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

[Nathan Chancellor <nathan@kernel.org>]

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.

[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..ff172cbe5881a074f9d9430c37071992a4c8beac 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>
>