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Date: Sat, 25 Apr 2020 13:30:01 +1000
From: Nicholas Piggin <npiggin@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: Adhemerval Zanella <adhemerval.zanella@...aro.org>,
libc-alpha@...rceware.org, libc-dev@...ts.llvm.org,
linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com
Subject: Re: Powerpc Linux 'scv' system call ABI proposal take 2
Excerpts from Rich Felker's message of April 23, 2020 12:36 pm:
> On Wed, Apr 22, 2020 at 04:18:36PM +1000, Nicholas Piggin wrote:
>> Yeah I had a bit of a play around with musl (which is very nice code I
>> must say). The powerpc64 syscall asm is missing ctr clobber by the way.
>> Fortunately adding it doesn't change code generation for me, but it
>> should be fixed. glibc had the same bug at one point I think (probably
>> due to syscall ABI documentation not existing -- something now lives in
>> linux/Documentation/powerpc/syscall64-abi.rst).
>
> Do you know anywhere I can read about the ctr issue, possibly the
> relevant glibc bug report? I'm not particularly familiar with ppc
> register file (at least I have to refamiliarize myself every time I
> work on this stuff) so it'd be nice to understand what's
> potentially-wrong now.
Ah I was misremembering, glibc was (and still is) actually missing cr
clobbers from its "vsyscall", probably because it copied syscall which
only clobbers cr0, but vsyscall clobbers cr0-1,5-7 like a normal
function call.
musl is missing the ctr register clobber from syscalls.
powerpc has gpr0-31 GPRs, cr0-7 condition regs, and lr and ctr branch
registers (lr is generally used for function returns, ctr for other
indirect branches). ctr is volatile (caller saved) across C function
calls, and sc system calls on Linux.
Thanks,
Nick
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