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cmd/internal/obj/arm64: minor mistake in pre-increment example #39367

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fwessels opened this issue Jun 2, 2020 · 5 comments
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cmd/internal/obj/arm64: minor mistake in pre-increment example #39367

fwessels opened this issue Jun 2, 2020 · 5 comments
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arch-arm64 Documentation Issues describing a change to documentation. FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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@fwessels
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fwessels commented Jun 2, 2020

The following URL contains a minor mistake: https://golang.org/pkg/cmd/internal/obj/arm64/

Under point 2 ("Go uses .P and .W suffixes to indicate post-increment and pre-increment.") there is a mismatch between the Go assembly and the GNU syntax (for the pre-increment with .W suffix).

Specifically Go assembly loads a signed-byte with MOVB whereas the GNU syntax loads an (unsigned) 64-bit double-word with ldr.

So either the example should be this

MOVB.W 16(R16), R10        <=>      ldrsb x10, [x16,#16]!

or

MOVD.W 16(R16), R10        <=>      ldr x10, [x16,#16]!

or even

MOVBU.W 16(R16), R10        <=>     ldrb x10, [x16,#16]!
@gopherbot gopherbot added this to the Unreleased milestone Jun 2, 2020
@dmitshur dmitshur changed the title x/website: Package arm64: minor mistake in pre-increment example cmd/internal/obj/arm64: minor mistake in pre-increment example Jun 2, 2020
@dmitshur dmitshur added arch-arm64 NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jun 2, 2020
@dmitshur dmitshur modified the milestones: Unreleased, Backlog Jun 2, 2020
@dmitshur
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dmitshur commented Jun 2, 2020

/cc @dr2chase @cherrymui

@dmitshur dmitshur added the Documentation Issues describing a change to documentation. label Jun 2, 2020
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cc @zhangfannie

@zhangfannie
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@fwessels Thank you for pointing out the mistake in https://golang.org/pkg/cmd/internal/obj/arm64/. The arm64 go assembler has the correct encoding for instruction " MOVB.W 16(R16), R10 ", we just have a typo in this document. I will submit a patch to fix this error. Thanks.

@fwessels
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fwessels commented Jun 3, 2020

Yes, correct, it is just a documentation error, luckily not in the assembler itself.

@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jun 3, 2020
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Change https://golang.org/cl/236217 mentions this issue: cmd/internal/obj/arm64: fix typos in document

@golang golang locked and limited conversation to collaborators Jun 3, 2021
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