[llvm] r255386 - Avoid buffered reads of /dev/urandom

[Rafael Espindola via llvm-commits]

Author: rafael
Date: Fri Dec 11 16:52:32 2015
New Revision: 255386

URL: http://llvm.org/viewvc/llvm-project?rev=255386&view=rev
Log:
Avoid buffered reads of /dev/urandom

I am seeing disappointing clang performance on a large PowerPC64
Linux box. GetRandomNumberSeed() does a buffered read from
/dev/urandom to seed its PRNG. As a result we read an entire page
even though we only need 4 bytes.

With every clang task reading a page worth of /dev/urandom we
end up spending a large amount of time stuck on kernel spinlock.

Patch by Anton Blanchard!

Modified:
    llvm/trunk/lib/Support/Unix/Process.inc

Modified: llvm/trunk/lib/Support/Unix/Process.inc
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Unix/Process.inc?rev=255386&r1=255385&r2=255386&view=diff
==============================================================================
--- llvm/trunk/lib/Support/Unix/Process.inc (original)
+++ llvm/trunk/lib/Support/Unix/Process.inc Fri Dec 11 16:52:32 2015
@@ -430,13 +430,18 @@ const char *Process::ResetColor() {
 #if !defined(HAVE_DECL_ARC4RANDOM) || !HAVE_DECL_ARC4RANDOM
 static unsigned GetRandomNumberSeed() {
   // Attempt to get the initial seed from /dev/urandom, if possible.
-  if (FILE *RandomSource = ::fopen("/dev/urandom", "r")) {
+  int urandomFD = open("/dev/urandom", O_RDONLY);
+
+  if (urandomFD != -1) {
     unsigned seed;
-    int count = ::fread((void *)&seed, sizeof(seed), 1, RandomSource);
-    ::fclose(RandomSource);
+    // Don't use a buffered read to avoid reading more data
+    // from /dev/urandom than we need.
+    int count = read(urandomFD, (void *)&seed, sizeof(seed));
+
+    close(urandomFD);
 
     // Return the seed if the read was successful.
-    if (count == 1)
+    if (count == sizeof(seed))
       return seed;
   }