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[MemorySSA] Small fix for the clobber limit. 

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.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60479

llvm-svn: 358303
This commit is contained in:
Alina Sbirlea 2019-04-12 18:48:46 +00:00
parent 11bbb5831b
commit 57769382b1
2 changed files with 143 additions and 4 deletions
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16
llvm/lib/Analysis/MemorySSA.cpp
View File

@ -544,10 +544,15 @@ template <class AliasAnalysisType> class ClobberWalker {
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 = 1;
LimitAlreadyReached = true;
}
for (MemoryAccess *Current : def_chain(Desc.Last)) {
Desc.Last = Current;
@ -568,6 +573,9 @@ template <class AliasAnalysisType> class ClobberWalker {
}
}
if (LimitAlreadyReached)
*UpwardWalkLimit = 0;
assert(isa<MemoryPhi>(Desc.Last) &&
"Ended at a non-clobber that's not a phi?");
return {Desc.Last, false, MayAlias};

131
llvm/test/Analysis/MemorySSA/reduce_clobber_limit.ll Normal file
View File

@ -0,0 +1,131 @@
; 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 }