Re: How to keep statement order in VC optimization?

"Doug Harrison [MVP]" <>
Sat, 01 Aug 2009 14:23:17 -0500
On Sat, 1 Aug 2009 11:36:45 +0800, "zhu.pi" <> wrote:

Yes, when funcXs are declared as funcX(volatile unsigned long long *t),
cl /O1 keep the order (while cl /O2, func3 is still out of order),

then I let func3 read *t

unsigned long long ll = *t;

both cl /O1 and cl /O2 keep the statement order,

Yes, it's essential that funcX all use *t in some way.

both cl /O1 and cl /O2 compile ll=*t to instruction codes,
they should "optimize out" this useless ll=*t statement :-)

But they won't if it's declared volatile.

Thank you very much!
By the way, I wonder if VC could add some keyword like __declspec(onorder) to keep
statement order, it should be better than this technique.

That's an interesting idea, but perhaps surprisingly, this sort of thing is
only rarely an issue. Off the top of my head, I've never seen it affect
anything but timing/benchmark code. Of course, multithreading has its own
ordering issues, but for the most part, they're dealt with at a higher
conceptual level in terms of mutex and other synchronization object usage.

Doug Harrison
Visual C++ MVP

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