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[LAA] Revert a small part of r239295

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This commit ([LAA] Fix estimation of number of memchecks) regressed the
logic a bit.  We shouldn't quit the analysis if we encounter a pointer
without known bounds *unless* we actually need to emit a memcheck for
it.

The original code was using NumComparisons which is now computed
differently.  Instead I compute NeedRTCheck from NumReadPtrChecks and
NumWritePtrChecks.

As side note, I find the separation of NeedRTCheck and CanDoRT
confusing, so I will try to merge them in a follow-up patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241756 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Adam Nemet 2015-07-08 22:58:48 +00:00
parent 9a39681fd9
commit 7a6f54545f
2 changed files with 62 additions and 6 deletions
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26
lib/Analysis/LoopAccessAnalysis.cpp
View File

@ -479,6 +479,9 @@ bool AccessAnalysis::canCheckPtrAtRT(
// Accesses between different groups doesn't need to be checked.
unsigned ASId = 1;
for (auto &AS : AST) {
int NumReadPtrChecks = 0;
int NumWritePtrChecks = 0;
// We assign consecutive id to access from different dependence sets.
// Accesses within the same set don't need a runtime check.
unsigned RunningDepId = 1;
@ -489,6 +492,11 @@ bool AccessAnalysis::canCheckPtrAtRT(
bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
MemAccessInfo Access(Ptr, IsWrite);
if (IsWrite)
++NumWritePtrChecks;
else
++NumReadPtrChecks;
if (hasComputableBounds(SE, StridesMap, Ptr) &&
// When we run after a failing dependency check we have to make sure
// we don't have wrapping pointers.
@ -516,15 +524,21 @@ bool AccessAnalysis::canCheckPtrAtRT(
}
}
// If we have at least two writes or one write and a read then we need to
// check them. But there is no need to checks if there is only one
// dependence set for this alias set.
//
// Note that this function computes CanDoRT and NeedRTCheck independently.
// For example CanDoRT=false, NeedRTCheck=false means that we have a pointer
// for which we couldn't find the bounds but we don't actually need to emit
// any checks so it does not matter.
if (!(IsDepCheckNeeded && CanDoRT && RunningDepId == 2))
NeedRTCheck |= (NumWritePtrChecks >= 2 || (NumReadPtrChecks >= 1 &&
NumWritePtrChecks >= 1));
++ASId;
}
// We need a runtime check if there are any accesses that need checking.
// However, some accesses cannot be checked (for example because we
// can't determine their bounds). In these cases we would need a check
// but wouldn't be able to add it.
NeedRTCheck = !CanDoRT || RtCheck.needsAnyChecking(nullptr);
// If the pointers that we would use for the bounds comparison have different
// address spaces, assume the values aren't directly comparable, so we can't
// use them for the runtime check. We also have to assume they could

42
test/Analysis/LoopAccessAnalysis/pointer-with-unknown-bounds.ll Normal file
View File

@ -0,0 +1,42 @@
; RUN: opt -loop-accesses -analyze < %s | FileCheck %s
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
; We shouldn't quit the analysis if we encounter a pointer without known
; bounds *unless* we actually need to emit a memcheck for it. (We only
; compute bounds for SCEVAddRecs so A[i*I] is deemed not having known bounds.)
;
; for (i = 0; i < 20; ++i)
; A[i*i] *= 2;
; CHECK: for.body:
; CHECK: Report: unsafe dependent memory operations in loop
; CHECK-NOT: Report: cannot identify array bounds
; CHECK: Interesting Dependences:
; CHECK: Unknown:
; CHECK: %loadA = load i16, i16* %arrayidxA, align 2 ->
; CHECK: store i16 %mul, i16* %arrayidxA, align 2
define void @f(i16* %a) {
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%ind = phi i64 [ 0, %entry ], [ %add, %for.body ]
%access_ind = mul i64 %ind, %ind
%arrayidxA = getelementptr inbounds i16, i16* %a, i64 %access_ind
%loadA = load i16, i16* %arrayidxA, align 2
%mul = mul i16 %loadA, 2
store i16 %mul, i16* %arrayidxA, align 2
%add = add nuw nsw i64 %ind, 1
%exitcond = icmp eq i64 %add, 20
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
ret void
}