[01/14] lib/string.c: implement a basic bcmp

Message ID 20190406013845.SWPXm4z6E%akpm@linux-foundation.org
State New
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  • [01/14] lib/string.c: implement a basic bcmp
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Commit Message

Andrew Morton April 6, 2019, 1:38 a.m.
From: Nick Desaulniers <ndesaulniers@google.com>

Subject: lib/string.c: implement a basic bcmp

A recent optimization in Clang (r355672) lowers comparisons of the return
value of memcmp against zero to comparisons of the return value of bcmp
against zero.  This helps some platforms that implement bcmp more
efficiently than memcmp.  glibc simply aliases bcmp to memcmp, but an
optimized implementation is in the works.

This results in linkage failures for all targets with Clang due to the
undefined symbol.  For now, just implement bcmp as a tailcail to memcmp to
unbreak the build.  This routine can be further optimized in the future.

Other ideas discussed:
* A weak alias was discussed, but breaks for architectures that define
their own implementations of memcmp since aliases to declarations are
not permitted (only definitions).  Arch-specific memcmp implementations
typically declare memcmp in C headers, but implement them in assembly.
* -ffreestanding also is used sporadically throughout the kernel.
* -fno-builtin-bcmp doesn't work when doing LTO.

Link: https://bugs.llvm.org/show_bug.cgi?id=41035
Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp
Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13
Link: https://github.com/ClangBuiltLinux/linux/issues/416
Link: http://lkml.kernel.org/r/20190313211335.165605-1-ndesaulniers@google.com
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: James Y Knight <jyknight@google.com>
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>


 include/linux/string.h |    3 +++
 lib/string.c           |   20 ++++++++++++++++++++
 2 files changed, 23 insertions(+)


--- a/include/linux/string.h~lib-stringc-implement-a-basic-bcmp
+++ a/include/linux/string.h
@@ -150,6 +150,9 @@  extern void * memscan(void *,int,__kerne
 extern int memcmp(const void *,const void *,__kernel_size_t);
+#ifndef __HAVE_ARCH_BCMP
+extern int bcmp(const void *,const void *,__kernel_size_t);
 extern void * memchr(const void *,int,__kernel_size_t);
--- a/lib/string.c~lib-stringc-implement-a-basic-bcmp
+++ a/lib/string.c
@@ -866,6 +866,26 @@  __visible int memcmp(const void *cs, con
+#ifndef __HAVE_ARCH_BCMP
+ * bcmp - returns 0 if and only if the buffers have identical contents.
+ * @a: pointer to first buffer.
+ * @b: pointer to second buffer.
+ * @len: size of buffers.
+ *
+ * The sign or magnitude of a non-zero return value has no particular
+ * meaning, and architectures may implement their own more efficient bcmp(). So
+ * while this particular implementation is a simple (tail) call to memcmp, do
+ * not rely on anything but whether the return value is zero or non-zero.
+ */
+#undef bcmp
+int bcmp(const void *a, const void *b, size_t len)
+	return memcmp(a, b, len);
  * memscan - Find a character in an area of memory.