On Mon, Feb 19, 2007 at 12:52:10PM -0800, Shankar Unni wrote: > Andrew Makhorin wrote: > > >{ double t0 = get_time(), t1 = get_time(); > > [Maybe OT?] > > 1. I can't remember if C guarantees that comma-separated *declarations* > are initialized in order or not.. And to think I used to be an ANSI C > guru :-(.
Should be fine in this case. > 2. The reason that the "t0 > t1" fails, but t0 and t1 get dumped to be > the same, is that C allows the implementation to use larger-than-64-bit > (for 64-bit) intermediate double representations. In the case of X86, > the CPU's floating-point registers are 80 bits wide. > > When they get written to stack, the value is rounded (or truncated?) to > 64 bits. I don't understand why they just didn't write: double t0, t1; t0 = t1 = get_time(); Not everything *has* to be initialized at declaration time. > > In the optimized code, I'll bet you that the two locals (t0 and t1) are > kept entirely in registers, at least until the "&t0" and "&t1" calls. So > at the point of comparison, it's comparing two 80-bit values, but when > you flush them to memory to dump them as integer values, they get > truncated to the (same) 64-bit value. Possible. Consider SSE ops (64-bit vs 80-bit on x87) and use of fast-math as well. -cl -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Problem reports: http://cygwin.com/problems.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/