[GVN] Don't replace constants with constants.

This fixes PR34908. Patch by Alex Crichton!

Differential Revision:  https://reviews.llvm.org/D38765

llvm-svn: 315429

2 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 4e24d755c26..c3cc2375e3b 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1362,6 +1362,11 @@ bool GVN::processAssumeIntrinsic(IntrinsicInst *IntrinsicI) {
     }
     markInstructionForDeletion(IntrinsicI);
     return false;
+  } else if (isa<Constant>(V)) {
+    // If it's not false, and constant, it must evaluate to true. This means our
+    // assume is assume(true), and thus, pointless, and we don't want to do
+    // anything more here.
+    return false;
   }
 
   Constant *True = ConstantInt::getTrue(V->getContext());
diff --git a/llvm/test/Transforms/GVN/pr34908.ll b/llvm/test/Transforms/GVN/pr34908.ll
new file mode 100644
index 00000000000..c2b58ad34a6
--- /dev/null
+++ b/llvm/test/Transforms/GVN/pr34908.ll
@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -gvn -S | FileCheck %s
+
+define i1 @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    call void @llvm.assume(i1 undef)
+; CHECK-NEXT:    ret i1 undef
+;
+  call void @llvm.assume(i1 undef)
+  ret i1 undef
+}
+
+declare void @llvm.assume(i1)