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https://github.com/vercel-labs/scriptc.git
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Malte Ubl
3467efac73
Program builds that import runtime bitcode parsed and merged every unit's whole module, although the import can only ever take small bodies that need no second copy of unit-local state. The helper now reduces a unit's bitcode to exactly that set when the runtime pack is built: the importable bodies within the size limit at their call distance from any public function, and the constants they copy. Every other definition becomes a declaration, as the import would treat it. A program's selection starts from public functions, so it is always a subset of the pruned set, and programs import the same bodies as before. `runtime-unit` writes the pruned bitcode for speed units; the new `prune-import` command prunes a unit's bitcode as compiled, for units whose objects the runtime compiler emits itself. Linux x64 speed bitcode shrinks from 17.0 MB to 8.6 MB. Runtime objects are byte-identical, and import lists match the unpruned bitcode exactly. Interleaved cold builds of 53 programs in a Linux Vercel Sandbox: speed went from +21.4% to +12.1% over release.
40 lines
1.7 KiB
C++
40 lines
1.7 KiB
C++
#pragma once
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#include "llvm/IR/Module.h"
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#include <optional>
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#include <string>
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#include <vector>
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namespace scriptc {
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// Makes small runtime functions visible to the program optimizer. Each input
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// is LLVM bitcode for a runtime-pack unit that is also linked as a native
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// object: speed units emit both from one module by `runtime-unit`, which
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// promotes unit-local symbols to hidden unit-qualified globals; release
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// units ship the bitcode of the same compilation as their object, reduced
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// by `prune-import` and with unit-local symbols left local. Selected external
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// definitions are linked into the program module as available_externally,
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// exactly like ThinLTO function import: the optimizer may inline or
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// specialize them, and every remaining reference resolves to the runtime
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// object at link time. Selection starts at the runtime functions the
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// program declares and follows calls with a size limit that decays per hop.
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// Bodies that would need a second copy of unit-local state or a second
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// address for a local function (possible only with unpromoted bitcode) are
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// never imported.
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//
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// Returns an error message on failure.
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std::optional<std::string>
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importRuntimeBitcode(llvm::Module &Program,
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const std::vector<std::string> &BitcodePaths);
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// Reduces one runtime unit's bitcode (promoted or as compiled) to what
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// importRuntimeBitcode can ever take from it: the bodies of importable
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// functions and the constants they copy. Every other definition becomes a
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// declaration, exactly as the import would treat it, so importing from the
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// reduced unit selects and links the same bodies as importing from the
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// whole one, without reading the rest.
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void pruneRuntimeImport(llvm::Module &Unit);
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} // namespace scriptc
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