this
transformation actually made sense (bugs are mine).

llvm-svn: 292531

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 9c5fa5c1426..334642fe35d 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -910,6 +910,12 @@ void SCCPSolver::visitBinaryOperator(Instruction &I) {
 
   // Otherwise, one of our operands is overdefined.  Try to produce something
   // better than overdefined with some tricks.
+  // If this is 0 / Y, it doesn't matter that the second operand is
+  // overdefined, and we can replace it with zero.
+  if (I.getOpcode() == Instruction::UDiv || I.getOpcode() == Instruction::SDiv)
+    if (V1State.isConstant() && V1State.getConstant()->isNullValue())
+      return markConstant(IV, &I, V1State.getConstant());
+
   // If this is:
   // -> AND/MUL with 0
   // -> OR with -1