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On linux, __rust_probestack shouldn't be called on smallish stack allocations #74405

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KyleSiefring opened this issue Jul 16, 2020 · 4 comments
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On linux, __rust_probestack shouldn't be called on smallish stack allocations #74405

KyleSiefring opened this issue Jul 16, 2020 · 4 comments
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@KyleSiefring
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On linux (x86_64), had a stack allocation that was "only" 8*4096 bytes causing __rust_probestack to be called. From what I understand from this, #43072, linux only needs to have the stack probed for much larger sizes.

Compiler version: Tested on 1.44.0, but a cursory glance in perf still shows __rust_probestack.
@nagisa
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nagisa commented Jul 17, 2020

__rust_probestack being called for anything more than 1 typical page on the platform (4k on Linux) is expected. That's because when the guard page is 1 page large (true at least for non-main threads), then not probing could allow functions to jump over the guard.

All the linked PR does is not manually allocating said guard page on the main thread. I’m not sure what the relation between it and probing functionality is, other than the fact that both involve stacks and their guard pages.

@cuviper
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cuviper commented Jul 17, 2020

With LLVM 11, I hope to switch to "probe-stack"="inline-asm", which avoids any call and supports more targets. That code uses a loop for large stack allocations, but also does unroll to a series of probes for small cases.

@cuviper cuviper added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. C-enhancement Category: An issue proposing an enhancement or a PR with one. labels Jul 17, 2020
@erikdesjardins erikdesjardins mentioned this issue Oct 13, 2020
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bors added a commit to rust-lang-ci/rust that referenced this issue Jan 16, 2021
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Auto merge of rust-lang#77885 - erikdesjardins:probeasm, r=cuviper
635ccfe 
Use probe-stack=inline-asm in LLVM 11+

Fixes (?) rust-lang#74405, related to rust-lang#43241

r? `@cuviper`
@vtjnash
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vtjnash commented Mar 16, 2021

Fixed?

@nagisa
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nagisa commented Mar 16, 2021

We still probe at 4k increments, but I don't think this issue is otherwise actionable given the reasoning outlined above.

Thank you, Kyle, for the report and Jameson for a reminder.

@nagisa nagisa closed this as completed Mar 16, 2021
@KyleSiefring KyleSiefring mentioned this issue Mar 19, 2021
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@ojeda ojeda mentioned this issue Mar 9, 2022
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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