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[LazyValueInfo] Insert an Overdefined placeholder to prevent infinite recursion 

getValueFromCondition() uses a Visited set to record the intermediate value.
However, it uses a postorder way to compute the value first and update the
Visited set later. Thus it will be trapped into an infinite recursion if there
exists IRs that use no dominated by its def as in this example:

  %tmp3 = or i1 undef, %tmp4
  %tmp4 = or i1 undef, %tmp3

To prevent this, we can insert an Overdefined placeholder into the set
before computing the actual value.

Reviewed by: nikic

Differential Revision: https://reviews.llvm.org/D101273
This commit is contained in:
Peilin Guo 2021-05-07 16:05:50 +08:00
parent 9deb7eeaf7
commit 911a541620
2 changed files with 88 additions and 10 deletions
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18
llvm/lib/Analysis/LazyValueInfo.cpp
View File

@ -1175,13 +1175,6 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
else
return ValueLatticeElement::getOverdefined();
// Prevent infinite recursion if Cond references itself as in this example:
// Cond: "%tmp4 = and i1 %tmp4, undef"
// BL: "%tmp4 = and i1 %tmp4, undef"
// BR: "i1 undef"
if (L == Cond || R == Cond)
return ValueLatticeElement::getOverdefined();
// if (L && R) -> intersect L and R
// if (!(L || R)) -> intersect L and R
// if (L || R) -> union L and R
@ -1201,9 +1194,14 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
static ValueLatticeElement
getValueFromCondition(Value *Val, Value *Cond, bool isTrueDest,
SmallDenseMap<Value*, ValueLatticeElement> &Visited) {
auto I = Visited.find(Cond);
if (I != Visited.end())
return I->second;
// Insert an Overdefined placeholder into the set to prevent
// infinite recursion if there exists IRs that use not
// dominated by its def as in this example:
// "%tmp3 = or i1 undef, %tmp4"
// "%tmp4 = or i1 undef, %tmp3"
auto Iter = Visited.try_emplace(Cond, ValueLatticeElement::getOverdefined());
if (!Iter.second)
return Iter.first->getSecond();
auto Result = getValueFromConditionImpl(Val, Cond, isTrueDest, Visited);
Visited[Cond] = Result;

80
llvm/test/Transforms/JumpThreading/insert-placeholder-to-prevent-infinite-recursion.ll Normal file
View File

@ -0,0 +1,80 @@
; RUN: opt -jump-threading -S < %s | FileCheck %s
@a = common dso_local local_unnamed_addr global i16 0, align 2
; Function Attrs: nofree norecurse nounwind
define internal fastcc void @s() unnamed_addr {
; CHECK-LABEL: @s(
; CHECK-NEXT: for.cond1.preheader.lr.ph:
; CHECK-NEXT: br label [[FOR_COND1_PREHEADER:%.*]]
; CHECK: for.cond1.preheader:
; CHECK-NEXT: [[DOTPR_I_19:%.*]] = phi i32 [ undef, [[FOR_COND1_PREHEADER_LR_PH:%.*]] ], [ 0, [[FOR_INC_SPLIT_1:%.*]] ]
; CHECK-NEXT: [[TMP0:%.*]] = phi i16 [ undef, [[FOR_COND1_PREHEADER_LR_PH]] ], [ [[INC9:%.*]], [[FOR_INC_SPLIT_1]] ]
; CHECK-NEXT: [[TOBOOL4_I:%.*]] = icmp eq i32 [[DOTPR_I_19]], 0
; CHECK-NEXT: br i1 [[TOBOOL4_I]], label [[FOR_INC_SPLIT_1]], label [[FOR_BODY_LR_PH_I:%.*]]
; CHECK: for.body.lr.ph.i:
; CHECK-NEXT: ret void
; CHECK: for.cond.for.end10_crit_edge:
; CHECK-NEXT: ret void
; CHECK: for.inc.split.1:
; CHECK-NEXT: [[INC9]] = add i16 [[TMP0]], 1
; CHECK-NEXT: [[TOBOOL:%.*]] = icmp eq i16 [[INC9]], 0
; CHECK-NEXT: br i1 [[TOBOOL]], label [[FOR_COND_FOR_END10_CRIT_EDGE:%.*]], label [[FOR_COND1_PREHEADER]]
;
for.cond1.preheader.lr.ph:
br label %for.cond1.preheader
for.cond1.preheader: ; preds = %for.inc.split.1, %for.cond1.preheader.lr.ph
%.pr.i.19 = phi i32 [ undef, %for.cond1.preheader.lr.ph ], [ 0, %for.inc.split.1 ]
%0 = phi i16 [ undef, %for.cond1.preheader.lr.ph ], [ %inc9, %for.inc.split.1 ]
%tobool4.i = icmp eq i32 %.pr.i.19, 0
br i1 %tobool4.i, label %t.exit, label %for.body.lr.ph.i
for.body.lr.ph.i: ; preds = %for.cond1.preheader
ret void
t.exit: ; preds = %for.cond1.preheader
br label %for.inc.split
for.inc.split: ; preds = %t.exit
%tobool4.i.1 = icmp eq i32 %.pr.i.19, 0
br i1 %tobool4.i.1, label %for.inc.split.1, label %for.body.lr.ph.i.1
for.cond.for.end10_crit_edge: ; preds = %for.inc.split.1
ret void
for.body.lr.ph.i.1: ; preds = %for.inc.split
br label %for.body.lr.ph.split.i.1
for.body.lr.ph.split.i.1: ; preds = %for.body.lr.ph.i.1
br label %for.body.preheader.i.1
for.body.preheader.i.1: ; preds = %for.body.lr.ph.split.i.1
br label %for.body.i.us.1.preheader
for.body.i.us.1.preheader: ; preds = %for.body.preheader.i.1
br label %for.body.i.us.1
for.body.i.us.1: ; preds = %lor.end.i.us.1, %for.body.i.us.1.preheader
%.b.i.us.1 = phi i1 [ %spec.select44.i.us.1, %lor.end.i.us.1 ], [ undef, %for.body.i.us.1.preheader ]
br i1 %.b.i.us.1, label %lor.end.i.us.1, label %lor.rhs.i.us.1
lor.rhs.i.us.1: ; preds = %for.body.i.us.1
%conv5.i.us.1 = trunc i32 undef to i8
br label %lor.end.i.us.1
lor.end.i.us.1: ; preds = %lor.rhs.i.us.1, %for.body.i.us.1
%.b31.i.us.1 = or i1 undef, %.b.i.us.1
%spec.select44.i.us.1 = or i1 undef, %.b31.i.us.1
br i1 undef, label %for.end18.loopexit42.i.1.loopexit, label %for.body.i.us.1
for.end18.loopexit42.i.1.loopexit: ; preds = %lor.end.i.us.1
br label %for.end18.loopexit42.i.1
for.end18.loopexit42.i.1: ; preds = %for.end18.loopexit42.i.1.loopexit
br label %for.inc.split.1
for.inc.split.1: ; preds = %for.end18.loopexit42.i.1, %for.inc.split
%inc9 = add i16 %0, 1
%tobool = icmp eq i16 %inc9, 0
br i1 %tobool, label %for.cond.for.end10_crit_edge, label %for.cond1.preheader
}