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cmd/compile: R12 can be clobbered for write barrier call on PPC64 [1.14 backport] #40938

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cherrymui opened this issue Aug 20, 2020 · 3 comments
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cmd/compile: R12 can be clobbered for write barrier call on PPC64 [1.14 backport] #40938

cherrymui opened this issue Aug 20, 2020 · 3 comments
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CherryPickApproved Used during the release process for point releases FrozenDueToAge
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Go1.14.9

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cherrymui commented Aug 20, 2020 •
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I think we may want to consider backporting the fix of issue #40851, as it could cause runtime crash or data corruption without workaround.
@gopherbot gopherbot added this to the Go1.14.8 milestone Aug 20, 2020
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Change https://golang.org/cl/249697 mentions this issue: [release-branch.go1.14] cmd/compile, runtime: mark R12 clobbered for write barrier call on PPC64

@dmitshur dmitshur added the CherryPickCandidate Used during the release process for point releases label Aug 21, 2020
@dmitshur dmitshur modified the milestones: Go1.14.8, Go1.14.9 Sep 1, 2020
@dmitshur dmitshur mentioned this issue Sep 2, 2020
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@dmitshur dmitshur added CherryPickApproved Used during the release process for point releases and removed CherryPickCandidate Used during the release process for point releases labels Sep 2, 2020
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dmitshur commented Sep 2, 2020

Approving per discussion in a release meeting. This backport applies to both 1.15 (#40868) and 1.14 (this issue).

gopherbot pushed a commit that referenced this issue Sep 3, 2020
@cherrymui @dmitshur
[release-branch.go1.14] cmd/compile, runtime: mark R12 clobbered for …
652aebf 
…write barrier call on PPC64

When external linking, for large binaries, the external linker
may insert a trampoline for the write barrier call, which looks

0000000005a98cc8 <__long_branch_runtime.gcWriteBarrier>:
 5a98cc8:       86 01 82 3d     addis   r12,r2,390
 5a98ccc:       d8 bd 8c e9     ld      r12,-16936(r12)
 5a98cd0:       a6 03 89 7d     mtctr   r12
 5a98cd4:       20 04 80 4e     bctr

It clobbers R12 (and CTR, which is never live across a call).

As at compile time we don't know whether the binary is big and
what link mode will be used, I think we need to mark R12 as
clobbered for write barrier call. For extra safety (future-proof)
we mark caller-saved register that cannot be used for function
arguments, which includes R11, as potentially clobbered as well.

Updates #40851.
Fixes #40938.

Change-Id: Iedd901c5072f1127cc59b0a48cfeb4aaec81b519
Reviewed-on: https://go-review.googlesource.com/c/go/+/248917
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
(cherry picked from commit b58d297)
Reviewed-on: https://go-review.googlesource.com/c/go/+/249697
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Closed by merging 652aebf to release-branch.go1.14.

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