RE: [PATCH v2] hexagon: Disable constant extender optimization for LLVM prior to 19.1.0

Brian Cain
2024-12-18T04:19:32Z

> -----Original Message-----
> From: Brian Cain <bcain@quicinc.com>
> Sent: Tuesday, December 17, 2024 8:11 PM
> To: Nathan Chancellor <nathan@kernel.org>; Andrew Morton <akpm@linux-
> foundation.org>; Linus Torvalds <torvalds@linux-foundation.org>
> Cc: linux-hexagon@vger.kernel.org; patches@lists.linux.dev;
> llvm@lists.linux.dev; stable@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: RE: [PATCH v2] hexagon: Disable constant extender optimization for
> LLVM prior to 19.1.0
> >
> > -----Original Message-----
> > From: Nathan Chancellor <nathan@kernel.org>
> > Sent: Tuesday, December 17, 2024 11:44 AM
> > To: Brian Cain <bcain@quicinc.com>; Andrew Morton <akpm@linux-
> > foundation.org>; Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: linux-hexagon@vger.kernel.org; patches@lists.linux.dev;
> > llvm@lists.linux.dev; stable@vger.kernel.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v2] hexagon: Disable constant extender optimization for
> > LLVM prior to 19.1.0
> >
> >
> > Hi Linus,
> >
> > Could you apply this change [1] directly? Brian has reviewed it and I have
> > sent this patch twice plus another ping for application [2]. I would
> > like to stop applying this in our CI and some other people have hit it
> > as well.
>
> Nathan,
>
> I'm sorry. You should not have to send these changes to Linus. I should have
> carried it in my tree and proposed it to Linus.
>
> I'll do that, if you don't mind.

I see it's landed. Sorry, all. I'll resolve to do a better job here.

> > [1]: https://lore.kernel.org/all/20241121-hexagon-disable-constant-
> > expander-pass-v2-1-1a92e9afb0f4@kernel.org/
> > [2]: https://lore.kernel.org/all/20241001185848.GA562711@thelio-
> 3990X/
> >
> > Cheers,
> > Nathan
> >
> > On Thu, Nov 21, 2024 at 11:22:18AM -0700, Nathan Chancellor wrote:
> > > The Hexagon-specific constant extender optimization in LLVM may crash
> on
> > > Linux kernel code [1], such as fs/bcache/btree_io.c after
> > > commit 32ed4a620c54 ("bcachefs: Btree path tracepoints") in 6.12:
> > >
> > > clang: llvm/lib/Target/Hexagon/HexagonConstExtenders.cpp:745: bool
> > (anonymous
> namespace)::HexagonConstExtenders::ExtRoot::operator<(const
> > HCE::ExtRoot &) const: Assertion `ThisB->getParent() == OtherB-
> >getParent()'
> > failed.
> > > Stack dump:
> > > 0. Program arguments: clang --target=hexagon-linux-musl ...
> > fs/bcachefs/btree_io.c
> > > 1. <eof> parser at end of file
> > > 2. Code generation
> > > 3. Running pass 'Function Pass Manager' on module
> > 'fs/bcachefs/btree_io.c'.
> > > 4. Running pass 'Hexagon constant-extender optimization' on function
> > '@__btree_node_lock_nopath'
> > >
> > > Without assertions enabled, there is just a hang during compilation.
> > >
> > > This has been resolved in LLVM main (20.0.0) [2] and backported to LLVM
> > > 19.1.0 but the kernel supports LLVM 13.0.1 and newer, so disable the
> > > constant expander optimization using the '-mllvm' option when using a
> > > toolchain that is not fixed.
> > >
> > > Cc: stable@vger.kernel.org
> > > Link: https://github.com/llvm/llvm-project/issues/99714 [1]
> > > Link: https://github.com/llvm/llvm-
> > project/commit/68df06a0b2998765cb0a41353fcf0919bbf57ddb [2]
> > > Link: https://github.com/llvm/llvm-
> > project/commit/2ab8d93061581edad3501561722ebd5632d73892 [3]
> > > Reviewed-by: Brian Cain <bcain@quicinc.com>
> > > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > > ---
> > > Andrew, can you please take this for 6.13? Our CI continues to hit this.
> > >
> > > Changes in v2:
> > > - Rebase on 6.12 to make sure it is still applicable
> > > - Name exact bcachefs commit that introduces crash now that it is
> > > merged
> > > - Add 'Cc: stable' as this is now visible in a stable release
> > > - Carry forward Brian's reviewed-by
> > > - Link to v1: https://lore.kernel.org/r/20240819-hexagon-disable-
> constant-
> > expander-pass-v1-1-36a734e9527d@kernel.org
> > > ---
> > > arch/hexagon/Makefile | 6 ++++++
> > > 1 file changed, 6 insertions(+)
> > >
> > > diff --git a/arch/hexagon/Makefile b/arch/hexagon/Makefile
> > > index
> >
> 92d005958dfb232d48a4ca843b46262a84a08eb4..ff172cbe5881a074f9d94
> > 30c37071992a4c8beac 100644
> > > --- a/arch/hexagon/Makefile
> > > +++ b/arch/hexagon/Makefile
> > > @@ -32,3 +32,9 @@ KBUILD_LDFLAGS += $(ldflags-y)
> > > TIR_NAME := r19
> > > KBUILD_CFLAGS += -ffixed-$(TIR_NAME) -
> > DTHREADINFO_REG=$(TIR_NAME) -D__linux__
> > > KBUILD_AFLAGS += -DTHREADINFO_REG=$(TIR_NAME)
> > > +
> > > +# Disable HexagonConstExtenders pass for LLVM versions prior to 19.1.0
> > > +# https://github.com/llvm/llvm-project/issues/99714
> > > +ifneq ($(call clang-min-version, 190100),y)
> > > +KBUILD_CFLAGS += -mllvm -hexagon-cext=false
> > > +endif
> > >
> > > ---
> > > base-commit: adc218676eef25575469234709c2d87185ca223a
> > > change-id: 20240802-hexagon-disable-constant-expander-pass-
> > 8b6b61db6afc
> > >
> > > Best regards,
> > > --
> > > Nathan Chancellor <nathan@kernel.org>
> > >