Jakub Jelinek wrote:
On Sat, Aug 02, 2014 at 12:09:24AM +0300, Janne Blomqvist wrote:
--- libgfortran/runtime/memory.c.jj 2014-06-18 08:50:33.000000000 +0200
+++ libgfortran/runtime/memory.c 2014-08-01 14:41:08.385856116 +0200
@@ -56,7 +56,9 @@ xmallocarray (size_t nmemb, size_t size)
if (!nmemb || !size)
size = nmemb = 1;
- else if (nmemb > SIZE_MAX / size)
+#define HALF_SIZE_T (((size_t) 1) << (__CHAR_BIT__ * sizeof (size_t) / 2))
+ else if (__builtin_expect ((nmemb | size) >= HALF_SIZE_T, 0)
+ && nmemb > SIZE_MAX / size)
{
errno = ENOMEM;
os_error ("Integer overflow in xmallocarray");
Nice, though as os_error() has the _Noreturn specifier the
__builtin_expect() is not necessary, right? In libgfortran.h we have
The reason for __builtin_expect here was to make already the
nmemb > SIZE_MAX / size
computation as unlikely, the noreturn predictor will of course DTRT with the
{} block.
But there is a difference in probability between __builtin_expect and
"noreturn". __builtin_expect had until two years ago a probability of
99%, now it has a probability of only 90% (which is tunable with a
-param)* – while "noreturn" has a higher probability. Thus, at least if
you had used
else if (unlikely(... & ...))
os_error
you would have made the basic block with os_error more likely than
without the "unlikely" (alias __builtin_expect). However, I don't know
what happens with using "unlikely(cond1) && cond2".
Tobias
* Or internally in the compiler only, by passing a third argument.
