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clang's missing -ffixed-REG, -fcall-used-REG, -fcall-saved-REG #9829

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pageexec mannequin opened this issue Mar 11, 2011 · 3 comments
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clang's missing -ffixed-REG, -fcall-used-REG, -fcall-saved-REG #9829

pageexec mannequin opened this issue Mar 11, 2011 · 3 comments
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bugzilla Issues migrated from bugzilla clang Clang issues not falling into any other category

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@pageexec
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Mannequin

pageexec mannequin commented Mar 11, 2011

Bugzilla Link 9457
Resolution FIXED
Resolved on Sep 25, 2018 10:04
Version trunk
OS Linux
Blocks #4440
CC @asl,@emaste,@jyknight,@nickdesaulniers,@pageexec,@stephenhines

Extended Description

in particular, -fcall-saved-REG is used by linux on x86 (i.e., both i386 and amd64) for an optimized ABI for a few specific functions. since clang ignores this switch (for completeness, i added the other two related switches too) the generated machine code is not correct and results in runtime failure. the workaround on the linux side is to disable CONFIG_ARCH_HWEIGHT_CFLAGS for now.
@jyknight
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Not used anymore on x86 or x86-64 now, only arm64.

I'd recommend not implementing this in clang and having the kernel use one of the alternate calling conventions for these functions instead (e.g. "coldcc" or "preserve_allcc"), if desired.

@nickdesaulniers
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aarch64:
-ffixed-* for LLVM: https://reviews.llvm.org/rL341706
-ffixed-* for Clang: https://reviews.llvm.org/rL342100

@nickdesaulniers
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aarch64:
-fcall-saved: https://reviews.llvm.org/rL342824
-fcall-saved: https://reviews.llvm.org/rL342990

For aarch64, the arm64 kernel maintainers have agreed to drop the use of -fcall-used: http://lists.infradead.org/pipermail/linux-arm-kernel/2018-September/602609.html

The Clang/LLVM flags I've linked in this bug will ship in Clang-8. The above kernel patch will land in Linux-4.20.

Since these flags are no longer used for other archs, and only in the kernel, I'm closing this bug. Please open a new one if you'd like support for other archs or -fcall-used.

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021
This issue was closed.
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