[PATCH] kbuild: rust: pass AutoFDO flags to Clang with inline helpers

[Jenhao Chen]

When CONFIG_RUST_INLINE_HELPERS=y, cmd_rustc_o_rs and cmd_rustc_library
compile Rust sources to LLVM bitcode (.bc), link rust/helpers/helpers.bc
via llvm-link, and then invoke $(CC) to compile the combined bitcode
into the final ELF object (.o).

When commit a960c2cdb19e ("kbuild: rust: add AutoFDO support") was
tested with CONFIG_RUST_INLINE_HELPERS=y and CONFIG_LTO_CLANG_THIN=y,
cmd_rustc_o_rs skipped the post-llvm-link $(CC) step under
CONFIG_LTO_CLANG and handed the linked bitcode directly to ld.lld, where
the ThinLTO backend applied --lto-sample-profile,
-plugin-opt=-split-machine-functions, and LLD's default function
sections. Non-LTO builds (!CONFIG_LTO_CLANG), however, ran the
post-llvm-link $(CC) step without $(CFLAGS_AUTOFDO_CLANG) ever since
commit 3a2486cc1da5 ("kbuild: rust: provide an option to inline C
helpers into Rust").

Subsequently, commit 5febf432df1c ("kbuild: rust: keep Rust objects out
of Clang LTO with inline helpers") updated cmd_rustc_o_rs and
rust/Makefile to always invoke $(CC) on the linked bitcode even when
CONFIG_LTO_CLANG=y. Because that $(CC) invocation only passes
$(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS)) and does not include
$(CFLAGS_AUTOFDO_CLANG) (which is normally appended to _c_flags in
scripts/Makefile.lib), the final code generation step for Rust objects
is compiled without -ffunction-sections, -fsplit-machine-functions,
-fprofile-sample-use, or flow-sensitive discriminator flags whenever
CONFIG_RUST_INLINE_HELPERS=y.

As a result, Rust crates built with both CONFIG_RUST_INLINE_HELPERS=y
and CONFIG_AUTOFDO_CLANG=y emit all functions into a single monolithic
.text section (0 .text.<func> sections instead of 417 in
rust_binder_main.o and 424 in rust/kernel.o on arm64 with
CLANG_AUTOFDO_PROFILE set), preventing the linker from performing
AutoFDO hot/cold function clustering and discarding SamplePGO weights on
the inlined C helpers during final code generation.

Factor out _autofdo_c_flags in scripts/Makefile.lib (preserving per-file
and per-directory AUTOFDO_PROFILE opt-outs) and pass it to the
post-llvm-link $(CC) invocation in scripts/Makefile.build and
rust/Makefile.

Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Fixes: 5febf432df1c ("kbuild: rust: keep Rust objects out of Clang LTO with inline helpers")
Signed-off-by: Jenhao Chen <jenhaochen@xxxxxxxxxx>
---
 rust/Makefile          | 2 +-
 scripts/Makefile.build | 2 +-
 scripts/Makefile.lib   | 3 ++-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile
index 0ac9ab28cc2b..f2d9b4411720 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -658,7 +658,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 	$(if $(link_helper),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) \
 		$(obj)/helpers/helpers$(if $(part-of-module),_module).bc -o $(patsubst %.o,%.m.bc,$@); \
 		$(CC) $(CLANG_FLAGS) $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS)) \
-		$(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
+		$(_autofdo_c_flags) $(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
 		$(cmd_ld_single)) \
 	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
 	$(cmd_objtool)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 6127feb970c6..bd7bd05e9fe9 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -369,7 +369,7 @@ quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
 	$(if $(CONFIG_RUST_INLINE_HELPERS),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) \
 		$(objtree)/rust/helpers/helpers$(if $(part-of-module),_module).bc -o $(patsubst %.o,%.m.bc,$@); \
 		$(CC) $(CLANG_FLAGS) $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS)) \
-		$(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
+		$(_autofdo_c_flags) $(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
 		$(cmd_ld_single)) \
 	$(cmd_objtool)
 
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 19e96b30a122..e72efa5c6c05 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -120,9 +120,10 @@ endif
 # enable (depends on variables AUTOFDO_PROFILE_obj.o and AUTOFDO_PROFILE).
 #
 ifeq ($(CONFIG_AUTOFDO_CLANG),y)
-_c_flags += $(if $(patsubst n%,, \
+_autofdo_c_flags = $(if $(patsubst n%,, \
 	$(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \
 	$(CFLAGS_AUTOFDO_CLANG))
+_c_flags += $(_autofdo_c_flags)
 _rust_flags += $(if $(patsubst n%,, \
 	$(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \
 	$(RUSTFLAGS_AUTOFDO_CLANG))
-- 
2.56.0.317.g454f8302e1-goog