For short builtin calls, the builtins are copied on the heap when they
cannot be put close enough to be in range of relative calls. This costs
memory, as the embedded builtins are part of the binary, and mapped from
the binary, and as a consequence shared with all running processes.
Rather than copying the memory, we can remap it at a different address,
avoiding the memory cost. This CL does that, on ARM64 macOS only for
now.
This saves at least ~1.4MiB of memory per V8 process. See below the
output of vmmap <PID>:
[...]
Memory Tag 255 7408308000-740833c000 [ 208K 144K 144K 0K] r-x/rwx SM=ZER
Memory Tag 255 740833c000-7408340000 [ 16K 0K 0K 0K] ---/rwx SM=ZER
Memory Tag 255 7408344000-7408348000 [ 16K 0K 0K 0K] ---/rwx SM=ZER
Memory Tag 255 7408348000-740837c000 [ 208K 144K 144K 0K] r-x/rwx SM=ZER
Memory Tag 255 740837c000-740fe80000 [123.0M 0K 0K 0K] ---/rwx SM=ZER
mapped file 740fe80000-740ffe4000 [ 1424K 1328K 0K 0K] r-x/rwx SM=COW ...pp/Contents/Frameworks/Chromium Framework.framework/Versions/102.0.4958.0/Chromium Framework
Memory Tag 255 740ffe4000-7410000000 [ 112K 0K 0K 0K] ---/rwx SM=ZER
The "208K" regions are 256kiB code pages, minus the header and guard
pages, meaning that they are code chunks. The mapped file are the
remapped builtins, showing that they aren't copied, but remapped from
the binary.
Bug: chromium:1298417
Change-Id: Ia30a43e671726d01450a7db0ecb7777b34763053
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3553006
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79716}
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