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.

[1]: https://lore.kernel.org/all/20241121-hexagon-disable-constant-expander-pass-... [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/68df06a0b2998765cb0a41353fcf0919... [2] Link: https://github.com/llvm/llvm-project/commit/2ab8d93061581edad3501561722ebd56... [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...

---

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