forked from lix-project/lix
bulk-allocate Value instances in the evaluator
calling GC_malloc for each value is significantly more expensive than allocating a bunch of values at once with GC_malloc_many. "a bunch" here is a GC block size, ie 16KiB or less. this gives a 1.5% performance boost when evaluating our nixos system. tested with nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system' # on master Time (mean ± σ): 3.335 s ± 0.007 s [User: 2.774 s, System: 0.293 s] Range (min … max): 3.315 s … 3.347 s 50 runs # with this change Time (mean ± σ): 3.288 s ± 0.006 s [User: 2.728 s, System: 0.292 s] Range (min … max): 3.274 s … 3.307 s 50 runs
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@ -821,8 +821,23 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
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Value * EvalState::allocValue()
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{
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/* We use the boehm batch allocator to speed up allocations of Values (of which there are many).
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GC_malloc_many returns a linked list of objects of the given size, where the first word
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of each object is also the pointer to the next object in the list. This also means that we
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have to explicitly clear the first word of every object we take. */
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if (!valueAllocCache) {
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valueAllocCache = GC_malloc_many(sizeof(Value));
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if (!valueAllocCache) throw std::bad_alloc();
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}
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/* GC_NEXT is a convenience macro for accessing the first word of an object.
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Take the first list item, advance the list to the next item, and clear the next pointer. */
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void * p = valueAllocCache;
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GC_PTR_STORE_AND_DIRTY(&valueAllocCache, GC_NEXT(p));
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GC_NEXT(p) = nullptr;
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nrValues++;
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auto v = (Value *) allocBytes(sizeof(Value));
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auto v = (Value *) p;
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return v;
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}
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@ -133,6 +133,9 @@ private:
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/* Cache used by prim_match(). */
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std::shared_ptr<RegexCache> regexCache;
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/* Allocation cache for GC'd Value objects. */
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void * valueAllocCache = nullptr;
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public:
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EvalState(
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