More conservatively report status from LoopIdiomRecognize

Being "precise" here is getting us into trouble with one of the
EXPENSIVE_CHECKS buildbots, see [1].  Rather than reporting IR additions that
later get rolled back as "no change", instead we now conservatively report that
there was.

1: http://lists.llvm.org/pipermail/llvm-dev/2020-July/143509.html

Differential Revision: https://reviews.llvm.org/D84071

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

@@ -912,6 +912,7 @@ bool LoopIdiomRecognize::processLoopStridedStore(
   Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS);
   Type *IntIdxTy = DL->getIndexType(DestPtr->getType());
 
+  bool Changed = false;
   const SCEV *Start = Ev->getStart();
   // Handle negative strided loops.
   if (NegStride)
@@ -920,7 +921,7 @@ bool LoopIdiomRecognize::processLoopStridedStore(
   // TODO: ideally we should still be able to generate memset if SCEV expander
   // is taught to generate the dependencies at the latest point.
   if (!isSafeToExpand(Start, *SE))
-    return false;
+    return Changed;
 
   // Okay, we have a strided store "p[i]" of a splattable value.  We can turn
   // this into a memset in the loop preheader now if we want.  However, this
@@ -929,16 +930,26 @@ bool LoopIdiomRecognize::processLoopStridedStore(
   // base pointer and checking the region.
   Value *BasePtr =
       Expander.expandCodeFor(Start, DestInt8PtrTy, Preheader->getTerminator());
+
+  // From here on out, conservatively report to the pass manager that we've
+  // changed the IR, even if we later clean up these added instructions. There
+  // may be structural differences e.g. in the order of use lists not accounted
+  // for in just a textual dump of the IR. This is written as a variable, even
+  // though statically all the places this dominates could be replaced with
+  // 'true', with the hope that anyone trying to be clever / "more precise" with
+  // the return value will read this comment, and leave them alone.
+  Changed = true;
+
   if (mayLoopAccessLocation(BasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                             StoreSize, *AA, Stores)) {
     Expander.clear();
     // If we generated new code for the base pointer, clean up.
     RecursivelyDeleteTriviallyDeadInstructions(BasePtr, TLI);
-    return false;
+    return Changed;
   }
 
   if (avoidLIRForMultiBlockLoop(/*IsMemset=*/true, IsLoopMemset))
-    return false;
+    return Changed;
 
   // Okay, everything looks good, insert the memset.
 
@@ -948,7 +959,7 @@ bool LoopIdiomRecognize::processLoopStridedStore(
   // TODO: ideally we should still be able to generate memset if SCEV expander
   // is taught to generate the dependencies at the latest point.
   if (!isSafeToExpand(NumBytesS, *SE))
-    return false;
+    return Changed;
 
   Value *NumBytes =
       Expander.expandCodeFor(NumBytesS, IntIdxTy, Preheader->getTerminator());
@@ -1067,6 +1078,7 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(StoreInst *SI,
 
   ExpandedValuesCleaner EVC(Expander, TLI);
 
+  bool Changed = false;
   const SCEV *StrStart = StoreEv->getStart();
   unsigned StrAS = SI->getPointerAddressSpace();
   Type *IntIdxTy = Builder.getIntNTy(DL->getIndexSizeInBits(StrAS));
@@ -1085,11 +1097,20 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(StoreInst *SI,
       StrStart, Builder.getInt8PtrTy(StrAS), Preheader->getTerminator());
   EVC.add(StoreBasePtr);
 
+  // From here on out, conservatively report to the pass manager that we've
+  // changed the IR, even if we later clean up these added instructions. There
+  // may be structural differences e.g. in the order of use lists not accounted
+  // for in just a textual dump of the IR. This is written as a variable, even
+  // though statically all the places this dominates could be replaced with
+  // 'true', with the hope that anyone trying to be clever / "more precise" with
+  // the return value will read this comment, and leave them alone.
+  Changed = true;
+
   SmallPtrSet<Instruction *, 1> Stores;
   Stores.insert(SI);
   if (mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                             StoreSize, *AA, Stores))
-    return false;
+    return Changed;
 
   const SCEV *LdStart = LoadEv->getStart();
   unsigned LdAS = LI->getPointerAddressSpace();
@@ -1106,10 +1127,10 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(StoreInst *SI,
 
   if (mayLoopAccessLocation(LoadBasePtr, ModRefInfo::Mod, CurLoop, BECount,
                             StoreSize, *AA, Stores))
-    return false;
+    return Changed;
 
   if (avoidLIRForMultiBlockLoop())
-    return false;
+    return Changed;
 
   // Okay, everything is safe, we can transform this!
 
@@ -1133,14 +1154,14 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(StoreInst *SI,
     const Align StoreAlign = SI->getAlign();
     const Align LoadAlign = LI->getAlign();
     if (StoreAlign < StoreSize || LoadAlign < StoreSize)
-      return false;
+      return Changed;
 
     // If the element.atomic memcpy is not lowered into explicit
     // loads/stores later, then it will be lowered into an element-size
     // specific lib call. If the lib call doesn't exist for our store size, then
     // we shouldn't generate the memcpy.
     if (StoreSize > TTI->getAtomicMemIntrinsicMaxElementSize())
-      return false;
+      return Changed;
 
     // Create the call.
     // Note that unordered atomic loads/stores are *required* by the spec to