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Differential D10305

Do not use b.cs instruction to jump to cerror
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Authored by kan on Apr 7 2017, 3:28 PM.
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andrew
emaste
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arm64
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rS316624: Do not use b.cs instruction to jump to cerror.
Summary

The conditional jump can only be performed tp targets up to 1MB in
either direction and does not work too well when linker places cerror
further that that from the caller. In that case linker will complain
about relocation overflows.

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rS FreeBSD src repository - subversion
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Event Timeline

kan created this revision.Apr 7 2017, 3:28 PM
Herald added a subscriber: imp. · View Herald TranscriptApr 7 2017, 3:28 PM
andrew edited edge metadata.Apr 7 2017, 3:32 PM

Can you add a comment to point out why we use the b.cs ; ret ; b sequence.

Harbormaster failed remote builds in B8583: Diff 27179!Apr 7 2017, 3:32 PM
imp added inline comments.Apr 7 2017, 3:34 PM
lib/libc/aarch64/SYS.h
54 ↗(On Diff #27179)

why not use b cerror directly?

andrew added inline comments.Apr 7 2017, 3:38 PM
lib/libc/aarch64/SYS.h
54 ↗(On Diff #27179)

How? We still need to check the process state as that's how syscalls signal failure.

emaste accepted this revision as: emaste.Apr 7 2017, 3:54 PM

This LGTM with @andrew's note addressed, probably just make a comment out of a short version of the explanation that's in the Phabricator summary?

Also, with luck a future version of the linker will insert a thunk by itself when necessary, and we could then revert this.

This revision is now accepted and ready to land.Apr 7 2017, 3:54 PM
emaste added a child revision: D10310: do not require binutils port if using lld as ld.Apr 7 2017, 6:33 PM
Closed by commit rS316624: Do not use b.cs instruction to jump to cerror. (authored by kan). · Explain WhyApr 7 2017, 10:58 PM
This revision was automatically updated to reflect the committed changes.

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head/
lib/
libc/
aarch64/
12 lines

Diff 27206

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head/lib/libc/aarch64/SYS.h

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