improve validity check to handle constant-trip-count loops more

aggressively.  In practice, this doesn't help anything though,
see the todo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122660 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index 56a35112aa1c2eaeeb60c4c26922325f0574ef2b..9878a2e4e8390e2479768c9a7de0c792e8748050 100644 (file)
--- a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -193,8 +193,7 @@ bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
     return processLoopStoreOfSplatValue(SI, StoreSize, SplatValue, Ev, BECount);
 
   // Handle the memcpy case here.
-  errs() << "Found strided store: " << *Ev << "\n";
-  
+ // errs() << "Found strided store: " << *Ev << "\n";
 
   return false;
 }
@@ -202,13 +201,23 @@ bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
 /// mayLoopModRefLocation - Return true if the specified loop might do a load or
 /// store to the same location that the specified store could store to, which is
 /// a loop-strided access. 
-static bool mayLoopModRefLocation(StoreInst *SI, Loop *L, AliasAnalysis &AA) {
+static bool mayLoopModRefLocation(StoreInst *SI, Loop *L, const SCEV *BECount,
+                                  unsigned StoreSize, AliasAnalysis &AA) {
   // Get the location that may be stored across the loop.  Since the access is
   // strided positively through memory, we say that the modified location starts
   // at the pointer and has infinite size.
-  // TODO: Could improve this for constant trip-count loops.
-  AliasAnalysis::Location StoreLoc =
-    AliasAnalysis::Location(SI->getPointerOperand());
+  uint64_t AccessSize = AliasAnalysis::UnknownSize;
+
+  // If the loop iterates a fixed number of times, we can refine the access size
+  // to be exactly the size of the memset, which is (BECount+1)*StoreSize
+  if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
+    AccessSize = (BECst->getValue()->getZExtValue()+1)*StoreSize;
+  
+  // TODO: For this to be really effective, we have to dive into the pointer
+  // operand in the store.  Store to &A[i] of 100 will always return may alias
+  // with store of &A[100], we need to StoreLoc to be "A" with size of 100,
+  // which will then no-alias a store to &A[100].
+  AliasAnalysis::Location StoreLoc(SI->getPointerOperand(), AccessSize);
 
   for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
        ++BI)
@@ -234,7 +243,8 @@ processLoopStoreOfSplatValue(StoreInst *SI, unsigned StoreSize,
   // this into a memset in the loop preheader now if we want.  However, this
   // would be unsafe to do if there is anything else in the loop that may read
   // or write to the aliased location.  Check for an alias.
-  bool Unsafe=mayLoopModRefLocation(SI, CurLoop, getAnalysis<AliasAnalysis>());
+  bool Unsafe = mayLoopModRefLocation(SI, CurLoop, BECount, StoreSize,
+                                      getAnalysis<AliasAnalysis>());
 
   SI->insertBefore(InstAfterStore);
   

diff --git a/test/Transforms/LoopIdiom/basic.ll b/test/Transforms/LoopIdiom/basic.ll
index 7f96b3c669ccbaac702ca50bf7ba816f10447a11..ef37b1dfdf7229f9e9cd4f0e779eb65c10560a90 100644 (file)
--- a/test/Transforms/LoopIdiom/basic.ll
+++ b/test/Transforms/LoopIdiom/basic.ll
@@ -1,4 +1,4 @@
-; RUN: opt -loop-idiom < %s -S | FileCheck %s
+; RUN: opt -basicaa -loop-idiom < %s -S | FileCheck %s
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
 target triple = "x86_64-apple-darwin10.0.0"
 
@@ -65,3 +65,29 @@ for.end:                                          ; preds = %entry
 ; CHECK-NOT: memset
 ; CHECK: ret void
 }
+
+
+;; TODO: We should be able to promote this memset.  Not yet though.
+define void @test4(i8* %Base) nounwind ssp {
+bb.nph:                                           ; preds = %entry
+  %Base100 = getelementptr i8* %Base, i64 1000
+  br label %for.body
+
+for.body:                                         ; preds = %bb.nph, %for.body
+  %indvar = phi i64 [ 0, %bb.nph ], [ %indvar.next, %for.body ]
+  %I.0.014 = getelementptr i8* %Base, i64 %indvar
+  store i8 0, i8* %I.0.014, align 1
+  
+  ;; Store beyond the range memset, should be safe to promote.
+  store i8 42, i8* %Base100
+  
+  %indvar.next = add i64 %indvar, 1
+  %exitcond = icmp eq i64 %indvar.next, 100
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+; CHECK-TODO: @test4
+; CHECK-TODO: call void @llvm.memset.p0i8.i64(i8* %Base, i8 0, i64 100, i32 1, i1 false)
+; CHECK-TODO-NOT: store
+}