[llvm] 406b601 - [InstCombine] Allow to limit the max number of iterations
Jakub Kuderski via llvm-commits
llvm-commits at lists.llvm.org
Wed Dec 18 10:55:44 PST 2019
Author: Jakub Kuderski
Date: 2019-12-18T13:48:54-05:00
New Revision: 406b6019cd2bd50924be11c634b058c01053fbd3
URL: https://github.com/llvm/llvm-project/commit/406b6019cd2bd50924be11c634b058c01053fbd3
DIFF: https://github.com/llvm/llvm-project/commit/406b6019cd2bd50924be11c634b058c01053fbd3.diff
LOG: [InstCombine] Allow to limit the max number of iterations
Summary:
This patch teaches InstCombine to accept a new parameter: maximum number of iterations over functions.
InstCombine tries to simplify instructions by iterating over the whole function until the function stops changing. As a consequence, the last iteration before reaching a fixpoint visits all instructions in the worklist and never performs any rewrites.
Bounding the number of iterations can have 2 benefits:
* In case the users of the pass can make a good guess about the number of required iterations, we can save the time normally spent on the last iteration that doesn't change anything.
* When the wants to use InstCombine as a cleanup pass, it may be enough to run just a few iterations and stop even before reaching a fixpoint. This can be also useful for implementing a lightweight pass pipeline (think `-O1`).
This patch does not change the behavior of opt or Clang -- limiting the number of iterations is entirely opt-in.
Reviewers: fhahn, davide, spatel, foad, nlopes, grosser, lebedev.ri, nikic, xbolva00
Reviewed By: spatel
Subscribers: craig.topper, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71145
Added:
llvm/test/Transforms/InstCombine/limit-max-iterations.ll
Modified:
llvm/include/llvm/Transforms/InstCombine/InstCombine.h
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/Transforms/InstCombine/InstCombine.h b/llvm/include/llvm/Transforms/InstCombine/InstCombine.h
index ed075192126c..d7a6662510d3 100644
--- a/llvm/include/llvm/Transforms/InstCombine/InstCombine.h
+++ b/llvm/include/llvm/Transforms/InstCombine/InstCombine.h
@@ -24,13 +24,14 @@ namespace llvm {
class InstCombinePass : public PassInfoMixin<InstCombinePass> {
InstCombineWorklist Worklist;
- bool ExpensiveCombines;
+ const bool ExpensiveCombines;
+ const unsigned MaxIterations;
public:
static StringRef name() { return "InstCombinePass"; }
- explicit InstCombinePass(bool ExpensiveCombines = true)
- : ExpensiveCombines(ExpensiveCombines) {}
+ explicit InstCombinePass(bool ExpensiveCombines = true);
+ explicit InstCombinePass(bool ExpensiveCombines, unsigned MaxIterations);
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
@@ -42,11 +43,14 @@ class InstCombinePass : public PassInfoMixin<InstCombinePass> {
class InstructionCombiningPass : public FunctionPass {
InstCombineWorklist Worklist;
const bool ExpensiveCombines;
+ const unsigned MaxIterations;
public:
static char ID; // Pass identification, replacement for typeid
- InstructionCombiningPass(bool ExpensiveCombines = true);
+ explicit InstructionCombiningPass(bool ExpensiveCombines = true);
+ explicit InstructionCombiningPass(bool ExpensiveCombines,
+ unsigned MaxIterations);
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnFunction(Function &F) override;
