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Fix memcheck interval ends for pointers with negative strides

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Summary:
The checking pointer grouping algorithm assumes that the
starts/ends of the pointers are well formed (start <= end).

The runtime memory checking algorithm also assumes this by doing:

 start0 < end1 && start1 < end0

to detect conflicts. This check only works if start0 <= end0 and
start1 <= end1.

This change correctly orders the interval ends by either checking
the stride (if it is constant) or by using min/max SCEV expressions.

Reviewers: anemet, rengolin

Subscribers: rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D11149

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242400 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Silviu Baranga 2015-07-16 14:02:58 +00:00
parent 5649b37b0e
commit e4877f25bd
2 changed files with 107 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
lib/Analysis/LoopAccessAnalysis.cpp
View File

@ -127,9 +127,25 @@ void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, bool WritePtr,
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Sc);
assert(AR && "Invalid addrec expression");
const SCEV *Ex = SE->getBackedgeTakenCount(Lp);
const SCEV *ScStart = AR->getStart();
const SCEV *ScEnd = AR->evaluateAtIteration(Ex, *SE);
Pointers.emplace_back(Ptr, AR->getStart(), ScEnd, WritePtr, DepSetId, ASId,
Sc);
const SCEV *Step = AR->getStepRecurrence(*SE);
// For expressions with negative step, the upper bound is ScStart and the
// lower bound is ScEnd.
if (const SCEVConstant *CStep = dyn_cast<const SCEVConstant>(Step)) {
if (CStep->getValue()->isNegative())
std::swap(ScStart, ScEnd);
} else {
// Fallback case: the step is not constant, but the we can still
// get the upper and lower bounds of the interval by using min/max
// expressions.
ScStart = SE->getUMinExpr(ScStart, ScEnd);
ScEnd = SE->getUMaxExpr(AR->getStart(), ScEnd);
}
Pointers.emplace_back(Ptr, ScStart, ScEnd, WritePtr, DepSetId, ASId, Sc);
}
bool RuntimePointerChecking::needsChecking(

89
test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll Normal file
View File

@ -0,0 +1,89 @@
; RUN: opt -loop-accesses -analyze < %s | FileCheck %s
; The runtime memory check code and the access grouping
; algorithm both assume that the start and end values
; for an access range are ordered (start <= stop).
; When generating checks for accesses with negative stride
; we need to take this into account and swap the interval
; ends.
;
; for (i = 0; i < 10000; i++) {
; B[i] = A[15000 - i] * 3;
; }
target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
target triple = "aarch64--linux-gnueabi"
; CHECK: function 'f':
; CHECK: (Low: (20000 + %a) High: (60000 + %a))
@B = common global i32* null, align 8
@A = common global i32* null, align 8
define void @f() {
entry:
%a = load i32*, i32** @A, align 8
%b = load i32*, i32** @B, align 8
br label %for.body
for.body: ; preds = %for.body, %entry
%idx = phi i64 [ 0, %entry ], [ %add, %for.body ]
%negidx = sub i64 15000, %idx
%arrayidxA0 = getelementptr inbounds i32, i32* %a, i64 %negidx
%loadA0 = load i32, i32* %arrayidxA0, align 2
%res = mul i32 %loadA0, 3
%add = add nuw nsw i64 %idx, 1
%arrayidxB = getelementptr inbounds i32, i32* %b, i64 %idx
store i32 %res, i32* %arrayidxB, align 2
%exitcond = icmp eq i64 %idx, 10000
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
ret void
}
; CHECK: function 'g':
; When the stride is not constant, we are forced to do umin/umax to get
; the interval limits.
; for (i = 0; i < 10000; i++) {
; B[i] = A[15000 - step * i] * 3;
; }
; Here it is not obvious what the limits are, since 'step' could be negative.
; CHECK: Low: (-1 + (-1 * ((-60001 + (-1 * %a)) umax (-60001 + (40000 * %step) + (-1 * %a)))))
; CHECK: High: ((60000 + %a) umax (60000 + (-40000 * %step) + %a))
define void @g(i64 %step) {
entry:
%a = load i32*, i32** @A, align 8
%b = load i32*, i32** @B, align 8
br label %for.body
for.body: ; preds = %for.body, %entry
%idx = phi i64 [ 0, %entry ], [ %add, %for.body ]
%idx_mul = mul i64 %idx, %step
%negidx = sub i64 15000, %idx_mul
%arrayidxA0 = getelementptr inbounds i32, i32* %a, i64 %negidx
%loadA0 = load i32, i32* %arrayidxA0, align 2
%res = mul i32 %loadA0, 3
%add = add nuw nsw i64 %idx, 1
%arrayidxB = getelementptr inbounds i32, i32* %b, i64 %idx
store i32 %res, i32* %arrayidxB, align 2
%exitcond = icmp eq i64 %idx, 10000
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
ret void
}