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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158399 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 17 insertions, 1 deletions

diff --git a/lib/Transforms/Scalar/Reassociate.cpp b/lib/Transforms/Scalar/Reassociate.cpp
index ed7340f7e6..66fa0744b8 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/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:
diff --git a/test/Transforms/Reassociate/absorption.ll b/test/Transforms/Reassociate/absorption.ll
new file mode 100644
index 0000000000..2ccc2b5794
--- /dev/null
+++ b/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
+}