[llvm-commits] [llvm] r117636 - in /llvm/trunk: lib/Analysis/MemoryDependenceAnalysis.cpp test/Analysis/TypeBasedAliasAnalysis/dse.ll

[Dan Gohman]

Author: djg
Date: Thu Oct 28 20:14:04 2010
New Revision: 117636

URL: http://llvm.org/viewvc/llvm-project?rev=117636&view=rev
Log:
Teach memdep to use pointsToConstantMemory to determine that loads
from constant memory don't alias any stores.

Modified:
    llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp
    llvm/trunk/test/Analysis/TypeBasedAliasAnalysis/dse.ll

Modified: llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp?rev=117636&r1=117635&r2=117636&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp Thu Oct 28 20:14:04 2010
@@ -161,8 +161,9 @@
 }
 
 /// 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 @@
       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);

Modified: llvm/trunk/test/Analysis/TypeBasedAliasAnalysis/dse.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/TypeBasedAliasAnalysis/dse.ll?rev=117636&r1=117635&r2=117636&view=diff
==============================================================================
--- llvm/trunk/test/Analysis/TypeBasedAliasAnalysis/dse.ll (original)
+++ llvm/trunk/test/Analysis/TypeBasedAliasAnalysis/dse.ll Thu Oct 28 20:14:04 2010
@@ -25,6 +25,29 @@
   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.