Add a description to the test from r211433 explaining why it's written that way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211465 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Transforms/IndVarSimplify/2014-06-21-congruent-constant.ll b/test/Transforms/IndVarSimplify/2014-06-21-congruent-constant.ll
index 3df4c210b4adf6ac458ce12080d69f32eccdd28b..2c738de32e0a3a146170934891ed5cf307c3f5f3 100644 (file)
--- a/test/Transforms/IndVarSimplify/2014-06-21-congruent-constant.ll
+++ b/test/Transforms/IndVarSimplify/2014-06-21-congruent-constant.ll
@@ -1,5 +1,11 @@
 ; RUN: opt -S -loop-unswitch -instcombine -indvars < %s | FileCheck %s
 
 ; RUN: opt -S -loop-unswitch -instcombine -indvars < %s | FileCheck %s
 

+; This used to crash in SCEVExpander when there were congruent phis with and
+; undef incoming value from the loop header. The -loop-unswitch -instcombine is
+; necessary to create just this pattern, which is essentially a nop and gets
+; folded away aggressively if spelled out in IR directly.
+; PR 20093
+

 @c = external global i32**, align 8
 
 define void @test1() {
 @c = external global i32**, align 8
 
 define void @test1() {