Add trivial support for the invariance intrinsics to memdep. This logic is

purely local for now.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85378 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/MemoryDependenceAnalysis.cpp

@@ -171,13 +171,42 @@ getCallSiteDependencyFrom(CallSite CS, bool isReadOnlyCall,
 /// location depends.  If isLoad is true, this routine ignore may-aliases with
 /// read-only operations.
 MemDepResult MemoryDependenceAnalysis::
-getPointerDependencyFrom(Value *MemPtr, uint64_t MemSize, bool isLoad,
+getPointerDependencyFrom(Value *MemPtr, uint64_t MemSize, bool isLoad, 
                          BasicBlock::iterator ScanIt, BasicBlock *BB) {
 
+  Value* invariantTag = 0;
+
   // Walk backwards through the basic block, looking for dependencies.
   while (ScanIt != BB->begin()) {
     Instruction *Inst = --ScanIt;
 
+    // If we're in an invariant region, no dependencies can be found before
+    // we pass an invariant-begin marker.
+    if (invariantTag == Inst) {
+      invariantTag = 0;
+      continue;
+    
+    // If we pass an invariant-end marker, then we've just entered an invariant
+    // region and can start ignoring dependencies.
+    } else if (IntrinsicInst* II = dyn_cast<IntrinsicInst>(Inst)) {
+      if (II->getIntrinsicID() == Intrinsic::invariant_end) {
+        uint64_t invariantSize = ~0ULL;
+        if (ConstantInt* CI = dyn_cast<ConstantInt>(II->getOperand(2)))
+          invariantSize = CI->getZExtValue();
+        
+        AliasAnalysis::AliasResult R =
+          AA->alias(II->getOperand(3), invariantSize, MemPtr, MemSize);
+        if (R == AliasAnalysis::MustAlias) {
+          invariantTag = II->getOperand(1);
+          continue;
+        }
+      }
+    }
+
+    // If we're querying on a load and we're in an invariant region, we're done
+    // at this point. Nothing a load depends on can live in an invariant region.
+    if (isLoad && invariantTag) continue;
+
     // Debug intrinsics don't cause dependences.
     if (isa<DbgInfoIntrinsic>(Inst)) continue;
 
@@ -201,6 +230,11 @@ getPointerDependencyFrom(Value *MemPtr, uint64_t MemSize, bool isLoad,
       return MemDepResult::getDef(Inst);
     }
     
+    // If we're querying on a store and we're in an invariant region, we're done
+    // at this point. The only things that stores depend on that could exist in
+    // an invariant region are loads, which we've already checked.
+    if (invariantTag) continue;
+    
     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
       // If alias analysis can tell that this store is guaranteed to not modify
       // the query pointer, ignore it.  Use getModRefInfo to handle cases where

test/Transforms/GVN/invariant-simple.ll

@@ -0,0 +1,21 @@
+; RUN: opt < %s -gvn -S | FileCheck %s
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
+target triple = "i386-apple-darwin7"
+
+define i8 @test(i8* %P) nounwind {
+; CHECK: @test
+; CHECK-NOT: load
+; CHECK: ret i8
+entry:
+  store i8 1, i8* %P
+  %0 = call {}* @llvm.invariant.start(i64 32, i8* %P)
+  %1 = tail call i32 @foo(i8* %P)
+  call void @llvm.invariant.end({}* %0, i64 32, i8* %P)
+  %2 = load i8* %P
+  ret i8 %2
+}
+
+declare i32 @foo(i8*) nounwind 
+declare {}* @llvm.invariant.start(i64 %S, i8* nocapture %P) readonly
+declare void @llvm.invariant.end({}* %S, i64 %SS, i8* nocapture %P)