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cmd/compile: R12 can be clobbered for write barrier call on PPC64 [1.15 backport] #40868

gopherbot opened this issue Aug 18, 2020 · 4 comments


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@gopherbot gopherbot commented Aug 18, 2020

@cherrymui requested issue #40851 to be considered for backport to the next 1.15 minor release.

@gopherbot please consider backporting this to Go 1.15. Thanks.

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@cherrymui cherrymui commented Aug 18, 2020

(Copied from the original issue)
I think we may want to back port, as this could cause runtime crash or data corruption without workaround.

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@gopherbot gopherbot commented Aug 18, 2020

Change mentions this issue: [release-branch.go1.15] cmd/compile, runtime: mark R12 clobbered for write barrier call on PPC64

@dmitshur dmitshur modified the milestones: Go1.15.1, Go1.15.2 Sep 1, 2020
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@dmitshur dmitshur commented Sep 2, 2020

Approving per discussion in a release meeting. This backport applies to both 1.15 (this issue) and 1.14 (#40938).

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@gopherbot gopherbot commented Sep 3, 2020

Closed by merging f50cde9 to release-branch.go1.15.

@gopherbot gopherbot closed this Sep 3, 2020
gopherbot pushed a commit that referenced this issue Sep 3, 2020
…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 #40868.

Change-Id: Iedd901c5072f1127cc59b0a48cfeb4aaec81b519
Run-TryBot: Cherry Zhang <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Austin Clements <>
(cherry picked from commit b58d297)
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