[MemorySSA] Small fix for the clobber limit.

[asbirlea]

Summary

After introducing the limit for clobber walking, walkToPhiOrClobber would assert that the limit is at least 1 on entry.
The test included triggered that assert.

The callsite in tryOptimizePhi making the calls to walkToPhiOrClobber is structured like this:

while (true) {
   if (getBlockingAccess()) { // calls walkToPhiOrClobber
   }
   for (...) {
     walkToPhiOrClobber();
   }
}

The cleanest fix is to check if the limit was reached inside walkToPhiOrClobber, and give an allowence of 1.
This approach not make any alias() calls (no calls to instructionClobbersQuery), so the performance condition is enforced.
The limit is set back to 0 if not used, as this provides info on the fact that we stopped before reaching a true clobber.

[george.burgess.iv]

LGTM after a nit is addressed. Thanks!

lib/Analysis/MemorySSA.cpp

553

For clarity's sake, can we please explicitly = 1 here (and = 0 in the new conditional below), rather than incrementing/decrementing?

lib/Analysis/MemorySSA.cpp

template <class AliasAnalysisType> class ClobberWalker {
   /// StopAt.
   ///
   /// This does not test for whether StopAt is a clobber
   UpwardsWalkResult
   walkToPhiOrClobber(DefPath &Desc, const MemoryAccess *StopAt = nullptr,
                      const MemoryAccess *SkipStopAt = nullptr) const {
     assert(!isa<MemoryUse>(Desc.Last) && "Uses don't exist in my world");
     assert(UpwardWalkLimit && "Need a valid walk limit");
-    // This will not do any alias() calls. It returns in the first iteration in
-    // the loop below.
-    if (*UpwardWalkLimit == 0)
-      (*UpwardWalkLimit)++;
+    bool LimitAlreadyReached = false;
+    // (*UpwardWalkLimit) may be 0 here, due to the loop in tryOptimizePhi. Set
+    // it to 1. This will not do any alias() calls. It either returns in the
+    // first iteration in the loop below, or is set back to 0 if all def chains
+    // are free of MemoryDefs.
+    if (!*UpwardWalkLimit) {
+      ++*UpwardWalkLimit;
+      LimitAlreadyReached = true;
+    }
 
     for (MemoryAccess *Current : def_chain(Desc.Last)) {
       Desc.Last = Current;
       if (Current == StopAt || Current == SkipStopAt)
         return {Current, false, MayAlias};
 
       if (auto *MD = dyn_cast<MemoryDef>(Current)) {
         if (MSSA.isLiveOnEntryDef(MD))
           return {MD, true, MustAlias};
 
         if (!--*UpwardWalkLimit)
           return {Current, true, MayAlias};
 
         ClobberAlias CA =
             instructionClobbersQuery(MD, Desc.Loc, Query->Inst, AA);
         if (CA.IsClobber)
           return {MD, true, CA.AR};
       }
     }
 
+    if (LimitAlreadyReached)
+      --*UpwardWalkLimit;
+
     assert(isa<MemoryPhi>(Desc.Last) &&
            "Ended at a non-clobber that's not a phi?");
     return {Desc.Last, false, MayAlias};
   }
 
   void addSearches(MemoryPhi *Phi, SmallVectorImpl<ListIndex> &PausedSearches,
                    ListIndex PriorNode) {
     auto UpwardDefs = make_range(upward_defs_begin({Phi, Paths[PriorNode].Loc}),

test/Analysis/MemorySSA/reduce_clobber_limit.ll

+; RUN: opt -S -memoryssa %s | FileCheck %s
+; REQUIRES: asserts
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @func()
+; Function Attrs: noinline
+define dso_local void @func() unnamed_addr #0 align 2 {
+entry:
+  %NoFinalize.addr = alloca i8, align 1
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  %call8 = call zeroext i1 @foo()
+  br i1 %call8, label %if.then9, label %while.cond
+
+if.then9:                                         ; preds = %entry
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  br label %while.cond
+
+while.cond:                                       ; preds = %cleanup, %if.then9, %entry
+  %call34 = call zeroext i1 @foo()
+  call void @blah()
+  br i1 %call34, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %call35 = call zeroext i1 @foo()
+  br i1 %call35, label %if.end37, label %if.then36
+
+if.then36:                                        ; preds = %while.body
+  store i32 2, i32* undef, align 4
+  br label %cleanup
+
+if.end37:                                         ; preds = %while.body
+  %call38 = call zeroext i1 @foo()
+  br i1 %call38, label %if.end46, label %land.lhs.true
+
+land.lhs.true:                                    ; preds = %if.end37
+  call void @blah()
+  %call41 = call zeroext i1 @foo()
+  br i1 %call41, label %if.then42, label %if.end46
+
+if.then42:                                        ; preds = %land.lhs.true
+  call void @blah()
+  br label %if.end46
+
+if.end46:                                         ; preds = %if.then42, %land.lhs.true, %if.end37
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  br label %cleanup
+
+cleanup:                                          ; preds = %if.end46, %if.then36
+  call void @blah()
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  %call93 = call zeroext i1 @foo()
+  br i1 %call93, label %if.end120, label %if.then94
+
+if.then94:                                        ; preds = %while.end
+  store i32 0, i32* undef, align 4
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %if.then94
+  br i1 undef, label %for.body, label %if.end120
+
+for.body:                                         ; preds = %for.cond
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  call void @blah()
+  br label %for.cond
+
+if.end120:                                        ; preds = %for.cond, %while.end
+  %val = load i8, i8* %NoFinalize.addr, align 1
+  ret void
+}
+
+; Function Attrs: noinline
+declare hidden void @blah() unnamed_addr #0 align 2
+
+; Function Attrs: noinline
+declare hidden i1 @foo() local_unnamed_addr #0 align 2
+
+attributes #0 = { noinline }
+