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pulley: Implement get_frame_pointer #9658

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merged 2 commits into from
Dec 3, 2024

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This commit implements the CLIF get_frame_pointer instruction which will be needed by trampolines in Wasmtime. This is implemented by generalizing the preexisting GetSp MInst into a "get special". This the additionally removes the extended get_sp opcode in Pulley as it's not necessary as xmov can operate on this register.

@alexcrichton alexcrichton requested review from a team as code owners November 22, 2024 20:27
@alexcrichton alexcrichton requested review from cfallin and removed request for a team November 22, 2024 20:27
@github-actions github-actions bot added cranelift Issues related to the Cranelift code generator isle Related to the ISLE domain-specific language pulley Issues related to the Pulley interpreter labels Nov 22, 2024
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@alexcrichton alexcrichton force-pushed the pulley-get-fp branch 2 times, most recently from 35a0761 to 93a7c06 Compare November 26, 2024 01:10
This commit implements the CLIF `get_frame_pointer` instruction which
will be needed by trampolines in Wasmtime. This is implemented by
generalizing the preexisting `GetSp` `MInst` into a "get special". This
the additionally removes the extended `get_sp` opcode in Pulley as it's
not necessary as `xmov` can operate on this register.
@alexcrichton
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ping @cfallin, mind taking a look at this?

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@cfallin cfallin left a comment

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Sorry, missed this review request! LGTM with a few minor bits below.

cranelift/codegen/src/isa/pulley_shared/inst.isle Outdated Show resolved Hide resolved
cranelift/codegen/src/isa/pulley_shared/inst/mod.rs Outdated Show resolved Hide resolved
cranelift/codegen/src/isa/pulley_shared/inst/mod.rs Outdated Show resolved Hide resolved
@alexcrichton alexcrichton added this pull request to the merge queue Dec 3, 2024
Merged via the queue into bytecodealliance:main with commit 75a4d27 Dec 3, 2024
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@alexcrichton alexcrichton deleted the pulley-get-fp branch December 3, 2024 18:49
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@fitzgen fitzgen left a comment

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FWIW, I kind of wanted to have the special registers not accessible by regular xmov/xadd/etc... so we could have 32 regular x registers (and the compact 5 bit representaton for all regular x registers) and then for our handful of special registers we would just have special instructions for manipulating.

This would trade using additional (presumably extended) opcodes for gaining more GPRs, which should help us avoid that many more regalloc spills. Spills result in more instructions, which means more turns of the interpreter loop, which is (expected to be) the most expensive part of wasm execution under pulley.

Not something we need to do now but it's something to think about as we design the instruction set, and maybe we should avoid similar changes as those included in this PR which move us away from that vision in the future where we can.

@alexcrichton alexcrichton mentioned this pull request Dec 5, 2024
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Yeah I've been thinking similarly that I'd prefer to take these registers out of the gpr set, I've opened #9747 and added this there.

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3 participants