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[SimplifyCFG] avoid sinking insts within an infinite-loop

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The test is reduced from a C source example in:
https://llvm.org/PR49541

It's possible that the test could be reduced further or
the predicate generalized further, but it seems to require
a few ingredients (including the "late" SimplifyCFG options
on the RUN line) to fall into the infinite-loop trap.
This commit is contained in:
Sanjay Patel 2021-03-12 07:56:54 -05:00
parent 5d515df178
commit 95bd4a26fb
2 changed files with 61 additions and 7 deletions
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19
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -1631,6 +1631,11 @@ static bool canSinkInstructions(
I->getType()->isTokenTy())
return false;
// Do not try to sink an instruction in an infinite loop - it can cause
// this algorithm to infinite loop.
if (I->getParent()->getSingleSuccessor() == I->getParent())
return false;
// Conservatively return false if I is an inline-asm instruction. Sinking
// and merging inline-asm instructions can potentially create arguments
// that cannot satisfy the inline-asm constraints.
@ -1717,13 +1722,13 @@ static bool canSinkInstructions(
return true;
}
// Assuming canSinkLastInstruction(Blocks) has returned true, sink the last
// Assuming canSinkInstructions(Blocks) has returned true, sink the last
// instruction of every block in Blocks to their common successor, commoning
// into one instruction.
static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
auto *BBEnd = Blocks[0]->getTerminator()->getSuccessor(0);
// canSinkLastInstruction returning true guarantees that every block has at
// canSinkInstructions returning true guarantees that every block has at
// least one non-terminator instruction.
SmallVector<Instruction*,4> Insts;
for (auto *BB : Blocks) {
@ -1736,9 +1741,9 @@ static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
}
// The only checking we need to do now is that all users of all instructions
// are the same PHI node. canSinkLastInstruction should have checked this but
// it is slightly over-aggressive - it gets confused by commutative instructions
// so double-check it here.
// are the same PHI node. canSinkInstructions should have checked this but
// it is slightly over-aggressive - it gets confused by commutative
// instructions so double-check it here.
Instruction *I0 = Insts.front();
if (!I0->user_empty()) {
auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
@ -1749,11 +1754,11 @@ static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
return false;
}
// We don't need to do any more checking here; canSinkLastInstruction should
// We don't need to do any more checking here; canSinkInstructions should
// have done it all for us.
SmallVector<Value*, 4> NewOperands;
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O) {
// This check is different to that in canSinkLastInstruction. There, we
// This check is different to that in canSinkInstructions. There, we
// cared about the global view once simplifycfg (and instcombine) have
// completed - it takes into account PHIs that become trivially
// simplifiable. However here we need a more local view; if an operand

49
test/Transforms/SimplifyCFG/sink-inf-loop.ll Normal file
View File

@ -0,0 +1,49 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt %s -simplifycfg -simplifycfg-require-and-preserve-domtree=1 -keep-loops=false -sink-common-insts=true -S | FileCheck %s
; This would infinite-loop because we allowed code sinking to examine an infinite-loop block (%j).
define void @PR49541(i32* %t1, i32 %a, i1 %bool) {
; CHECK-LABEL: @PR49541(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[I:%.*]]
; CHECK: j:
; CHECK-NEXT: [[T3:%.*]] = phi i32 [ [[B:%.*]], [[J:%.*]] ], [ [[A:%.*]], [[COND_TRUE:%.*]] ], [ [[A]], [[COND_FALSE:%.*]] ]
; CHECK-NEXT: [[T2:%.*]] = phi i32 [ [[T2]], [[J]] ], [ [[PRE2:%.*]], [[COND_TRUE]] ], [ 0, [[COND_FALSE]] ]
; CHECK-NEXT: [[B]] = load i32, i32* [[T1:%.*]], align 4
; CHECK-NEXT: br label [[J]]
; CHECK: i:
; CHECK-NEXT: [[G_1:%.*]] = phi i16 [ undef, [[ENTRY:%.*]] ], [ [[G_1]], [[COND_FALSE]] ]
; CHECK-NEXT: br i1 [[BOOL:%.*]], label [[COND_FALSE]], label [[COND_TRUE]]
; CHECK: cond.true:
; CHECK-NEXT: [[TOBOOL9_NOT:%.*]] = icmp eq i16 [[G_1]], 0
; CHECK-NEXT: [[PRE2]] = load i32, i32* [[T1]], align 4
; CHECK-NEXT: br label [[J]]
; CHECK: cond.false:
; CHECK-NEXT: [[T5:%.*]] = load i32, i32* [[T1]], align 4
; CHECK-NEXT: [[B2:%.*]] = icmp eq i32 [[T5]], 0
; CHECK-NEXT: br i1 [[B2]], label [[J]], label [[I]]
;
entry:
br label %i
j:
%t3 = phi i32 [ %b, %j ], [ %a, %cond.true ], [ %a, %cond.false ]
%t2 = phi i32 [ %t2, %j ], [ %pre2, %cond.true ], [ 0, %cond.false ]
%b = load i32, i32* %t1, align 4
br label %j
i:
%g.1 = phi i16 [ undef, %entry ], [ %g.1, %cond.false ]
br i1 %bool, label %cond.false, label %cond.true
cond.true:
%tobool9.not = icmp eq i16 %g.1, 0
%pre2 = load i32, i32* %t1, align 4
br label %j
cond.false:
%t5 = load i32, i32* %t1, align 4
%b2 = icmp eq i32 %t5, 0
br i1 %b2, label %j, label %i
}