I can't tell if it would be useful to do this:
x = get_x();
__builtin_expect(x&MASK,KNOWNBITS);
__builtin_expect(x<=KNOWNMAX,1);
__builtin_expect(x>=KNOWNMIN,1);
// code which uses x would follow this
Given two booleans that are 2/3 likely, the chance of both is 4/9 likely. So
maybe it is appropriate to do this:
if(__builtin_expect((__builtin_expect(x,1) && __builtin_expect(y,1)), 0))
Yes? No?
Suppose that I am quite certain of how likely something will be, and that I
care deeply about optimization. At what point should I mark my code with
__builtin_expect()? The obvious choice is whenever the liklihood is not exactly
50%, but I hear that this varies by architecture. ARM supposedly uses a very
strong branch prediction bit that forces a major stall when the expectation is
wrong, while x86 is not so punishing. No hint of this is in the documentation.
--
Summary: __builtin_expect needs better documentation
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: acahalan at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26366
