It is possible for several constants which aren't individually absorbing to

combine to the absorbing element.  Thanks to nbjoerg on IRC for pointing this 
out.

llvm-svn: 158399

llvm/lib/Transforms/Scalar/Reassociate.cpp

@@ -629,8 +629,13 @@ static bool LinearizeExprTree(BinaryOperator *I,
   // Add any constants back into Ops, all globbed together and reduced to having
   // weight 1 for the convenience of users.
   Constant *Identity = ConstantExpr::getBinOpIdentity(Opcode, I->getType());
-  if (Cst && Cst != Identity)
+  if (Cst && Cst != Identity) {
+    // If combining multiple constants resulted in the absorber then the entire
+    // expression must evaluate to the absorber.
+    if (Cst == Absorber)
+      Ops.clear();
     Ops.push_back(std::make_pair(Cst, APInt(Bitwidth, 1)));
+  }
 
   // For nilpotent operations or addition there may be no operands, for example
   // because the expression was "X xor X" or consisted of 2^Bitwidth additions:

llvm/test/Transforms/Reassociate/absorption.ll

@@ -0,0 +1,11 @@
+; RUN: opt -S -reassociate < %s | FileCheck %s
+
+; Check that if constants combine to an absorbing value then the expression is
+; evaluated as the absorbing value.
+define i8 @foo(i8 %x) {
+  %tmp1 = or i8 %x, 127
+  %tmp2 = or i8 %tmp1, 128
+  ret i8 %tmp2
+; CHECK: @foo
+; CHECK: ret i8 -1
+}