[PATCH v2 04/14] kbuild: Update assembler calls to use proper flags and language target

From: Nathan Chancellor <nathan@kernel.org>
To: masahiroy@kernel.org
Cc: ndesaulniers@google.com, nicolas@fjasle.eu, trix@redhat.com,
	 linux-kbuild@vger.kernel.org, llvm@lists.linux.dev,
	 Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH v2 04/14] kbuild: Update assembler calls to use proper flags and language target
Date: Wed, 11 Jan 2023 20:05:01 -0700	[thread overview]
Message-ID: <20221228-drop-qunused-arguments-v2-4-9adbddd20d86@kernel.org> (raw)
In-Reply-To: <20221228-drop-qunused-arguments-v2-0-9adbddd20d86@kernel.org>

From: Nick Desaulniers <ndesaulniers@google.com>

as-instr uses KBUILD_AFLAGS, but as-option uses KBUILD_CFLAGS. This can
cause as-option to fail unexpectedly when CONFIG_WERROR is set, because
clang will emit -Werror,-Wunused-command-line-argument for various -m
and -f flags in KBUILD_CFLAGS for assembler sources.

Callers of as-option and as-instr should be adding flags to
KBUILD_AFLAGS / aflags-y, not KBUILD_CFLAGS / cflags-y. Use
KBUILD_AFLAGS in all macros to clear up the initial problem.

Unfortunately, -Wunused-command-line-argument can still be triggered
with clang by the presence of warning flags or macro definitions because
'-x assembler' is used, instead of '-x assembler-with-cpp', which will
consume these flags. Switch to '-x assembler-with-cpp' in places where
'-x assembler' is used, as the compiler is always used as the driver for
out of line assembler sources in the kernel.

Finally, add -Werror to these macros so that they behave consistently
whether or not CONFIG_WERROR is set.

Link: https://github.com/ClangBuiltLinux/linux/issues/1699
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
[nathan: Reworded and expanded on problems in commit message
         Use '-x assembler-with-cpp' in a couple more places]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
Benchmarking '-x assembler' vs. '-x assembler-with-cpp' does show that
invoking the preprocessor incurs some overhead but my kernel builds do
not show any noticeable slowdowns due to how infrequently these macros
are used.

$ hyperfine -L lang assembler,assembler-with-cpp -N -w 5 "/usr/bin/gcc -x {lang} -c -o /dev/null /dev/null"
Benchmark 1: /usr/bin/gcc -x assembler -c -o /dev/null /dev/null
  Time (mean ± σ):       5.5 ms ±   0.5 ms    [User: 4.1 ms, System: 1.3 ms]
  Range (min … max):     4.3 ms …   6.3 ms    472 runs

Benchmark 2: /usr/bin/gcc -x assembler-with-cpp -c -o /dev/null /dev/null
  Time (mean ± σ):      12.7 ms ±   1.0 ms    [User: 9.1 ms, System: 3.6 ms]
  Range (min … max):     9.6 ms …  14.2 ms    225 runs

Summary
  '/usr/bin/gcc -x assembler -c -o /dev/null /dev/null' ran
    2.29 ± 0.28 times faster than '/usr/bin/gcc -x assembler-with-cpp -c -o /dev/null /dev/null'

$ hyperfine -L lang assembler,assembler-with-cpp -N -w 5 "/usr/bin/clang -x {lang} -c -o /dev/null /dev/null"
Benchmark 1: /usr/bin/clang -x assembler -c -o /dev/null /dev/null
  Time (mean ± σ):      21.0 ms ±   1.1 ms    [User: 9.6 ms, System: 11.1 ms]
  Range (min … max):    13.9 ms …  22.0 ms    138 runs

Benchmark 2: /usr/bin/clang -x assembler-with-cpp -c -o /dev/null /dev/null
  Time (mean ± σ):      56.9 ms ±   3.2 ms    [User: 27.2 ms, System: 29.5 ms]
  Range (min … max):    53.9 ms …  62.1 ms    48 runs

Summary
  '/usr/bin/clang -x assembler -c -o /dev/null /dev/null' ran
    2.71 ± 0.20 times faster than '/usr/bin/clang -x assembler-with-cpp -c -o /dev/null /dev/null'
---
 scripts/Kconfig.include   | 2 +-
 scripts/Makefile.compiler | 8 ++++----
 scripts/as-version.sh     | 2 +-
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index 274125307ebd..5a84b6443875 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -33,7 +33,7 @@ ld-option = $(success,$(LD) -v $(1))
 
 # $(as-instr,<instr>)
 # Return y if the assembler supports <instr>, n otherwise
-as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x assembler -o /dev/null -)
+as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x assembler-with-cpp -o /dev/null -)
 
 # check if $(CC) and $(LD) exist
 $(error-if,$(failure,command -v $(CC)),C compiler '$(CC)' not found)
diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index 3d8adfd34af1..7aa1fbc4aafe 100644
--- a/scripts/Makefile.compiler
+++ b/scripts/Makefile.compiler
@@ -29,16 +29,16 @@ try-run = $(shell set -e;		\
 	fi)
 
 # as-option
-# Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,)
+# Usage: aflags-y += $(call as-option,-Wa$(comma)-isa=foo,)
 
 as-option = $(call try-run,\
-	$(CC) $(KBUILD_CFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2))
+	$(CC) -Werror $(KBUILD_AFLAGS) $(1) -c -x assembler-with-cpp /dev/null -o "$$TMP",$(1),$(2))
 
 # as-instr
-# Usage: cflags-y += $(call as-instr,instr,option1,option2)
+# Usage: aflags-y += $(call as-instr,instr,option1,option2)
 
 as-instr = $(call try-run,\
-	printf "%b\n" "$(1)" | $(CC) $(KBUILD_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
+	printf "%b\n" "$(1)" | $(CC) -Werror $(KBUILD_AFLAGS) -c -x assembler-with-cpp -o "$$TMP" -,$(2),$(3))
 
 # __cc-option
 # Usage: MY_CFLAGS += $(call __cc-option,$(CC),$(MY_CFLAGS),-march=winchip-c6,-march=i586)
diff --git a/scripts/as-version.sh b/scripts/as-version.sh
index 1a21495e9ff0..af717476152d 100755
--- a/scripts/as-version.sh
+++ b/scripts/as-version.sh
@@ -45,7 +45,7 @@ orig_args="$@"
 # Get the first line of the --version output.
 IFS='
 '
-set -- $(LC_ALL=C "$@" -Wa,--version -c -x assembler /dev/null -o /dev/null 2>/dev/null)
+set -- $(LC_ALL=C "$@" -Wa,--version -c -x assembler-with-cpp /dev/null -o /dev/null 2>/dev/null)
 
 # Split the line on spaces.
 IFS=' '

-- 
2.39.0

