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Bug 9457 - clang's missing -ffixed-REG, -fcall-used-REG, -fcall-saved-REG
Summary: clang's missing -ffixed-REG, -fcall-used-REG, -fcall-saved-REG
Status: RESOLVED FIXED
Alias: None
Product: clang
Classification: Unclassified
Component: -New Bugs (show other bugs)
Version: trunk
Hardware: PC Linux
: P normal
Assignee: Unassigned Clang Bugs
URL:
Keywords:
Depends on:
Blocks: 4068
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Reported: 2011-03-11 07:27 PST by PaX Team
Modified: 2018-09-25 10:04 PDT (History)
7 users (show)

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Description PaX Team 2011-03-11 07:27:32 PST 
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.
Comment 1 James Y Knight 2017-03-21 07:16:04 PDT 
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.
Comment 2 Nick Desaulniers 2018-09-19 15:19:24 PDT 
aarch64:
-ffixed-* for LLVM: https://reviews.llvm.org/rL341706
-ffixed-* for Clang: https://reviews.llvm.org/rL342100
Comment 3 Nick Desaulniers 2018-09-25 10:04:35 PDT 
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.