Performance improvement.

Using fwrite and fread was very *very* slow. The resulting code was multiple
times slower than GCC's implementation of gcov. Replace the fwrite/fread system
with an mmap() version.

If the `.gcda' file doesn't exist, we (re)allocate a buffer that we write
into. That gets written to the `.gcda' file in one chunk. If the `.gcda' file
already exists, we simply mmap() the file, modify the mapped data, and use
msync() to write the contents out to disk. It's much easier than implementing
our own buffering scheme, and we don't have to use fwrite's and fread's
buffering.

For those who are numbers-oriented, here are some timings:

GCC Verison
-----------

`.gcda' files don't exist:  23s
`.gcda' files do exist:     14s

LLVM Version (before this change)
---------------------------------

`.gcda' files don't exist:  28s
`.gcda' files do exist:     28s

LLVM Version (with this change)
-------------------------------

`.gcda' files don't exist:  18s
`.gcda' files do exist:      4s

It's a win-win-win-win-lose-win-win scenario!

<rdar://problem/13466086>

llvm-svn: 182563

1 files changed, 21 insertions, 3 deletions

diff --git a/compiler-rt/SDKs/linux/usr/include/sys/mman.h b/compiler-rt/SDKs/linux/usr/include/sys/mman.h
index 7c4d05181f5..bfb7f8bb02d 100644
--- a/compiler-rt/SDKs/linux/usr/include/sys/mman.h
+++ b/compiler-rt/SDKs/linux/usr/include/sys/mman.h
@@ -19,10 +19,28 @@
 
 typedef __SIZE_TYPE__ size_t;
 
-#define PROT_READ 0x1
-#define PROT_WRITE 0x2
-#define PROT_EXEC 0x4
+#define PROT_NONE     0x00
+#define PROT_READ     0x01
+#define PROT_WRITE    0x02
+#define PROT_EXEC     0x04
 
+#define MAP_SHARED    0x0001
+#define MAP_PRIVATE   0x0002
+
+#define MAP_FILE      0x0000
+#define MAP_ANON      0x1000
+
+#define MS_ASYNC      0x0001
+#define MS_INVALIDATE 0x0002
+#define MS_SYNC       0x0010
+
+extern void *mmap(void *addr, size_t len, int prot, int flags, int fd,
+                  long long offset)
+  __attribute__((__nothrow__));
+extern int munmap(void *addr, size_t len)
+  __attribute__((__nothrow__));
+extern int msync(void *addr, size_t len, int flags)
+  __attribute__((__nothrow__));
 extern int mprotect (void *__addr, size_t __len, int __prot)
   __attribute__((__nothrow__));