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Inline many (#1125)
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* Improve inlining pass to inline single-use functions that are fairly small, which makes it useful for removing unnecessary global constructors from clang.

 * Add an inlining-optimizing pass that also optimizes where it inlined, as new opportunities arise. enable that it by default in O2+

* In addition, in -O3+ also inline small functions with multiple uses. This helps a lot with things like safe-int-divide functions (where each int divide is replaced by a safe divide that won't trap). Inlining gets rid of around half of the overhead there.
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kripken authored Aug 22, 2017
1 parent ad8bc65 commit cfeddd7
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156 changes: 94 additions & 62 deletions src/passes/Inlining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -17,10 +17,15 @@
//
// Inlining.
//
// For now, this does a conservative inlining of all functions that have
// By default, this does a conservative inlining of all functions that have
// exactly one use, and are fairly small. That should not increase code
// size, and may have speed benefits.
//
// When opt level is 3+ (-O3 or above), we more aggressively inline
// even functions with more than one use, that seem to be "lightweight"
// (no loops or calls etc.), so inlining them may get rid of call overhead
// that would be noticeable otherwise
//

#include <atomic>

Expand All @@ -33,42 +38,66 @@

namespace wasm {

// A limit on how big a function to inline.
static const int INLINING_SIZE_LIMIT = 15;
// A limit on how big a function to inline when being careful about size
static const int CAREFUL_SIZE_LIMIT = 15;

// A limit on how big a function to inline when being more flexible. In
// particular it's nice that with this limit we can inline the clamp
// functions (i32s-div, f64-to-int, etc.), that can affect perf.
static const int FLEXIBLE_SIZE_LIMIT = 20;

// Useful into on a function, helping us decide if we can inline it
struct FunctionInfo {
std::atomic<Index> calls;
Index size;
bool lightweight = true;
bool usedGlobally = false; // in a table or export

bool worthInlining(PassOptions& options, bool allowMultipleInliningsPerFunction) {
// if it's big, it's just not worth doing (TODO: investigate more)
if (size > FLEXIBLE_SIZE_LIMIT) return false;
// if it has one use, then inlining it would likely reduce code size
// since we are just moving code around, + optimizing, so worth it
// if small enough that we are pretty sure its ok
if (calls == 1 && !usedGlobally && size <= CAREFUL_SIZE_LIMIT) return true;
if (!allowMultipleInliningsPerFunction) return false;
// more than one use, so we can't eliminate it after inlining,
// so only worth it if we really care about speed and don't care
// about size, and if it's lightweight so a good candidate for
// speeding us up
return options.optimizeLevel >= 3 && options.shrinkLevel == 0 && lightweight;
}
};

// We only inline a function with a single use.
static const int SINGLE_USE = 1;
typedef std::unordered_map<Name, FunctionInfo> NameInfoMap;

// A number of uses of a function that is too high for us to
// inline it to all those locations.
static const int TOO_MANY_USES_TO_INLINE = SINGLE_USE + 1;
struct FunctionInfoScanner : public WalkerPass<PostWalker<FunctionInfoScanner>> {
bool isFunctionParallel() override { return true; }

// Map of function name => number of uses. We build the values in
// parallel, using atomic increments. This is safe because we never
// update the map itself in parallel, we only update the values,
// and so the map never allocates or moves values which could be
// a problem with atomics (in fact it would be a problem in general
// as well, not just with atomics, as we don't use a lock in
// parallel access, we depend on the map itself being constant
// when running multiple threads).
typedef std::map<Name, std::atomic<Index>> NameToAtomicIndexMap;
FunctionInfoScanner(NameInfoMap* infos) : infos(infos) {}

struct FunctionUseCounter : public WalkerPass<PostWalker<FunctionUseCounter>> {
bool isFunctionParallel() override { return true; }
FunctionInfoScanner* create() override {
return new FunctionInfoScanner(infos);
}

FunctionUseCounter(NameToAtomicIndexMap* uses) : uses(uses) {}
void visitLoop(Loop* curr) {
// having a loop is not lightweight
(*infos)[getFunction()->name].lightweight = false;
}

