| Message ID | 20190313203741.142380-1-ndesaulniers@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3] lib/string.c: implement a basic bcmp | expand |
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.
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. + */ +#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.