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

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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@xxxxxxxxxxxxxxx
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@xxxxxxxxxxx>
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
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@xxxxxxxxxx
---
 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@xxxxxxxxxx>





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