Array accesses are independent if the underlying arrays differ.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74499 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/LoopDependenceAnalysis.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Instructions.h"
+#include "llvm/Support/Debug.h"
 using namespace llvm;
 
 LoopPass *llvm::createLoopDependenceAnalysisPass() {
@@ -51,6 +52,20 @@ static void GetMemRefInstrs(
         memrefs.push_back(i);
 }
 
+static bool IsLoadOrStoreInst(Value *I) {
+  return isa<LoadInst>(I) || isa<StoreInst>(I);
+}
+
+static Value *GetPointerOperand(Value *I) {
+  if (LoadInst *i = dyn_cast<LoadInst>(I))
+    return i->getPointerOperand();
+  if (StoreInst *i = dyn_cast<StoreInst>(I))
+    return i->getPointerOperand();
+  assert(0 && "Value is no load or store instruction!");
+  // Never reached.
+  return 0;
+}
+
 //===----------------------------------------------------------------------===//
 //                             Dependence Testing
 //===----------------------------------------------------------------------===//
@@ -65,6 +80,38 @@ bool LoopDependenceAnalysis::isDependencePair(const Value *x,
 
 bool LoopDependenceAnalysis::depends(Value *src, Value *dst) {
   assert(isDependencePair(src, dst) && "Values form no dependence pair!");
+  DOUT << "== LDA test ==\n" << *src << *dst;
+
+  // We only analyse loads and stores; for possible memory accesses by e.g.
+  // free, call, or invoke instructions we conservatively assume dependence.
+  if (!IsLoadOrStoreInst(src) || !IsLoadOrStoreInst(dst))
+    return true;
+
+  Value *srcPtr = GetPointerOperand(src);
+  Value *dstPtr = GetPointerOperand(dst);
+  const Value *srcObj = srcPtr->getUnderlyingObject();
+  const Value *dstObj = dstPtr->getUnderlyingObject();
+  const Type *srcTy = srcObj->getType();
+  const Type *dstTy = dstObj->getType();
+
+  // For now, we only work on (pointers to) global or stack-allocated array
+  // values, as we know that their underlying memory areas will not overlap.
+  // MAYBE: relax this and test for aliasing?
+  if (!((isa<GlobalVariable>(srcObj) || isa<AllocaInst>(srcObj)) &&
+        (isa<GlobalVariable>(dstObj) || isa<AllocaInst>(dstObj)) &&
+        isa<PointerType>(srcTy) &&
+        isa<PointerType>(dstTy) &&
+        isa<ArrayType>(cast<PointerType>(srcTy)->getElementType()) &&
+        isa<ArrayType>(cast<PointerType>(dstTy)->getElementType())))
+    return true;
+
+  // If the arrays are different, the underlying memory areas do not overlap
+  // and the memory accesses are therefore independent.
+  if (srcObj != dstObj)
+    return false;
+
+  // We couldn't establish a more precise result, so we have to conservatively
+  // assume full dependence.
   return true;
 }
 

test/Analysis/LoopDependenceAnalysis/local-array.ll

@@ -0,0 +1,24 @@
+; RUN: llvm-as < %s | opt -disable-output -analyze -lda > %t
+; RUN: grep {instructions: 2} %t | count 1
+; RUN: grep {0,1: dependent} %t | count 1
+
+; x[5] = x[6] // with x being an array on the stack
+
+define void @foo(...) nounwind {
+entry:
+  %xptr = alloca [256 x i32], align 4
+  %x.ld.addr = getelementptr [256 x i32]* %xptr, i64 0, i64 6
+  %x.st.addr = getelementptr [256 x i32]* %xptr, i64 0, i64 5
+  br label %for.body
+
+for.body:
+  %i = phi i64 [ 0, %entry ], [ %i.next, %for.body ]
+  %x = load i32* %x.ld.addr
+  store i32 %x, i32* %x.st.addr
+  %i.next = add i64 %i, 1
+  %exitcond = icmp eq i64 %i.next, 256
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
+}

test/Analysis/LoopDependenceAnalysis/no-array.ll

@@ -0,0 +1,23 @@
+; RUN: llvm-as < %s | opt -disable-output -analyze -lda > %t
+; RUN: grep {instructions: 2} %t | count 1
+; RUN: grep {0,1: dependent} %t | count 1
+
+; x[5] = x[6] // with x being a pointer passed as argument
+
+define void @foo(i32* nocapture %xptr) nounwind {
+entry:
+  %x.ld.addr = getelementptr i32* %xptr, i64 6
+  %x.st.addr = getelementptr i32* %xptr, i64 5
+  br label %for.body
+
+for.body:
+  %i = phi i64 [ 0, %entry ], [ %i.next, %for.body ]
+  %x = load i32* %x.ld.addr
+  store i32 %x, i32* %x.st.addr
+  %i.next = add i64 %i, 1
+  %exitcond = icmp eq i64 %i.next, 256
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
+}

test/Analysis/LoopDependenceAnalysis/siv-strong1.ll

@@ -1,7 +1,7 @@
 ; RUN: llvm-as < %s | opt -disable-output -analyze -lda > %t
 ; RUN: grep {instructions: 3} %t | count 1
 ; RUN: grep {0,2: dependent} %t | count 1
-; RUN: grep {1,2: dependent} %t | count 1
+; RUN: grep {1,2: independent} %t | count 1
 
 ; for (i = 0; i < 256; i++)
 ;   x[i] = x[i] + y[i]

test/Analysis/LoopDependenceAnalysis/siv-strong2.ll

@@ -1,7 +1,7 @@
 ; RUN: llvm-as < %s | opt -disable-output -analyze -lda > %t
 ; RUN: grep {instructions: 3} %t | count 1
 ; RUN: grep {0,2: dependent} %t | count 1
-; RUN: grep {1,2: dependent} %t | count 1
+; RUN: grep {1,2: independent} %t | count 1
 
 ; for (i = 0; i < 256; i++)
 ;   x[i+1] = x[i] + y[i]