From 6903a24f32e11d10ad7232b2c16c8d07f27a31c7 Mon Sep 17 00:00:00 2001
From: Eli Friedman <eli.friedman@gmail.com>
Date: Fri, 13 Jun 2008 22:02:12 +0000
Subject: [PATCH] Don't skip over instructions other than loads that might read
 memory when trying to sink stores.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52259 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../Scalar/InstructionCombining.cpp           | 11 ++++++-----
 .../2008-06-13-ReadOnlyCallStore.ll           | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 test/Transforms/InstCombine/2008-06-13-ReadOnlyCallStore.ll

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 8dbca0f2e8b..8a7128bf65c 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -10417,18 +10417,19 @@ bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) {
           return false;
         break;
       }
-      // If we find something that may be using the stored value, or if we run
-      // out of instructions, we can't do the xform.
-      if (isa<LoadInst>(BBI) || BBI->mayWriteToMemory() ||
+      // If we find something that may be using or overwriting the stored
+      // value, or if we run out of instructions, we can't do the xform.
+      if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() ||
           BBI == OtherBB->begin())
         return false;
     }
     
     // In order to eliminate the store in OtherBr, we have to
-    // make sure nothing reads the stored value in StoreBB.
+    // make sure nothing reads or overwrites the stored value in
+    // StoreBB.
     for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) {
       // FIXME: This should really be AA driven.
-      if (isa<LoadInst>(I) || I->mayWriteToMemory())
+      if (I->mayReadFromMemory() || I->mayWriteToMemory())
         return false;
     }
   }
diff --git a/test/Transforms/InstCombine/2008-06-13-ReadOnlyCallStore.ll b/test/Transforms/InstCombine/2008-06-13-ReadOnlyCallStore.ll
new file mode 100644
index 00000000000..3f23098225e
--- /dev/null
+++ b/test/Transforms/InstCombine/2008-06-13-ReadOnlyCallStore.ll
@@ -0,0 +1,19 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {store i8} | count 2
+
+define i32 @a(i8* %s) nounwind  {
+entry:
+	store i8 0, i8* %s, align 1 ; This store cannot be eliminated!
+	%tmp3 = call i32 @strlen( i8* %s ) nounwind readonly
+	%tmp5 = icmp ne i32 %tmp3, 0
+	br i1 %tmp5, label %bb, label %bb8
+
+bb:		; preds = %entry
+	store i8 0, i8* %s, align 1
+	br label %bb8
+
+bb8:
+	ret i32 %tmp3
+}
+
+declare i32 @strlen(i8*) nounwind readonly 
+