FunctionUseCounter* create() override {
return new FunctionUseCounter(uses);
void visitCall(Call* curr) {
assert(infos->count(curr->target) > 0); // can't add a new element in parallel
(*infos)[curr->target].calls++;
// having a call is not lightweight
(*infos)[getFunction()->name].lightweight = false;
}

void visitCall(Call *curr) {
assert(uses->count(curr->target) > 0); // can't add a new element in parallel
(*uses)[curr->target]++;
void visitFunction(Function* curr) {
(*infos)[curr->name].size = Measurer::measure(curr->body);
}

private:
NameToAtomicIndexMap* uses;
NameInfoMap* infos;
};

struct InliningAction {
Expand All @@ -79,8 +108,8 @@ struct InliningAction {
};

struct InliningState {
std::set<Name> canInline;
std::map<Name, std::vector<InliningAction>> actionsForFunction; // function name => actions that can be performed in it
std::unordered_set<Name> worthInlining;
std::unordered_map<Name, std::vector<InliningAction>> actionsForFunction; // function name => actions that can be performed in it
};

struct Planner : public WalkerPass<PostWalker<Planner>> {
Expand All @@ -95,7 +124,7 @@ struct Planner : public WalkerPass<PostWalker<Planner>> {
void visitCall(Call* curr) {
// plan to inline if we know this is valid to inline, and if the call is
// actually performed - if it is dead code, it's pointless to inline
if (state->canInline.count(curr->target) &&
if (state->worthInlining.count(curr->target) &&
curr->type != unreachable) {
// nest the call in a block. that way the location of the pointer to the call will not
// change even if we inline multiple times into the same function, otherwise
Expand All @@ -110,7 +139,7 @@ struct Planner : public WalkerPass<PostWalker<Planner>> {
void doWalkFunction(Function* func) {
// we shouldn't inline into us if we are to be inlined
// ourselves - that has the risk of cycles
if (state->canInline.count(func->name) == 0) {
if (state->worthInlining.count(func->name) == 0) {
walk(func->body);
}
}
Expand Down Expand Up @@ -169,33 +198,43 @@ struct Inlining : public Pass {
// whether to optimize where we inline
bool optimize = false;

NameToAtomicIndexMap uses;
NameInfoMap infos;

bool firstIteration;

void run(PassRunner* runner, Module* module) override {
// keep going while we inline, to handle nesting. TODO: optimize
calculateUses(module);
while (iteration(runner, module)) {}
firstIteration = true;
while (1) {
calculateInfos(module);
if (!iteration(runner, module)) {
return;
}
firstIteration = false;
}
}

