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Set an absolute maximum inlining limit #3959

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Jul 1, 2021
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6 changes: 6 additions & 0 deletions src/ir/utils.h
Original file line number Diff line number Diff line change
Expand Up @@ -33,11 +33,17 @@ struct Measurer

void visitExpression(Expression* curr) { size++; }

// Measure the number of expressions.
static Index measure(Expression* tree) {
Measurer measurer;
measurer.walk(tree);
return measurer.size;
}

// A rough estimate of average binary size per expression. The number here is
// based on measurements on real-world (MVP) wasm files, on which observed
// ratios were 2.2 - 2.8.
static constexpr double BytesPerExpr = 2.5;
};

struct ExpressionAnalyzer {
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21 changes: 21 additions & 0 deletions src/passes/Inlining.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -420,6 +420,9 @@ struct Inlining : public Pass {
continue;
}
Name inlinedName = inlinedFunction->name;
if (!isUnderSizeLimit(func->name, inlinedName)) {
continue;
}
#ifdef INLINING_DEBUG
std::cout << "inline " << inlinedName << " into " << func->name << '\n';
#endif
Expand All @@ -446,6 +449,24 @@ struct Inlining : public Pass {
// return whether we did any work
return inlinedUses.size() > 0;
}

// Checks if the combined size of the code after inlining is under the
// absolute size limit. We have an absolute limit in order to avoid
// extremely-large sizes after inlining, as they may hit limits in VMs and/or
// slow down startup (measurements there indicate something like ~1 second to
// optimize a 100K function). See e.g.
// https://github.com/WebAssembly/binaryen/pull/3730#issuecomment-867939138
// https://github.com/emscripten-core/emscripten/issues/13899#issuecomment-825073344
bool isUnderSizeLimit(Name target, Name source) {
// Estimate the combined binary size from the number of instructions.
auto combinedSize = infos[target].size + infos[source].size;
auto estimatedBinarySize = Measurer::BytesPerExpr * combinedSize;
// The limit is arbitrary, but based on the links above. It is a very high
// value that should appear very rarely in practice (for example, it does
// not occur on the Emscripten benchmark suite of real-world codebases).
const Index MaxCombinedBinarySize = 400 * 1024;
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Is this an arbitrary limit? Maybe good to add a single-line comment on what this is. If this is an arbitrary limit, stating that will help readers too, in case they think this number has a specific meaning.

return estimatedBinarySize < MaxCombinedBinarySize;
}
};

Pass* createInliningPass() { return new Inlining(); }
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