@@ -41,6 +41,12 @@ NEON_FLAGS += -isystem $(shell $(CC) -print-file-name=include)
ifeq ($(ARCH),arm)
NEON_FLAGS += -march=armv7-a -mfloat-abi=softfp -mfpu=neon
endif
+# https://github.com/ClangBuiltLinux/linux/issues/1603
+ifdef CONFIG_CPU_BIG_ENDIAN
+ifeq ($(shell test $(CONFIG_CLANG_VERSION) -lt 140001; echo $$?),0)
+NEON_FLAGS += -Wno-declaration-after-statement
+endif
+endif
CFLAGS_recov_neon_inner.o += $(NEON_FLAGS)
ifeq ($(ARCH),arm64)
CFLAGS_REMOVE_recov_neon_inner.o += -mgeneral-regs-only
After commit e8c07082a810 ("Kbuild: move to -std=gnu11"), there is a spew of warnings in lib/raid6/ when building arm64 defconfig plus CONFIG_CPU_BIG_ENDIAN=y with clang 14.0.0, which come from its arm_neon.h header: lib/raid6/recov_neon_inner.c:55:8: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement] vy = vshrq_n_u8(vx, 4); ^ .../lib/clang/14.0.0/include/arm_neon.h:25231:14: note: expanded from macro 'vshrq_n_u8' uint8x16_t __ret; \ ^ Looking at the header, the big endian version does mix declarations and code, due to the reversal of the first argument: #ifdef __LITTLE_ENDIAN__ #define vshrq_n_u8(__p0, __p1) __extension__ ({ \ uint8x16_t __s0 = __p0; \ uint8x16_t __ret; \ __ret = (uint8x16_t) __builtin_neon_vshrq_n_v((int8x16_t)__s0, __p1, 48); \ __ret; \ }) #else #define vshrq_n_u8(__p0, __p1) __extension__ ({ \ uint8x16_t __s0 = __p0; \ uint8x16_t __rev0; __rev0 = __builtin_shufflevector(__s0, __s0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0); \ uint8x16_t __ret; \ __ret = (uint8x16_t) __builtin_neon_vshrq_n_v((int8x16_t)__rev0, __p1, 48); \ __ret = __builtin_shufflevector(__ret, __ret, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0); \ __ret; \ }) #endif It was initially surprising that clang warned about a macro that originates from a system header, as this deviates from GCC. The machinery in clang to ignore warnings in macros from system headers is relatively new in clang (January 2022). When that change was landed, the code owner of clang was not confident that clang's diagnostic system provided enough information to avoid falsely disabling warnings that originated from user controlled code that was mixed with system macros, so the default behavior of warning within system macros remained unchanged. This deficiency is unlikely to further impact the kernel, as it does not use any system headers aside from SIMD intrinsic headers after commit 04e85bbf71c9 ("isystem: delete global -isystem compile option"). Another interesting aspect of this warning's appearance is how it only appeared when moving from gnu89 to gnu11, as the above problem has always been relevant for clang, regardless of the C standard version used to build the kernel. In clang 14.0.0, -Wdeclaration-after-statement was made available under newer C standard versions; prior to that change, it was only available for C standard versions older than C99. https://github.com/llvm/llvm-project/commit/118f966b46cfb60897b56a9878e1c68fd0e2afa4 When compiling for a C standard older than C99, -Wdeclaration-after-statement is an extension warning, which means it can be silenced with '__extension__', as the macro above does. However, starting with C99, using '__extension__' to silence the warning does not work, as mixing declarations and code is no longer an extension, which is why the warning appears for gnu11 and gnu89. Ultimately, this issue is resolved in clang 14.0.1 and 15.0.0 by making arm_neon.h fully compliant with -Wdeclaration-after-statement for all C standard versions. For older versions of clang, hide this warning for 32-bit and 64-bit ARM big endian in files that use arm_neon.h. Link: https://github.com/ClangBuiltLinux/linux/issues/1603 Link: https://github.com/llvm/llvm-project/issues/54062 Link: https://github.com/llvm/llvm-project/commit/5a2e56b70e2fa7ad0d82e54bc4c741b16f05e475 Signed-off-by: Nathan Chancellor <nathan@kernel.org> --- lib/raid6/Makefile | 6 ++++++ 1 file changed, 6 insertions(+) base-commit: 3123109284176b1532874591f7c81f3837bbdc17