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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
This commit is contained in:
Jakub Kuderski 2019-12-20 16:08:52 -05:00
parent 494b1318ca
commit c431c407eb
2 changed files with 18 additions and 4 deletions
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19
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
View File

@ -122,7 +122,8 @@ STATISTIC(NumReassoc , "Number of reassociations");
DEBUG_COUNTER(VisitCounter, "instcombine-visit",
"Controls which instructions are visited");
static constexpr unsigned InstCombineDefaultMaxIterations = UINT_MAX - 1;
static constexpr unsigned InstCombineDefaultMaxIterations = 1000;
static constexpr unsigned InstCombineDefaultInfiniteLoopThreshold = 1000;
static cl::opt<bool>
EnableCodeSinking("instcombine-code-sinking", cl::desc("Enable code sinking"),
@ -137,6 +138,12 @@ static cl::opt<unsigned> LimitMaxIterations(
cl::desc("Limit the maximum number of instruction combining iterations"),
cl::init(InstCombineDefaultMaxIterations));
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"));
@ -3571,13 +3578,17 @@ static bool combineInstructionsOverFunction(
unsigned Iteration = 0;
while (true) {
++Iteration;
if (Iteration > InfiniteLoopDetectionThreshold) {
report_fatal_error(
"Instruction Combining seems stuck in an infinite loop after " +
Twine(InfiniteLoopDetectionThreshold) + " iterations.");
}
if (Iteration > MaxIterations) {
LLVM_DEBUG(dbgs() << "\n\n[IC] Iteration limit #" << MaxIterations
<< " on " << F.getName()
<< " reached; stopping before reaching a fixpoint\n");
LLVM_DEBUG(dbgs().flush());
assert(Iteration <= InstCombineDefaultMaxIterations &&
"InstCombine stuck in an infinite loop?");
break;
}

3
llvm/test/Transforms/InstCombine/limit-max-iterations.ll
View File

@ -2,6 +2,7 @@
; RUN: opt < %s -instcombine --instcombine-max-iterations=0 -S | FileCheck %s --check-prefix=ZERO
; RUN: opt < %s -instcombine --instcombine-max-iterations=1 -S | FileCheck %s --check-prefix=ONE
; RUN: opt < %s -instcombine -S | FileCheck %s --check-prefix=FIXPOINT
; RUN: not opt < %s -instcombine -S --instcombine-infinite-loop-threshold=3 2>&1 | FileCheck %s --check-prefix=LOOP
; Based on xor-of-icmps-with-extra-uses.ll. This requires multiple iterations of
; InstCombine to reach a fixpoint.
@ -32,6 +33,8 @@ define i1 @v0_select_of_consts(i32 %X, i32* %selected) {
; FIXPOINT-NEXT: [[TMP1:%.*]] = icmp ult i32 [[X_OFF]], 65535
; FIXPOINT-NEXT: ret i1 [[TMP1]]
; LOOP: LLVM ERROR: Instruction Combining seems stuck in an infinite loop after 3 iterations.
%cond0 = icmp sgt i32 %X, 32767
%cond1 = icmp sgt i32 %X, -32768
%select = select i1 %cond0, i32 32767, i32 -32768