Re: [PATCH v3] lib/string.c: implement a basic bcmp

Steven Rostedt
2019-03-13T21:13:32Z

On March 13, 2019 4:37:37 PM EDT, Nick Desaulniers <ndesaulniers@google.com> wrote:
>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
>Cc: stable@vger.kernel.org
>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>
>Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
>---
>Changes V2 -> V3:
>* Adjust comment as per Steven to Rasmus' sugguestion.
>* Pick up Steven's Ack.
>Changes V1 -> V2:
>* Add declaration to include/linux/string.h.
>* Reword comment above bcmp.
>
> include/linux/string.h | 3 +++
> lib/string.c | 15 +++++++++++++++
> 2 files changed, 18 insertions(+)
>
>diff --git a/include/linux/string.h b/include/linux/string.h
>index 7927b875f80c..6ab0a6fa512e 100644
>--- a/include/linux/string.h
>+++ b/include/linux/string.h
>@@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kernel_size_t);
> #ifndef __HAVE_ARCH_MEMCMP
> extern int memcmp(const void *,const void *,__kernel_size_t);
> #endif
>+#ifndef __HAVE_ARCH_BCMP
>+extern int bcmp(const void *,const void *,__kernel_size_t);
>+#endif
> #ifndef __HAVE_ARCH_MEMCHR
> extern void * memchr(const void *,int,__kernel_size_t);
> #endif
>diff --git a/lib/string.c b/lib/string.c
>index 38e4ca08e757..48a43a42d3f2 100644
>--- a/lib/string.c
>+++ b/lib/string.c
>@@ -866,6 +866,21 @@ __visible int memcmp(const void *cs, const void
>*ct, size_t count)
> EXPORT_SYMBOL(memcmp);
> #endif
>
>+#ifndef __HAVE_ARCH_BCMP
>+/**
>+ * bcmp - returns 0 if and only if the buffers have identical
>contents.
>+ * @cs: One area of memory.
>+ * @ct: Another area of memory.
>+ * @count: The size of the areas.

Where's the rest of the comment that I gave my ack to?

-- Steve

>+ */
>+#undef bcmp
>+int bcmp(const void *cs, const void *ct, size_t count)
>+{
>+ return memcmp(cs, ct, count);
>+}
>+EXPORT_SYMBOL(bcmp);
>+#endif
>+
> #ifndef __HAVE_ARCH_MEMSCAN
> /**
> * memscan - Find a character in an area of memory.

--
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