@@ -65,6 +69,8 @@ class InstructionCombiningPass : public FunctionPass {
// %Z = add int 2, %X
//
FunctionPass *createInstructionCombiningPass(bool ExpensiveCombines = true);
+FunctionPass *createInstructionCombiningPass(bool ExpensiveCombines,
+ unsigned MaxIterations);
}
#endif
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 5383da8fd869..0c00d6521dd3 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -122,6 +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 cl::opt<bool>
EnableCodeSinking("instcombine-code-sinking", cl::desc("Enable code sinking"),
cl::init(true));
@@ -130,6 +132,11 @@ static cl::opt<bool>
EnableExpensiveCombines("expensive-combines",
cl::desc("Enable expensive instruction combines"));
+static cl::opt<unsigned> LimitMaxIterations(
+ "instcombine-max-iterations",
+ cl::desc("Limit the maximum number of instruction combining iterations"),
+ cl::init(InstCombineDefaultMaxIterations));
+
static cl::opt<unsigned>
MaxArraySize("instcombine-maxarray-size", cl::init(1024),
cl::desc("Maximum array size considered when doing a combine"));
@@ -3538,10 +3545,11 @@ static bool combineInstructionsOverFunction(
Function &F, InstCombineWorklist &Worklist, AliasAnalysis *AA,
AssumptionCache &AC, TargetLibraryInfo &TLI, DominatorTree &DT,
OptimizationRemarkEmitter &ORE, BlockFrequencyInfo *BFI,
- ProfileSummaryInfo *PSI, bool ExpensiveCombines = true,
- LoopInfo *LI = nullptr) {
+ ProfileSummaryInfo *PSI, bool ExpensiveCombines, unsigned MaxIterations,
+ LoopInfo *LI) {
auto &DL = F.getParent()->getDataLayout();
ExpensiveCombines |= EnableExpensiveCombines;
+ MaxIterations = std::min(MaxIterations, LimitMaxIterations.getValue());
/// Builder - This is an IRBuilder that automatically inserts new
/// instructions into the worklist when they are created.
@@ -3560,9 +3568,19 @@ static bool combineInstructionsOverFunction(
MadeIRChange = LowerDbgDeclare(F);
// Iterate while there is work to do.
- int Iteration = 0;
+ unsigned Iteration = 0;
while (true) {
++Iteration;
+ 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_DEBUG(dbgs() << "\n\nINSTCOMBINE ITERATION #" << Iteration << " on "
<< F.getName() << "\n");
@@ -3574,11 +3592,19 @@ static bool combineInstructionsOverFunction(
if (!IC.run())
break;
+
+ MadeIRChange = true;
}
- return MadeIRChange || Iteration > 1;
+ return MadeIRChange;
}
+InstCombinePass::InstCombinePass(bool ExpensiveCombines)
+ : ExpensiveCombines(ExpensiveCombines), MaxIterations(LimitMaxIterations) {}
+
+InstCombinePass::InstCombinePass(bool ExpensiveCombines, unsigned MaxIterations)
+ : ExpensiveCombines(ExpensiveCombines), MaxIterations(MaxIterations) {}
+
PreservedAnalyses InstCombinePass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &AC = AM.getResult<AssumptionAnalysis>(F);
@@ -3596,8 +3622,9 @@ PreservedAnalyses InstCombinePass::run(Function &F,
auto *BFI = (PSI && PSI->hasProfileSummary()) ?
&AM.getResult<BlockFrequencyAnalysis>(F) : nullptr;
- if (!combineInstructionsOverFunction(F, Worklist, AA, AC, TLI, DT, ORE,
- BFI, PSI, ExpensiveCombines, LI))
+ if (!combineInstructionsOverFunction(F, Worklist, AA, AC, TLI, DT, ORE, BFI,
+ PSI, ExpensiveCombines, MaxIterations,
+ LI))
// No changes, all analyses are preserved.
