- 04 Mar, 2019 1 commit
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Ross McIlroy authored
BUG=v8:8801 Change-Id: I9d9d9824c6c9ad0176bbfd3723da1b578b17c256 Reviewed-on: https://chromium-review.googlesource.com/c/1495555 Commit-Queue: Ross McIlroy <rmcilroy@chromium.org> Reviewed-by: Mythri Alle <mythria@chromium.org> Cr-Commit-Position: refs/heads/master@{#60001}
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- 08 Oct, 2018 1 commit
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Benedikt Meurer authored
This adds support to the escape analysis to allow scalar replacement of (small) FixedArrays with element accesses where the index is not a compile time constant. This happens quite often when inlining functions that operate on variable number of arguments. For example consider this little piece of code: ```js function sum(...args) { let s = 0; for (let i = 0; i < args.length; ++i) s += args[i]; return s; } function sum2(x, y) { return sum(x, y); } ``` This example is made up, of course, but it shows the problem. Let's assume that TurboFan inlines the function `sum` into it's call site at `sum2`. Now it has to materialize the `args` array with the two values `x` and `y`, and iterate through these `args` to sum them up. The escape analysis pass figures out that `args` doesn't escape (aka doesn't outlive) the optimized code for `sum2` now, but TurboFan still needs to materialize the elements backing store for `args` since there's a `LoadElement(args.elements,i)` in the graph now, and `i` is not a compile time constant. However the escape analysis has more information than just that. In particular the escape analysis knows exactly how many elements a non escaping object has, based on the fact that the allocation must be local to the function and that we only track objects with known size. So in the case above when we get to `args[i]` in the escape analysis the relevant part of the graph looks something like this: ``` elements = LoadField[elements](args) length = LoadField[length](args) index = CheckBounds(i, length) value = LoadElement(elements, index) ``` In particular the contract here is that `LoadElement(elements,index)` is guaranteed to have an `index` that is within the valid bounds for the `elements` (there must be a preceeding `CheckBounds` or some other guard in optimized code before it). And since `elements` is allocated inside of the optimized code object, the escape analysis also knows that `elements` has exactly two elements inside (namely the values of `x` and `y`). So we can use that information and replace the access with a `Select(index===0,x,y)` operation instead, which allows us to scalar replace the `elements`, since there's no escaping use anymore in the graph. We do this for the case that the number of elements is 2, as described above, but also for the case where elements length is one. In case of 0, we know that the `LoadElement` must be in dead code, but we can't just mark it for deletion from the graph (to make sure it doesn't block scalar replacement of non-dead code), so we don't handle this for now. And for one element it's even easier, since the `LoadElement` has to yield exactly said element. We could generalize this to handle arbitrary lengths, but since there's a cost to arbitrary decision trees here, it's unclear when this is still beneficial. Another possible solution for length > 2 would be to have special stack allocation for these backing stores and do variable index accesses to these stack areas. But that's way beyond the scope of this isolated change. This change shows a ~2% improvement on the EarleyBoyer benchmark in JetStream, since it benefits a lot from not having to materialize these small arguments backing stores. Drive-by-fix: Fix JSCreateLowering to properly initialize "elements" with StoreElement instead of StoreField (which violates the invariant in TurboFan that fields and elements never alias). Bug: v8:5267, v8:6200 Change-Id: Idd464a15a81e7c9653c48c814b406eb859841428 Reviewed-on: https://chromium-review.googlesource.com/c/1267935 Commit-Queue: Benedikt Meurer <bmeurer@chromium.org> Reviewed-by: Tobias Tebbi <tebbi@chromium.org> Cr-Commit-Position: refs/heads/master@{#56442}
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