From: Dan Gohman <gohman@apple.com>
Date: Fri, 29 Oct 2010 01:14:04 +0000 (+0000)
Subject: Teach memdep to use pointsToConstantMemory to determine that loads
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=cd5c123a1d8723dbd5d493210c0a56890bffe409;p=oota-llvm.git

Teach memdep to use pointsToConstantMemory to determine that loads
from constant memory don't alias any stores.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117636 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 5940105a3aa..c72cd1e83a8 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -161,8 +161,9 @@ getCallSiteDependencyFrom(CallSite CS, bool isReadOnlyCall,
 }
 
 /// getPointerDependencyFrom - Return the instruction on which a memory
-/// location depends.  If isLoad is true, this routine ignore may-aliases with
-/// read-only operations.
+/// location depends.  If isLoad is true, this routine ignores may-aliases with
+/// read-only operations.  If isLoad is false, this routine ignores may-aliases
+/// with reads from read-only locations.
 MemDepResult MemoryDependenceAnalysis::
 getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad, 
                          BasicBlock::iterator ScanIt, BasicBlock *BB) {
@@ -225,17 +226,21 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
       Value *Pointer = LI->getPointerOperand();
       uint64_t PointerSize = AA->getTypeStoreSize(LI->getType());
       MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
+      AliasAnalysis::Location LoadLoc(Pointer, PointerSize, TBAATag);
       
       // If we found a pointer, check if it could be the same as our pointer.
-      AliasAnalysis::AliasResult R =
-        AA->alias(AliasAnalysis::Location(Pointer, PointerSize, TBAATag),
-                  MemLoc);
+      AliasAnalysis::AliasResult R = AA->alias(LoadLoc, MemLoc);
       if (R == AliasAnalysis::NoAlias)
         continue;
       
       // May-alias loads don't depend on each other without a dependence.
       if (isLoad && R == AliasAnalysis::MayAlias)
         continue;
+
+      // Stores don't alias loads from read-only memory.
+      if (!isLoad && AA->pointsToConstantMemory(LoadLoc))
+        continue;
+
       // Stores depend on may and must aliased loads, loads depend on must-alias
       // loads.
       return MemDepResult::getDef(Inst);
diff --git a/test/Analysis/TypeBasedAliasAnalysis/dse.ll b/test/Analysis/TypeBasedAliasAnalysis/dse.ll
index 180149a7ad1..09a7f2c3b78 100644
--- a/test/Analysis/TypeBasedAliasAnalysis/dse.ll
+++ b/test/Analysis/TypeBasedAliasAnalysis/dse.ll
@@ -25,6 +25,29 @@ define i8 @test0_no(i8* %a, i8* %b) nounwind {
   ret i8 %y
 }
 
+; CHECK: @test1_yes
+; CHECK-NEXT: load i8* %b
+; CHECK-NEXT: store i8 1, i8* %a
+; CHECK-NEXT: ret i8 %y
+define i8 @test1_yes(i8* %a, i8* %b) nounwind {
+  store i8 0, i8* %a
+  %y = load i8* %b, !tbaa !5
+  store i8 1, i8* %a
+  ret i8 %y
+}
+
+; CHECK: @test1_no
+; CHECK-NEXT: store i8 0, i8* %a
+; CHECK-NEXT: load i8* %b
+; CHECK-NEXT: store i8 1, i8* %a
+; CHECK-NEXT: ret i8 %y
+define i8 @test1_no(i8* %a, i8* %b) nounwind {
+  store i8 0, i8* %a
+  %y = load i8* %b, !tbaa !6
+  store i8 1, i8* %a
+  ret i8 %y
+}
+
 ; Root note.
 !0 = metadata !{ }
 ; Some type.