8000 [InstCombine] Fix infinite loop due to bitcast <-> phi transforms by tmandry · Pull Request #34 · rust-lang/llvm-project · GitHub
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[InstCombine] Fix infinite loop due to bitcast <-> phi transforms #34

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Merged
merged 7 commits into from
Jan 21, 2020
Merged

[InstCombine] Fix infinite loop due to bitcast <-> phi transforms #34

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22a4d4f
[InstCombine] Add test for iterator invalidation bug; NFC
nikic Jan 13, 2020
bccfb33
[InstCombine] Fix user iterator invalidation in bitcast of phi transform
nikic Jan 13, 2020
e35eeb6
[InstCombine] Make combineLoadToNewType a method; NFC
nikic Jan 13, 2020
1beab05
[InstCombine] Fix infinite loop due to bitcast <-> phi transforms
nikic Jan 13, 2020
c6a5418
[InstCombine] Improve infinite loop detection
kuhar Dec 20, 2019
f6905cf
[InstCombine] Add tests for PR44242
cwabbott0 Dec 31, 2019
f88b689
[InstCombine] Don't rewrite phi-of-bitcast when the phi has other users
cwabbott0 Dec 31, 2019
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[InstCombine] Improve infinite loop detection 
Summary:
This patch limits the default number of iterations performed by InstCombine. It also exposes a new option that allows to specify how many iterations is considered getting stuck in an infinite loop.

Based on experiments performed on real-world C++ programs, InstCombine seems to perform at most ~8-20 iterations, so treating 1000 iterations as an infinite loop seems like a safe choice. See D71145 for details.

The two limits can be specified via command line options.

Reviewers: spatel, lebedev.ri, nikic, xbolva00, grosser

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71673
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@kuhar @tmandry
kuhar authored and tmandry committed Jan 17, 2020
commit c6a5418f76c308bcce1c7232e79bac301af81a7a
18 changes: 17 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -121,6 +121,9 @@ STATISTIC(NumReassoc , "Number of reassociations");
DEBUG_COUNTER(VisitCounter, "instcombine-visit",
"Controls which instructions are visited");

static constexpr unsigned InstCombineDefaultMaxIterations = 1000;
static constexpr unsigned InstCombineDefaultInfiniteLoopThreshold = 1000;

static cl::opt<bool>
EnableCodeSinking("instcombine-code-sinking", cl::desc("Enable code sinking"),
cl::init(true));
Expand All @@ -129,6 +132,12 @@ static cl::opt<bool>
EnableExpensiveCombines("expensive-combines",
cl::desc("Enable expensive instruction combines"));

static cl::opt<unsigned> InfiniteLoopDetectionThreshold(
"instcombine-infinite-loop-threshold",
cl::desc("Number of instruction combining iterations considered an "
"infinite loop"),
cl::init(InstCombineDefaultInfiniteLoopThreshold), cl::Hidden);

static cl::opt<unsigned>
MaxArraySize("instcombine-maxarray-size", cl::init(1024),
cl::desc("Maximum array size considered when doing a combine"));
Expand Down Expand Up @@ -3508,9 +3517,16 @@ static bool combineInstructionsOverFunction(
MadeIRChange = LowerDbgDeclare(F);

// Iterate while there is work to do.
int Iteration = 0;
unsigned Iteration = 0;
while (true) {
++Iteration;

if (Iteration > InfiniteLoopDetectionThreshold) {
report_fatal_error(
"Instruction Combining seems stuck in an infinite loop after " +
Twine(InfiniteLoopDetectionThreshold) + " iterations.");
}

LLVM_DEBUG(dbgs() << "\n\nINSTCOMBINE ITERATION #" << Iteration << " on "
<< F.getName() << "\n");

Expand Down
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