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Message-ID: <20251012030214.GL1827@brightrain.aerifal.cx>
Date: Sat, 11 Oct 2025 23:02:14 -0400
From: Rich Felker <dalias@...c.org>
To: Alex Rønne Petersen <alex@...xrp.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] s390x: manually inline __tls_get_addr in
__tls_get_offset
On Sun, Oct 12, 2025 at 04:56:02AM +0200, Alex Rønne Petersen wrote:
> On Sun, Oct 12, 2025, at 04:46, Rich Felker wrote:
> > On Fri, Jan 24, 2025 at 06:12:13AM +0100, Alex Rønne Petersen wrote:
> >> Calling __tls_get_addr with brasl is not valid since it's a global symbol; doing
> >> so results in an R_390_PC32DBL relocation error from lld. We could fix this by
> >> marking __tls_get_addr hidden since it is not part of the s390x ABI, or by using
> >> a different instruction. However, given its simplicity, it makes more sense to
> >> just manually inline it into __tls_get_offset for performance.
> >>
> >> The patch has been tested by applying to Zig's bundled musl copy and running the
> >> full Zig test suite under qemu-s390x.
> >> ---
> >> src/thread/s390x/__tls_get_offset.s | 20 ++++++++++----------
> >> 1 file changed, 10 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/src/thread/s390x/__tls_get_offset.s b/src/thread/s390x/__tls_get_offset.s
> >> index 8ee92de8..405f118b 100644
> >> --- a/src/thread/s390x/__tls_get_offset.s
> >> +++ b/src/thread/s390x/__tls_get_offset.s
> >> @@ -1,17 +1,17 @@
> >> .global __tls_get_offset
> >> .type __tls_get_offset,%function
> >> __tls_get_offset:
> >> - stmg %r14, %r15, 112(%r15)
> >> - aghi %r15, -160
> >> + ear %r0, %a0
> >> + sllg %r0, %r0, 32
> >> + ear %r0, %a1
> >>
> >> - la %r2, 0(%r2, %r12)
> >> - brasl %r14, __tls_get_addr
> >> + la %r1, 0(%r2, %r12)
> >>
> >> - ear %r1, %a0
> >> - sllg %r1, %r1, 32
> >> - ear %r1, %a1
> >> + lg %r3, 0(%r1)
> >> + sllg %r4, %r3, 3
> >> + lg %r5, 8(%r0)
> >> + lg %r2, 0(%r4, %r5)
> >> + ag %r2, 8(%r1)
> >> + sgr %r2, %r0
> >>
> >> - sgr %r2, %r1
> >> -
> >> - lmg %r14, %r15, 272(%r15)
> >> br %r14
> >> --
> >> 2.43.0
> >
> > This must not have been covered by tests because it doesn't work. The
> > problem is that there is apparently no such thing as 8(%r0); it looks
> > like r0 is special in addressing and means constant zero. So it should
> > be fixable just shuffling around which registers are used for what.
> >
> > I can try to prepare a fix if you don't have time for it.
>
> Oh, yikes, yes; I actually got bit by this exact s390x quirk
> recently in an unrelated context while working on Zig:
> https://github.com/ziglang/zig/commit/7b92d5f4052be651e9bc5cd4ad78a69ccbee865d
>
> And indeed, another thing I've since discovered is that our test
> suite has rather poor coverage of TLS usage. Something we'll
> definitely need work on.
>
> Sorry for the trouble; I'll send a v2 of this patch shortly.
Thanks! The commit is already in git history, so please just send fix
as a patch against the old rather than a v2 of the above..
Rich
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