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[SCEV] Consistently Handle Expressions That Cannot Be Divided 

This patch addresses the issue of SCEV division asserting on some
input expressions (e.g., non-affine expressions) and quietly giving
up on others.  When giving up, we set the quotient to be equal to
zero and the remainder to be equal to the numerator. With this
patch, we always quietly give up when we cannot perform the
division.

This patch also adds a test case for DependenceAnalysis that
previously caused an assertion.

Differential Revision: http://reviews.llvm.org/D11725

llvm-svn: 247314
This commit is contained in:
Matthew Simpson 2015-09-10 18:12:47 +00:00
parent fa946233b4
commit ddb4d9741f
2 changed files with 62 additions and 36 deletions
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60
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -782,17 +782,15 @@ public:
void visitAddRecExpr(const SCEVAddRecExpr *Numerator) {
const SCEV *StartQ, *StartR, *StepQ, *StepR;
assert(Numerator->isAffine() && "Numerator should be affine");
if (!Numerator->isAffine())
return cannotDivide(Numerator);
divide(SE, Numerator->getStart(), Denominator, &StartQ, &StartR);
divide(SE, Numerator->getStepRecurrence(SE), Denominator, &StepQ, &StepR);
// Bail out if the types do not match.
Type *Ty = Denominator->getType();
if (Ty != StartQ->getType() || Ty != StartR->getType() ||
Ty != StepQ->getType() || Ty != StepR->getType()) {
Quotient = Zero;
Remainder = Numerator;
return;
}
Ty != StepQ->getType() || Ty != StepR->getType())
return cannotDivide(Numerator);
Quotient = SE.getAddRecExpr(StartQ, StepQ, Numerator->getLoop(),
Numerator->getNoWrapFlags());
Remainder = SE.getAddRecExpr(StartR, StepR, Numerator->getLoop(),
@ -808,11 +806,8 @@ public:
divide(SE, Op, Denominator, &Q, &R);
// Bail out if types do not match.
if (Ty != Q->getType() || Ty != R->getType()) {
Quotient = Zero;
Remainder = Numerator;
return;
}
if (Ty != Q->getType() || Ty != R->getType())
return cannotDivide(Numerator);
Qs.push_back(Q);
Rs.push_back(R);
@ -835,11 +830,8 @@ public:
bool FoundDenominatorTerm = false;
for (const SCEV *Op : Numerator->operands()) {
// Bail out if types do not match.
if (Ty != Op->getType()) {
Quotient = Zero;
Remainder = Numerator;
return;
}
if (Ty != Op->getType())
return cannotDivide(Numerator);
if (FoundDenominatorTerm) {
Qs.push_back(Op);
@ -855,11 +847,8 @@ public:
}
// Bail out if types do not match.
if (Ty != Q->getType()) {
Quotient = Zero;
Remainder = Numerator;
return;
}
if (Ty != Q->getType())
return cannotDivide(Numerator);
FoundDenominatorTerm = true;
Qs.push_back(Q);
@ -874,11 +863,8 @@ public:
return;
}
if (!isa<SCEVUnknown>(Denominator)) {
Quotient = Zero;
Remainder = Numerator;
return;
}
if (!isa<SCEVUnknown>(Denominator))
return cannotDivide(Numerator);
// The Remainder is obtained by replacing Denominator by 0 in Numerator.
ValueToValueMap RewriteMap;
@ -898,15 +884,12 @@ public:
// Quotient is (Numerator - Remainder) divided by Denominator.
const SCEV *Q, *R;
const SCEV *Diff = SE.getMinusSCEV(Numerator, Remainder);
if (sizeOfSCEV(Diff) > sizeOfSCEV(Numerator)) {
// This SCEV does not seem to simplify: fail the division here.
Quotient = Zero;
Remainder = Numerator;
return;
}
if (sizeOfSCEV(Diff) > sizeOfSCEV(Numerator))
return cannotDivide(Numerator);
divide(SE, Diff, Denominator, &Q, &R);
assert(R == Zero &&
"(Numerator - Remainder) should evenly divide Denominator");
if (R != Zero)
return cannotDivide(Numerator);
Quotient = Q;
}
@ -917,8 +900,15 @@ private:
Zero = SE.getConstant(Denominator->getType(), 0);
One = SE.getConstant(Denominator->getType(), 1);
// By default, we don't know how to divide Expr by Denominator.
// Providing the default here simplifies the rest of the code.
// We generally do not know how to divide Expr by Denominator. We
// initialize the division to a "cannot divide" state to simplify the rest
// of the code.
cannotDivide(Numerator);
}
// Convenience function for giving up on the division. We set the quotient to
// be equal to zero and the remainder to be equal to the numerator.
void cannotDivide(const SCEV *Numerator) {
Quotient = Zero;
Remainder = Numerator;
}

36
llvm/test/Analysis/DependenceAnalysis/NonAffineExpr.ll Normal file
View File

@ -0,0 +1,36 @@
; RUN: opt < %s -analyze -basicaa -da-delinearize -da
;
; CHECK: da analyze - consistent input [S S]!
; CHECK: da analyze - confused!
; CHECK: da analyze - input [* *]!
;
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n8:16:32-S64"
target triple = "thumbv7--linux-gnueabi"
define void @f(i32** %a, i32 %n) align 2 {
for.preheader:
%t.0 = ashr exact i32 %n, 3
br label %for.body.1
for.body.1:
%i.1 = phi i32 [ %t.5, %for.inc ], [ 0, %for.preheader ]
%i.2 = phi i32 [ %i.5, %for.inc ], [ %t.0, %for.preheader ]
br i1 undef, label %for.inc, label %for.body.2
for.body.2:
%i.3 = phi i32 [ %t.1, %for.body.2 ], [ %i.1, %for.body.1 ]
%t.1 = add i32 %i.3, 1
%t.2 = load i32*, i32** %a, align 4
%t.3 = getelementptr inbounds i32, i32* %t.2, i32 %i.3
%t.4 = load i32, i32* %t.3, align 4
br i1 undef, label %for.inc, label %for.body.2
for.inc:
%i.4 = phi i32 [ %i.2, %for.body.1 ], [ %i.2, %for.body.2 ]
%t.5 = add i32 %i.1, %i.4
%i.5 = add i32 %i.2, -1
br i1 undef, label %for.exit, label %for.body.1
for.exit:
ret void
}