1527274 - Fix stack misalignment issues caused by 3-word size of wasm::Frame

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P2)

Description

[Julian Seward [:jseward]]

Currently wasm::Frame has a size of 3 words except on arm64 and mips(64?).
This has the inevitable effect that the spill/work area immediately below the
Frame, as it is used by at least the baseline compiler, can often be only
singleword-aligned. On 32 bit targets that's particularly bad as we could
wind up spilling 8-byte floats to a 4-byte aligned address. And if we want to
do SIMD in future we'll need to have at least 16-aligned spill slots
available.

One, but not necessarily the only, proposed fix is to make wasm::Frame have 4
words on all targets.

Really though, a good solution to this will require further study of frame
layouts and in particular the interactions of the various alignment shims that
are inserted by the various compilers.

Comment 1

[Julian Seward [:jseward]]

Changing the frame size to 4 words exposes the bug described in bug 1518785.

Comment 2

[Lars T Hansen [:lth]]

Luke is talking about taking the size down to two words instead, but we'll leave this opn for now anyway.

Comment 3

[Lars T Hansen [:lth]]

This is either P2 or fixed, so let's bump it.

Comment 4

[Lars T Hansen [:lth]]

wasm::Frame has size 2 words.