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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255386 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Support/Unix/Process.inc b/lib/Support/Unix/Process.inc
index df13bd2217390b46c859872aa88b7db2a9f1949f..27083eeb072d3dec8db8454e75e7539993c334b1 100644 (file)
--- a/lib/Support/Unix/Process.inc
+++ b/lib/Support/Unix/Process.inc
@@ -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;
   }