From 5745fbce1674b29f4dce6b6e31556c4c1e83dc89 Mon Sep 17 00:00:00 2001
From: Benjamin Kramer <benny.kra@googlemail.com>
Date: Tue, 22 Nov 2011 12:31:53 +0000
Subject: [PATCH] Add configure checking for pread(2) and use it to save a
 syscall when reading files.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145061 91177308-0d34-0410-b5e6-96231b3b80d8
---
 autoconf/configure.ac              | 2 +-
 cmake/config-ix.cmake              | 2 ++
 configure                          | 3 ++-
 include/llvm/Config/config.h.cmake | 3 +++
 include/llvm/Config/config.h.in    | 3 +++
 lib/Support/MemoryBuffer.cpp       | 7 +++++++
 6 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 090f5dc12d6..d7775533e56 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -1293,7 +1293,7 @@ AC_CHECK_FUNCS([backtrace ceilf floorf roundf rintf nearbyintf getcwd ])
 AC_CHECK_FUNCS([powf fmodf strtof round ])
 AC_CHECK_FUNCS([getpagesize getrusage getrlimit setrlimit gettimeofday ])
 AC_CHECK_FUNCS([isatty mkdtemp mkstemp ])
-AC_CHECK_FUNCS([mktemp posix_spawn realpath sbrk setrlimit strdup ])
+AC_CHECK_FUNCS([mktemp posix_spawn pread realpath sbrk setrlimit strdup ])
 AC_CHECK_FUNCS([strerror strerror_r setenv ])
 AC_CHECK_FUNCS([strtoll strtoq sysconf malloc_zone_statistics ])
 AC_CHECK_FUNCS([setjmp longjmp sigsetjmp siglongjmp writev])
diff --git a/cmake/config-ix.cmake b/cmake/config-ix.cmake
index 62699fa5aaf..0943ae71b83 100755
--- a/cmake/config-ix.cmake
+++ b/cmake/config-ix.cmake
@@ -126,6 +126,8 @@ check_symbol_exists(readdir "sys/types.h;dirent.h" HAVE_READDIR)
 check_symbol_exists(getcwd unistd.h HAVE_GETCWD)
 check_symbol_exists(gettimeofday sys/time.h HAVE_GETTIMEOFDAY)
 check_symbol_exists(getrlimit "sys/types.h;sys/time.h;sys/resource.h" HAVE_GETRLIMIT)
+check_symbol_exists(posix_spawn spawn.h HAVE_POSIX_SPAWN)
+check_symbol_exists(pread unistd.h HAVE_PREAD)
 check_symbol_exists(rindex strings.h HAVE_RINDEX)
 check_symbol_exists(strchr string.h HAVE_STRCHR)
 check_symbol_exists(strcmp string.h HAVE_STRCMP)
diff --git a/configure b/configure
index 09173d257f3..bfd61dd5e3e 100755
--- a/configure
+++ b/configure
@@ -17151,7 +17151,8 @@ done
 
 
 
-for ac_func in mktemp posix_spawn realpath sbrk setrlimit strdup
+
+for ac_func in mktemp posix_spawn pread realpath sbrk setrlimit strdup
 do
 as_ac_var=`echo "ac_cv_func_$ac_func" | $as_tr_sh`
 { echo "$as_me:$LINENO: checking for $ac_func" >&5
diff --git a/include/llvm/Config/config.h.cmake b/include/llvm/Config/config.h.cmake
index 69e3eefb116..11e5b5a8d03 100644
--- a/include/llvm/Config/config.h.cmake
+++ b/include/llvm/Config/config.h.cmake
@@ -297,6 +297,9 @@
 /* Define to 1 if you have the `powf' function. */
 #cmakedefine HAVE_POWF ${HAVE_POWF}
 
+/* Define to 1 if you have the `pread' function. */
+#cmakedefine HAVE_PREAD ${HAVE_PREAD}
+
 /* Define if libtool can extract symbol lists from object files. */
 #undef HAVE_PRELOADED_SYMBOLS
 
diff --git a/include/llvm/Config/config.h.in b/include/llvm/Config/config.h.in
index 813c6ebc383..b3a95908047 100644
--- a/include/llvm/Config/config.h.in
+++ b/include/llvm/Config/config.h.in
@@ -295,6 +295,9 @@
 /* Define to 1 if you have the `powf' function. */
 #undef HAVE_POWF
 
+/* Define to 1 if you have the `pread' function. */
+#undef HAVE_PREAD
+
 /* Define if libtool can extract symbol lists from object files. */
 #undef HAVE_PRELOADED_SYMBOLS
 
diff --git a/lib/Support/MemoryBuffer.cpp b/lib/Support/MemoryBuffer.cpp
index 580338117b8..1a409720c02 100644
--- a/lib/Support/MemoryBuffer.cpp
+++ b/lib/Support/MemoryBuffer.cpp
@@ -14,6 +14,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/ADT/OwningPtr.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Config/config.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Errno.h"
 #include "llvm/Support/Path.h"
@@ -320,11 +321,17 @@ error_code MemoryBuffer::getOpenFile(int FD, const char *Filename,
   char *BufPtr = const_cast<char*>(SB->getBufferStart());
 
   size_t BytesLeft = MapSize;
+#ifndef HAVE_PREAD
   if (lseek(FD, Offset, SEEK_SET) == -1)
     return error_code(errno, posix_category());
+#endif
 
   while (BytesLeft) {
+#ifdef HAVE_PREAD
+    ssize_t NumRead = ::pread(FD, BufPtr, BytesLeft, MapSize-BytesLeft+Offset);
+#else
     ssize_t NumRead = ::read(FD, BufPtr, BytesLeft);
+#endif
     if (NumRead == -1) {
       if (errno == EINTR)
         continue;
-- 
2.34.1