From 10e3ff7e5f0c11324cb4b028a75ce2b650f60849 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Sat, 29 Mar 2008 05:15:47 +0000
Subject: [PATCH] change iterator invalidation avoidance to just move the
 iterator backward when something changes, instead of moving forward.  This
 allows us to simplify memset lowering, inserting the memset at the end of the
 range of stuff we're touching instead of at the start.

This, in turn, allows us to make use of the addressing instructions already
used in the function instead of inserting our own.  For example, we now
codegen:

	%tmp41 = getelementptr [8 x i8]* %ref_idx, i32 0, i32 0		; <i8*> [#uses=2]
	call void @llvm.memset.i64( i8* %tmp41, i8 -1, i64 8, i32 1 )

instead of:

	%tmp20 = getelementptr [8 x i8]* %ref_idx, i32 0, i32 7		; <i8*> [#uses=1]
	%ptroffset = getelementptr i8* %tmp20, i64 -7		; <i8*> [#uses=1]
	call void @llvm.memset.i64( i8* %ptroffset, i8 -1, i64 8, i32 1 )

llvm-svn: 48940
---
 lib/Transforms/Scalar/GVN.cpp | 43 ++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 42418f0ac4e..12ce28660c2 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -1232,8 +1232,10 @@ void MemsetRanges::addStore(int64_t Start, StoreInst *SI) {
   // See if the range extends the start of the range.  In this case, it couldn't
   // possibly cause it to join the prior range, because otherwise we would have
   // stopped on *it*.
-  if (Start < I->Start)
+  if (Start < I->Start) {
     I->Start = Start;
+    I->StartPtr = SI->getPointerOperand();
+  }
     
   // Now we know that Start <= I->End and Start >= I->Start (so the startpoint
   // is in or right at the end of I), and that End >= I->Start.  Extend I out to
@@ -1345,18 +1347,17 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
       continue;
     
     // Otherwise, we do want to transform this!  Create a new memset.  We put
-    // the memset right after the first store that we found in this block.  This
-    // ensures that the caller will increment the iterator to the memset before
-    // it deletes all the stores.
-    BasicBlock::iterator InsertPt = SI; ++InsertPt;
+    // the memset right before the first instruction that isn't part of this
+    // memset block.  This ensure that the memset is dominated by any addressing
+    // instruction needed by the start of the block.
+    BasicBlock::iterator InsertPt = BI;
   
     if (MemSetF == 0)
       MemSetF = Intrinsic::getDeclaration(SI->getParent()->getParent()
                                           ->getParent(), Intrinsic::memset_i64);
     
-    // StartPtr may not dominate the starting point.  Instead of using it, base
-    // the destination pointer off the input to the first store in the block.
-    StartPtr = SI->getPointerOperand();
+    // Get the starting pointer of the block.
+    StartPtr = Range.StartPtr;
   
     // Cast the start ptr to be i8* as memset requires.
     const Type *i8Ptr = PointerType::getUnqual(Type::Int8Ty);
@@ -1364,12 +1365,6 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
       StartPtr = new BitCastInst(StartPtr, i8Ptr, StartPtr->getNameStart(),
                                  InsertPt);
   
-    // Offset the pointer if needed.
-    if (Range.Start)
-      StartPtr = new GetElementPtrInst(StartPtr, ConstantInt::get(Type::Int64Ty,
-                                                                  Range.Start),
-                                       "ptroffset", InsertPt);
-  
     Value *Ops[] = {
       StartPtr, ByteVal,   // Start, value
       ConstantInt::get(Type::Int64Ty, Range.End-Range.Start),  // size
@@ -1695,7 +1690,7 @@ bool GVN::iterateOnFunction(Function &F) {
   
   DominatorTree &DT = getAnalysis<DominatorTree>();   
   
-  SmallVector<Instruction*, 4> toErase;
+  SmallVector<Instruction*, 8> toErase;
   DenseMap<Value*, LoadInst*> lastSeenLoad;
 
   // Top-down walk of the dominator tree
@@ -1713,19 +1708,31 @@ bool GVN::iterateOnFunction(Function &F) {
       currAvail = availableOut[DI->getIDom()->getBlock()];
 
     for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
-         BI != BE; ) {
+         BI != BE;) {
       changed_function |= processInstruction(BI, currAvail,
                                              lastSeenLoad, toErase);
+      if (toErase.empty()) {
+        ++BI;
+        continue;
+      }
       
+      // If we need some instructions deleted, do it now.
       NumGVNInstr += toErase.size();
       
-      // Avoid iterator invalidation
-      ++BI;
+      // Avoid iterator invalidation.
+      bool AtStart = BI == BB->begin();
+      if (!AtStart)
+        --BI;
 
       for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
            E = toErase.end(); I != E; ++I)
         (*I)->eraseFromParent();
 
+      if (AtStart)
+        BI = BB->begin();
+      else
+        ++BI;
+      
       toErase.clear();
     }
   }