kbuild: Remove support for Clang's ThinLTO caching

There is an issue in clang's ThinLTO caching (enabled for the kernel via
'--thinlto-cache-dir') with .incbin, which the kernel occasionally uses
to include data within the kernel, such as the .config file for
/proc/config.gz. For example, when changing the .config and rebuilding
vmlinux, the copy of .config in vmlinux does not match the copy of
.config in the build folder:

  $ echo 'CONFIG_LTO_NONE=n
  CONFIG_LTO_CLANG_THIN=y
  CONFIG_IKCONFIG=y
  CONFIG_HEADERS_INSTALL=y' >kernel/configs/repro.config

  $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 clean defconfig repro.config vmlinux
  ...

  $ grep CONFIG_HEADERS_INSTALL .config
  CONFIG_HEADERS_INSTALL=y

  $ scripts/extract-ikconfig vmlinux | grep CONFIG_HEADERS_INSTALL
  CONFIG_HEADERS_INSTALL=y

  $ scripts/config -d HEADERS_INSTALL

  $ make -kj"$(nproc)" ARCH=x86_64 LLVM=1 vmlinux
  ...
    UPD     kernel/config_data
    GZIP    kernel/config_data.gz
    CC      kernel/configs.o
  ...
    LD      vmlinux
  ...

  $ grep CONFIG_HEADERS_INSTALL .config
  # CONFIG_HEADERS_INSTALL is not set

  $ scripts/extract-ikconfig vmlinux | grep CONFIG_HEADERS_INSTALL
  CONFIG_HEADERS_INSTALL=y

Without '--thinlto-cache-dir' or when using full LTO, this issue does
not occur.

Benchmarking incremental builds on a few different machines with and
without the cache shows a 20% increase in incremental build time without
the cache when measured by touching init/main.c and running 'make all'.

ARCH=arm64 defconfig + CONFIG_LTO_CLANG_THIN=y on an arm64 host:

  Benchmark 1: With ThinLTO cache
    Time (mean ± σ):     56.347 s ±  0.163 s    [User: 83.768 s, System: 24.661 s]
    Range (min … max):   56.109 s … 56.594 s    10 runs

  Benchmark 2: Without ThinLTO cache
    Time (mean ± σ):     67.740 s ±  0.479 s    [User: 718.458 s, System: 31.797 s]
    Range (min … max):   67.059 s … 68.556 s    10 runs

  Summary
    With ThinLTO cache ran
      1.20 ± 0.01 times faster than Without ThinLTO cache

ARCH=x86_64 defconfig + CONFIG_LTO_CLANG_THIN=y on an x86_64 host:

  Benchmark 1: With ThinLTO cache
    Time (mean ± σ):     85.772 s ±  0.252 s    [User: 91.505 s, System: 8.408 s]
    Range (min … max):   85.447 s … 86.244 s    10 runs

  Benchmark 2: Without ThinLTO cache
    Time (mean ± σ):     103.833 s ±  0.288 s    [User: 232.058 s, System: 8.569 s]
    Range (min … max):   103.286 s … 104.124 s    10 runs

  Summary
    With ThinLTO cache ran
      1.21 ± 0.00 times faster than Without ThinLTO cache

While it is unfortunate to take this performance improvement off the
table, correctness is more important. If/when this is fixed in LLVM, it
can potentially be brought back in a conditional manner. Alternatively,
a developer can just disable LTO if doing incremental compiles quickly
is important, as a full compile cycle can still take over a minute even
with the cache and it is unlikely that LTO will result in functional
differences for a kernel change.

Cc: [email protected]
Fixes: dc5723b ("kbuild: add support for Clang LTO")
Reported-by: Yifan Hong <[email protected]>
Closes: ClangBuiltLinux#2021
Reported-by: Masami Hiramatsu <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]/
Signed-off-by: Nathan Chancellor <[email protected]>

Makefile

@@ -942,7 +942,6 @@ endif
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
-KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod_prefix).thinlto-cache
 else
 CC_FLAGS_LTO	:= -flto
 endif
@@ -1477,7 +1476,7 @@ endif # CONFIG_MODULES
 # Directories & files removed with 'make clean'
 CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
-	       compile_commands.json .thinlto-cache rust/test \
+	       compile_commands.json rust/test \
 	       rust-project.json .vmlinux.objs .vmlinux.export.c
 
 # Directories & files removed with 'make mrproper'
@@ -1783,7 +1782,7 @@ PHONY += compile_commands.json
 
 clean-dirs := $(KBUILD_EXTMOD)
 clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
-	$(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
+	$(KBUILD_EXTMOD)/compile_commands.json
 
 PHONY += prepare
 # now expand this into a simple variable to reduce the cost of shell evaluations