From: Steven Rostedt <rostedt@goodmis.org>
To: Nick Desaulniers <ndesaulniers@google.com>, akpm@linux-foundation.org
Cc: clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org,
stable@vger.kernel.org,
Nathan Chancellor <natechancellor@gmail.com>,
Adhemerval Zanella <adhemerval.zanella@linaro.org>,
Arnd Bergmann <arnd@arndb.de>,
James Y Knight <jyknight@google.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Dan Williams <dan.j.williams@intel.com>,
Namhyung Kim <namhyung@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] lib/string.c: implement a basic bcmp
Date: Wed, 13 Mar 2019 17:03:57 -0400 [thread overview]
Message-ID: <7549EE7E-4172-467D-815A-63664A33D410@goodmis.org> (raw)
In-Reply-To: <20190313203741.142380-1-ndesaulniers@google.com>
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.
--
Sent from my Android device with K-9 Mail. Please excuse my brevity and top posting.
next prev parent reply other threads:[~2019-03-13 21:13 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-12 21:52 [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS Nathan Chancellor
2019-03-12 22:55 ` Nick Desaulniers
2019-03-13 8:13 ` Rasmus Villemoes
2019-03-13 10:58 ` Masahiro Yamada
2019-03-13 13:44 ` Nathan Chancellor
2019-03-13 13:48 ` Arnd Bergmann
2019-03-13 15:32 ` Nathan Chancellor
2019-03-13 17:21 ` Nick Desaulniers
2019-03-13 17:27 ` Nick Desaulniers
2019-03-13 18:02 ` [PATCH] lib/string.c: implement a basic bcmp Nick Desaulniers
2019-03-13 18:11 ` Nick Desaulniers
2019-03-13 18:17 ` [PATCH v2] " Nick Desaulniers
2019-03-13 18:40 ` Steven Rostedt
2019-03-13 18:51 ` Nick Desaulniers
2019-03-13 19:01 ` Steven Rostedt
2019-03-13 19:34 ` Rasmus Villemoes
2019-03-13 20:12 ` Steven Rostedt
2019-03-13 20:37 ` [PATCH v3] " Nick Desaulniers
2019-03-13 21:03 ` Steven Rostedt [this message]
2019-03-13 21:13 ` [PATCH v4] " Nick Desaulniers
2019-03-14 3:15 ` Nathan Chancellor
2019-03-14 5:00 ` Masahiro Yamada
2019-03-14 8:33 ` Andy Shevchenko
2019-03-14 9:57 ` David Laight
2019-03-14 11:07 ` Rasmus Villemoes
2019-03-21 2:11 ` Andrew Morton
2019-03-21 17:02 ` Nick Desaulniers
2019-03-21 17:20 ` Nick Desaulniers
2019-03-21 21:05 ` Andrew Morton
2019-03-13 19:38 ` [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS Arnd Bergmann
[not found] ` <20190325003834.2F24E2133F@mail.kernel.org>
2019-03-25 14:02 ` Nathan Chancellor
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