Commit c431c407 authored by Jakub Kuderski's avatar Jakub Kuderski
Browse files

[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
parent 494b1318

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

+15 −4
Original line number Diff line number Diff line
@@ -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;

    }

llvm/test/Transforms/InstCombine/limit-max-iterations.ll

+3 −0
Original line number Diff line number Diff line
@@ -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