enhance loop idiom recognition to scan *all* unconditionally executed

blocks in a loop, instead of just the header block.  This makes it more
aggressive, able to handle Duncan's Ada examples.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122704 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopIdiomRecognize.cpp

@@ -58,6 +58,7 @@ namespace {
   class LoopIdiomRecognize : public LoopPass {
     Loop *CurLoop;
     const TargetData *TD;
+    DominatorTree *DT;
     ScalarEvolution *SE;
   public:
     static char ID;
@@ -66,6 +67,8 @@ namespace {
     }
 
     bool runOnLoop(Loop *L, LPPassManager &LPM);
+    bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
+                        SmallVectorImpl<BasicBlock*> &ExitBlocks);
 
     bool processLoopStore(StoreInst *SI, const SCEV *BECount);
     
@@ -93,6 +96,7 @@ namespace {
       AU.addRequired<ScalarEvolution>();
       AU.addPreserved<ScalarEvolution>();
       AU.addPreserved<DominatorTree>();
+      AU.addRequired<DominatorTree>();
     }
   };
 }
@@ -101,6 +105,7 @@ char LoopIdiomRecognize::ID = 0;
 INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
                       false, false)
 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
+INITIALIZE_PASS_DEPENDENCY(DominatorTree)
 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
 INITIALIZE_PASS_DEPENDENCY(LCSSA)
 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
@@ -158,12 +163,37 @@ bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
   TD = getAnalysisIfAvailable<TargetData>();
   if (TD == 0) return false;
 
-  // TODO: We currently only scan the header of the loop, because it is the only
-  // part that is known to execute and we don't want to make a conditional store
-  // into an unconditional one in the preheader.  However, there can be diamonds
-  // and other things in the loop that would make other blocks "always executed"
-  // we should get the full set and scan each block.
-  BasicBlock *BB = L->getHeader();
+  DT = &getAnalysis<DominatorTree>();
+  LoopInfo &LI = getAnalysis<LoopInfo>();
+  
+  SmallVector<BasicBlock*, 8> ExitBlocks;
+  CurLoop->getUniqueExitBlocks(ExitBlocks);
+
+  bool MadeChange = false;
+  // Scan all the blocks in the loop that are not in subloops.
+  for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
+       ++BI) {
+    // Ignore blocks in subloops.
+    if (LI.getLoopFor(*BI) != CurLoop)
+      continue;
+    
+    MadeChange |= runOnLoopBlock(*BI, BECount, ExitBlocks);
+  }
+  return MadeChange;
+}
+
+/// runOnLoopBlock - Process the specified block, which lives in a counted loop
+/// with the specified backedge count.  This block is known to be in the current
+/// loop and not in any subloops.
+bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
+                                     SmallVectorImpl<BasicBlock*> &ExitBlocks) {
+  // We can only promote stores in this block if they are unconditionally
+  // executed in the loop.  For a block to be unconditionally executed, it has
+  // to dominate all the exit blocks of the loop.  Verify this now.
+  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
+    if (!DT->dominates(BB, ExitBlocks[i]))
+      return false;
+  
   DEBUG(dbgs() << "loop-idiom Scanning: F[" << BB->getParent()->getName()
         << "] Loop %" << BB->getName() << "\n");
   
@@ -187,6 +217,7 @@ bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
   return MadeChange;
 }
 
+
 /// scanBlock - Look over a block to see if we can promote anything out of it.
 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
   Value *StoredVal = SI->getValueOperand();

test/Transforms/LoopIdiom/basic.ll

@@ -165,4 +165,27 @@ for.end:                                          ; preds = %for.body, %entry
 }
 
 
+; This is a loop that was rotated but where the blocks weren't merged.  This
+; shouldn't perturb us.
+define void @test7(i8* %Base, i64 %Size) nounwind ssp {
+bb.nph:                                           ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %bb.nph, %for.body
+  %indvar = phi i64 [ 0, %bb.nph ], [ %indvar.next, %for.body.cont ]
+  br label %for.body.cont
+for.body.cont:
+  %I.0.014 = getelementptr i8* %Base, i64 %indvar
+  store i8 0, i8* %I.0.014, align 1
+  %indvar.next = add i64 %indvar, 1
+  %exitcond = icmp eq i64 %indvar.next, %Size
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+; CHECK: @test7
+; CHECK: call void @llvm.memset.p0i8.i64(i8* %Base, i8 0, i64 %Size, i32 1, i1 false)
+; CHECK-NOT: store
+}
+