From 9ad8c54aab51afec95ae4989e36850cddf40b606 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Fri, 13 Jun 2008 21:43:41 +0000
Subject: [PATCH] Protect ChangeCompareStride from situations in which it is
 possible for it to generate use-before-def IR, such as in this testcase.

llvm-svn: 52258
---
 .../Transforms/Scalar/LoopStrengthReduce.cpp  | 15 +++++++++++---
 .../change-compare-stride-trickiness.ll       | 20 +++++++++++++++++++
 2 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll

diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index cc5da5dd538d..1955fa5aa847 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1529,6 +1529,15 @@ namespace {
 ICmpInst *LoopStrengthReduce::ChangeCompareStride(Loop *L, ICmpInst *Cond,
                                                 IVStrideUse* &CondUse,
                                                 const SCEVHandle* &CondStride) {
+  // Forgo this transformation if the condition has multiple uses. This is
+  // over-conservative, but simpler than alternatives. It guards against
+  // comparisons with a use that occurs earlier than the add instruction for the
+  // new stride index.  See
+  // test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll
+  // for an example of this situation.
+  if (!Cond->hasOneUse())
+    return Cond;
+
   if (StrideOrder.size() < 2 ||
       IVUsesByStride[*CondStride].Users.size() != 1)
     return Cond;
@@ -1654,9 +1663,9 @@ ICmpInst *LoopStrengthReduce::ChangeCompareStride(Loop *L, ICmpInst *Cond,
       RHS = SCEVExpander::InsertCastOfTo(Instruction::IntToPtr, RHS, NewCmpTy);
     }
     // Insert new compare instruction.
-    Cond = new ICmpInst(Predicate, NewIncV, RHS);
-    Cond->setName(L->getHeader()->getName() + ".termcond");
-    OldCond->getParent()->getInstList().insert(OldCond, Cond);
+    Cond = new ICmpInst(Predicate, NewIncV, RHS,
+                        L->getHeader()->getName() + ".termcond",
+                        OldCond);
 
     // Remove the old compare instruction. The old indvar is probably dead too.
     DeadInsts.insert(cast<Instruction>(CondUse->OperandValToReplace));
diff --git a/llvm/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll b/llvm/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll
new file mode 100644
index 000000000000..9a54b7be1f58
--- /dev/null
+++ b/llvm/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll
@@ -0,0 +1,20 @@
+; RUN: llvm-as < %s | llc
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+target triple = "x86_64-apple-darwin9"
+
+define void @foo() {
+entry:
+	br label %loop
+
+loop:
+	%indvar = phi i32 [ 0, %entry ], [ %i.2.0.us1534, %loop ]		; <i32> [#uses=1]
+	%i.2.0.us1534 = add i32 %indvar, 1		; <i32> [#uses=3]
+	%tmp611.us1535 = icmp eq i32 %i.2.0.us1534, 4		; <i1> [#uses=2]
+	%tmp623.us1538 = select i1 %tmp611.us1535, i32 6, i32 0		; <i32> [#uses=0]
+	%tmp628.us1540 = shl i32 %i.2.0.us1534, 1		; <i32> [#uses=1]
+	%tmp645646647.us1547 = sext i32 %tmp628.us1540 to i64		; <i64> [#uses=0]
+	br i1 %tmp611.us1535, label %exit, label %loop
+
+exit:
+	ret void
+}