return PreservedAnalyses::all();
@@ -3646,14 +3673,23 @@ bool InstructionCombiningPass::runOnFunction(Function &F) {
&getAnalysis<LazyBlockFrequencyInfoPass>().getBFI() :
nullptr;
- return combineInstructionsOverFunction(F, Worklist, AA, AC, TLI, DT, ORE,
- BFI, PSI, ExpensiveCombines, LI);
+ return combineInstructionsOverFunction(F, Worklist, AA, AC, TLI, DT, ORE, BFI,
+ PSI, ExpensiveCombines, MaxIterations,
+ LI);
}
char InstructionCombiningPass::ID = 0;
InstructionCombiningPass::InstructionCombiningPass(bool ExpensiveCombines)
- : FunctionPass(ID), ExpensiveCombines(ExpensiveCombines) {
+ : FunctionPass(ID), ExpensiveCombines(ExpensiveCombines),
+ MaxIterations(InstCombineDefaultMaxIterations) {
+ initializeInstructionCombiningPassPass(*PassRegistry::getPassRegistry());
+}
+
+InstructionCombiningPass::InstructionCombiningPass(bool ExpensiveCombines,
+ unsigned MaxIterations)
+ : FunctionPass(ID), ExpensiveCombines(ExpensiveCombines),
+ MaxIterations(MaxIterations) {
initializeInstructionCombiningPassPass(*PassRegistry::getPassRegistry());
}
@@ -3683,6 +3719,11 @@ FunctionPass *llvm::createInstructionCombiningPass(bool ExpensiveCombines) {
return new InstructionCombiningPass(ExpensiveCombines);
}
+FunctionPass *llvm::createInstructionCombiningPass(bool ExpensiveCombines,
+ unsigned MaxIterations) {
+ return new InstructionCombiningPass(ExpensiveCombines, MaxIterations);
+}
+
void LLVMAddInstructionCombiningPass(LLVMPassManagerRef PM) {
unwrap(PM)->add(createInstructionCombiningPass());
}
diff --git a/llvm/test/Transforms/InstCombine/limit-max-iterations.ll b/llvm/test/Transforms/InstCombine/limit-max-iterations.ll
new file mode 100644
index 000000000000..22d02e1c1538
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/limit-max-iterations.ll
@@ -0,0 +1,41 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; 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
+
+; Based on xor-of-icmps-with-extra-uses.ll. This requires multiple iterations of
+; InstCombine to reach a fixpoint.
+
+define i1 @v0_select_of_consts(i32 %X, i32* %selected) {
+; ZERO-LABEL: @v0_select_of_consts(
+; ZERO-NEXT: [[COND0:%.*]] = icmp sgt i32 [[X:%.*]], 32767
+; ZERO-NEXT: [[COND1:%.*]] = icmp sgt i32 [[X]], -32768
+; ZERO-NEXT: [[SELECT:%.*]] = select i1 [[COND0]], i32 32767, i32 -32768
+; ZERO-NEXT: store i32 [[SELECT]], i32* [[SELECTED:%.*]]
+; ZERO-NEXT: [[RES:%.*]] = xor i1 [[COND0]], [[COND1]]
+; ZERO-NEXT: ret i1 [[RES]]
+
+; ONE-LABEL: @v0_select_of_consts(
+; ONE-NEXT: [[COND0:%.*]] = icmp sle i32 [[X:%.*]], 32767
+; ONE-NEXT: [[COND0_NOT:%.*]] = xor i1 [[COND0]], true
+; ONE-NEXT: [[COND1:%.*]] = icmp sgt i32 [[X]], -32768
+; ONE-NEXT: [[SELECT:%.*]] = select i1 [[COND0_NOT]], i32 32767, i32 -32768
+; ONE-NEXT: store i32 [[SELECT]], i32* [[SELECTED:%.*]], align 4
+; ONE-NEXT: [[TMP1:%.*]] = and i1 [[COND0]], [[COND1]]
+; ONE-NEXT: ret i1 [[TMP1]]
+
+; FIXPOINT-LABEL: @v0_select_of_consts(
+; FIXPOINT-NEXT: [[COND0_INV:%.*]] = icmp sgt i32 [[X:%.*]], 32767
+; FIXPOINT-NEXT: [[SELECT:%.*]] = select i1 [[COND0_INV]], i32 32767, i32 -32768
+; FIXPOINT-NEXT: store i32 [[SELECT]], i32* [[SELECTED:%.*]], align 4
+; FIXPOINT-NEXT: [[X_OFF:%.*]] = add i32 [[X]], 32767
+; FIXPOINT-NEXT: [[TMP1:%.*]] = icmp ult i32 [[X_OFF]], 65535
+; FIXPOINT-NEXT: ret i1 [[TMP1]]
+
+ %cond0 = icmp sgt i32 %X, 32767
+ %cond1 = icmp sgt i32 %X, -32768
+ %select = select i1 %cond0, i32 32767, i32 -32768
+ store i32 %select, i32* %selected
+ %res = xor i1 %cond0, %cond1
+ ret i1 %res
+}
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