1778466 - Preserve frame pointers when entering Wasm code

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, task, P3)

Description

[Jan de Mooij [:jandem]]

Wasm code mostly maintains frame pointers, but the main exceptions are:

1. The interpreter entry stub sets FP to nullptr.
2. Direct calls from JIT code use a tagged frame pointer.

I have some patches to fix these to preserve the caller's frame pointer. This will let native stack unwinders (profilers, crash dump analysis, etc) properly unwind through Wasm frames too.

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 1 - Use CodeRange kind to check for InterpEntry stub instead of null frame pointer. r?rhunt!

This will let us preserve the caller's frame pointer in later patches. It's also more
consistent with what we do for the JitEntry stub.

Comment 2

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 2 - Don't clobber FP with nullptr in GenerateInterpEntry. r?rhunt!

This will let native stack unwinders (such as 'perf' or crash-stats tooling) unwind
from Wasm to C++ frames.

Depends on D151246

Comment 3

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 3 - Tidy up InterpEntry handling in ProfilingFrameIterator, remove fake ExitReason. r?rhunt!

Treat the InterpEntry stub more like the JitEntry stub in ProfilingFrameIterator.
This lets us remove the FakeInterpEntry ExitReason, which was a bit of a hack because
this is not an exit.

Depends on D151247

Comment 4

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 4 - Simplify ProfilingFrameIterator::done. r?rhunt!

Depends on D151248

Comment 5

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 5 - Clean up unwoundJitCallerFP_ code in ProfilingFrameIterator. r?rhunt!

The unwoundJitCallerFP_ is only used when the iterator is done() (to transition
to the JS JIT frame iterator after either the JitEntry stub or a direct call).

With this patch, we only set unwoundJitCallerFP_ right before we're done iterating.

Depends on D151249

Comment 6

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 6 - Stop relying on tagged frame pointer for direct JIT => Wasm calls. r?rhunt!

If there's no CodeRange for a frame's return address, this must be a direct call
from JIT code.

Use AssertDirectJitCall in a few more places to help catch issues with this.

Depends on D151250

Comment 7

[Jan de Mooij [:jandem]]

Attachment: Bug 1778466 part 7 - Don't use a tagged frame pointer for direct JIT => Wasm calls. r?rhunt!

The tag bit is now only used for the activation's exitFP field.

Similar to part 2, this makes it easier for stack unwinders to unwind through Wasm frames.

Depends on D151251

Comment 8

[Ryan Hunt [:rhunt] (on leave until early May)]

Thanks for this, this really improves the readability of ProfilingFrameIterator.

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/6bbcd6e57809
part 1 - Use CodeRange kind to check for InterpEntry stub instead of null frame pointer. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/63e1dd659c57
part 2 - Don't clobber FP with nullptr in GenerateInterpEntry. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/94b62c2e8637
part 3 - Tidy up InterpEntry handling in ProfilingFrameIterator, remove fake ExitReason. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/78c4287d2c3c
part 4 - Simplify ProfilingFrameIterator::done. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/073da829bd32
part 5 - Clean up unwoundJitCallerFP_ code in ProfilingFrameIterator. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/7bba2760a2ea
part 6 - Stop relying on tagged frame pointer for direct JIT => Wasm calls. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/cd86be9642b7
part 7 - Don't use a tagged frame pointer for direct JIT => Wasm calls. r=rhunt

Comment 10

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/6bbcd6e57809
https://hg.mozilla.org/mozilla-central/rev/63e1dd659c57
https://hg.mozilla.org/mozilla-central/rev/94b62c2e8637
https://hg.mozilla.org/mozilla-central/rev/78c4287d2c3c
https://hg.mozilla.org/mozilla-central/rev/073da829bd32
https://hg.mozilla.org/mozilla-central/rev/7bba2760a2ea
https://hg.mozilla.org/mozilla-central/rev/cd86be9642b7