[LLVMbugs] [Bug 12750] New: clang is really bad at compiling OS X's / FreeBSD's memchr()

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Sun May 6 22:25:22 PDT 2012


             Bug #: 12750
           Summary: clang is really bad at compiling OS X's / FreeBSD's
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: nicolasweber at gmx.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 8500
  --> http://llvm.org/bugs/attachment.cgi?id=8500

The attached program contains a memchr() benchmark that tests two memchr()

void* stupid_memchr(const void* s_in, int c, size_t n) {
  const unsigned char* s = (const unsigned char*)s_in;
  for (int i = 0; i < n; ++i)
    if (s[i] == c)
      return (void*)(s + i);
  return NULL;

// FreeBSD,
// http://opensource.apple.com/source/Libc/Libc-763.12/string/memchr-fbsd.c
void *
bsd_memchr(const void *s, int c, size_t n)
    if (n != 0) {
        const unsigned char *p = (const unsigned char*)s;

        do {
            if (*p++ == (unsigned char)c)
                return ((void *)(p - 1));
        } while (--n != 0);
    return (NULL);

If I compile this program with gcc 4.2, the output is (for these 2 functions
and the system memchr(), my system is OS X):

tests-MacBook-Pro-2:src test$ gcc chr_bench.cc -o chr_bench -O2
tests-MacBook-Pro-2:src test$ ./chr_bench 
stupid_memchr: 391.0us
bsd_memchr: 394.0us
memchr: 563.0us

If do the same thing with clang:

tests-MacBook-Pro-2:src test$ third_party/llvm-build/Release+Asserts/bin/clang
chr_bench.cc -o chr_bench -O2
tests-MacBook-Pro-2:src test$ ./chr_bench 
stupid_memchr: 375.0us
bsd_memchr: 554.0us
memchr: 552.0us
tests-MacBook-Pro-2:src test$ third_party/llvm-build/Release+Asserts/bin/clang
clang version 3.2 (trunk 155352)
Target: x86_64-apple-darwin11.3.0
Thread model: posix

Note that bsd_memchr() becomes much much slower. Notice that the run time now
matches my the runtime of my system's memchr(), which was probably built by

(See also rdar 11393714)

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