void calculateUses(Module* module) {
// fill in uses, as we operate on it in parallel (each function to its own entry)
void calculateInfos(Module* module) {
infos.clear();
// fill in info, as we operate on it in parallel (each function to its own entry)
for (auto& func : module->functions) {
uses[func->name].store(0);
infos[func->name];
}
PassRunner runner(module);
runner.setIsNested(true);
runner.add<FunctionUseCounter>(&uses);
runner.add<FunctionInfoScanner>(&infos);
runner.run();
// fill in global uses
// anything exported or used in a table should not be inlined
for (auto& ex : module->exports) {
if (ex->kind == ExternalKind::Function) {
uses[ex->value].store(TOO_MANY_USES_TO_INLINE);
infos[ex->value].usedGlobally = true;
}
}
for (auto& segment : module->table.segments) {
for (auto name : segment.data) {
if (module->getFunctionOrNull(name)) {
uses[name].store(TOO_MANY_USES_TO_INLINE);
infos[name].usedGlobally = true;
}
}
}
Expand All @@ -205,12 +244,12 @@ struct Inlining : public Pass {
// decide which to inline
InliningState state;
for (auto& func : module->functions) {
auto name = func->name;
auto numUses = uses[name].load();
if (canInline(numUses) && worthInlining(module->getFunction(name))) {
state.canInline.insert(name);
// on the first iteration, allow multiple inlinings per function
if (infos[func->name].worthInlining(runner->options, firstIteration /* allowMultipleInliningsPerFunction */)) {
state.worthInlining.insert(func->name);
}
}
if (state.worthInlining.size() == 0) return false;
// fill in actionsForFunction, as we operate on it in parallel (each function to its own entry)
for (auto& func : module->functions) {
state.actionsForFunction[func->name];
Expand All @@ -222,17 +261,16 @@ struct Inlining : public Pass {
runner.add<Planner>(&state);
runner.run();
}
// perform inlinings
std::set<Name> inlined;
std::set<Function*> inlinedInto;
// perform inlinings TODO: parallelize
std::unordered_map<Name, Index> inlinedUses; // how many uses we inlined
std::unordered_set<Function*> inlinedInto; // which functions were inlined into
for (auto& func : module->functions) {
for (auto& action : state.actionsForFunction[func->name]) {
Name inlinedName = action.contents->name;
doInlining(module, func.get(), action);
inlined.insert(inlinedName);
inlinedUses[inlinedName]++;
inlinedInto.insert(func.get());
uses[inlinedName]--;
assert(uses[inlinedName].load() == 0);
assert(inlinedUses[inlinedName] <= infos[inlinedName].calls);
}
}
// anything we inlined into may now have non-unique label names, fix it up
Expand All @@ -242,26 +280,20 @@ struct Inlining : public Pass {
if (optimize && inlinedInto.size() > 0) {
doOptimize(inlinedInto, module, runner);
}
// remove functions that we managed to inline, their one use is gone
// remove functions that we no longer need after inlining
auto& funcs = module->functions;
funcs.erase(std::remove_if(funcs.begin(), funcs.end(), [&inlined](const std::unique_ptr<Function>& curr) {
return inlined.count(curr->name) > 0;
funcs.erase(std::remove_if(funcs.begin(), funcs.end(), [&](const std::unique_ptr<Function>& curr) {
auto name = curr->name;
auto& info = infos[name];
return inlinedUses.count(name) && inlinedUses[name] == info.calls && !info.usedGlobally;
}), funcs.end());
// return whether we did any work
return inlined.size() > 0;
}

bool canInline(int numUses) {
return numUses == SINGLE_USE;
}

bool worthInlining(Function* func) {
return Measurer::measure(func->body) <= INLINING_SIZE_LIMIT;
return inlinedUses.size() > 0;
}

// Run useful optimizations after inlining, things like removing
// unnecessary new blocks, sharing variables, etc.
void doOptimize(std::set<Function*>& funcs, Module* module, PassRunner* parentRunner) {
void doOptimize(std::unordered_set<Function*>& funcs, Module* module, PassRunner* parentRunner) {
// save the full list of functions on the side
std::vector<std::unique_ptr<Function>> all;
all.swap(module->functions);
Expand Down
4 changes: 2 additions & 2 deletions src/passes/pass.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -72,8 +72,8 @@ void PassRegistry::registerPasses() {
registerPass("duplicate-function-elimination", "removes duplicate functions", createDuplicateFunctionEliminationPass);
registerPass("extract-function", "leaves just one function (useful for debugging)", createExtractFunctionPass);
registerPass("flatten-control-flow", "flattens out control flow to be only on blocks, not nested as expressions", createFlattenControlFlowPass);
registerPass("inlining", "inlines functions (currently only ones with a single use)", createInliningPass);
registerPass("inlining-optimizing", "inlines functions (currently only ones with a single use) and optimizes where we inlined", createInliningOptimizingPass);
registerPass("inlining", "inlines functions", createInliningPass);
registerPass("inlining-optimizing", "inlines functions and optimizes where we inlined", createInliningOptimizingPass);
registerPass("legalize-js-interface", "legalizes i64 types on the import/export boundary", createLegalizeJSInterfacePass);
registerPass("local-cse", "common subexpression elimination inside basic blocks", createLocalCSEPass);
registerPass("log-execution", "instrument the build with logging of where execution goes", createLogExecutionPass);
Expand Down
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