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

[Nathan Chancellor <nathan@kernel.org>]

The Hexagon-specific constant extender optimization in LLVM may crash on
Linux kernel code [1] (such as with a bcachefs change in -next):

  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.

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]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/hexagon/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/hexagon/Makefile b/arch/hexagon/Makefile
index 92d005958dfb..ff172cbe5881 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: 47ac09b91befbb6a235ab620c32af719f8208399
change-id: 20240802-hexagon-disable-constant-expander-pass-8b6b61db6afc

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>