linux/compiler-clang.h: define HAVE_BUILTIN_BSWAP*

Separating compiler-clang.h from compiler-gcc.h inadventently dropped the
definitions of the three HAVE_BUILTIN_BSWAP macros, which requires falling
back to the open-coded version and hoping that the compiler detects it.

Since all versions of clang support the __builtin_bswap interfaces, add
back the flags and have the headers pick these up automatically.

This results in a 4% improvement of compilation speed for arm defconfig.

Note: it might also be worth revisiting which architectures set
CONFIG_ARCH_USE_BUILTIN_BSWAP for one compiler or the other, today this is
set on six architectures (arm32, csky, mips, powerpc, s390, x86), while
another ten architectures define custom helpers (alpha, arc, ia64, m68k,
mips, nios2, parisc, sh, sparc, xtensa), and the rest (arm64, h8300,
hexagon, microblaze, nds32, openrisc, riscv) just get the unoptimized
version and rely on the compiler to detect it.

A long time ago, the compiler builtins were architecture specific, but
nowadays, all compilers that are able to build the kernel have correct
implementations of them, though some may not be as optimized as the inline
asm versions.

The patch that dropped the optimization landed in v4.19, so as discussed
it would be fairly safe to backport this revert to stable kernels to the
4.19/5.4/5.10 stable kernels, but there is a remaining risk for
regressions, and it has no known side-effects besides compile speed.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h mutually exclusive")
Signed-off-by: Arnd Bergmann <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Miguel Ojeda <[email protected]>
Acked-by: Nick Desaulniers <[email protected]>
Acked-by: Luc Van Oostenryck <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Nick Hu <[email protected]>
Cc: Greentime Hu <[email protected]>
Cc: Vincent Chen <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Guo Ren <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Sami Tolvanen <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Arvind Sankar <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 04c0a5a717f7..d217c382b02d 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -31,6 +31,12 @@
 #define __no_sanitize_thread
 #endif
 
+#if defined(CONFIG_ARCH_USE_BUILTIN_BSWAP)
+#define __HAVE_BUILTIN_BSWAP32__
+#define __HAVE_BUILTIN_BSWAP64__
+#define __HAVE_BUILTIN_BSWAP16__
+#endif /* CONFIG_ARCH_USE_BUILTIN_BSWAP */
+
 #if __has_feature(undefined_behavior_sanitizer)
 /* GCC does not have __SANITIZE_UNDEFINED__ */
 #define __no_sanitize_undefined \