Re: [RFH] gcc constant expression warning...

From: Daniel Barkalow <barkalow@iabervon.org>
Date: Oct 28, 2007, 16:28 UTC
Message-ID: <Pine.LNX.4.64.0710281204350.7345@iabervon.org>
In-Reply-To: <87ir4rletv.fsf@mid.deneb.enyo.de>

On Sun, 28 Oct 2007, Florian Weimer wrote:

Show 12 quoted lines

> * Junio C. Hamano:
> 
> > The offending lines are:
> >
> >         if (idx_size != min_size) {
> >                 /* make sure we can deal with large pack offsets */
> >                 off_t x = 0x7fffffffUL, y = 0xffffffffUL;
> >                 if (x > (x + 1) || y > (y + 1)) {
> >                         munmap(idx_map, idx_size);
> 
> x and y must be unsigned for this test to work (signed overflow is
> undefined).

I believe the test is trying to determine if signed addition on numbers of a certain size is safe in this environment. Doing the test with unsigned variables would cause the test to give a predictable but irrelevant result. I think gcc is being annoying in assuming that signed overflow doesn't occur (even when it must), rather than assuming that the result of signed overflow is some arbitrary and likely not useful value. If we have an overflow possible with off_t in the way we'd use it, then one of those tests should be automatically true due to the limited size of the type (except that I think the test should be >= instead of >). I think we should be able to assume that the result of a signed overflow, whatever undefined value it is, is a possible value of its type and therefore not more than the maximum value of its type, but gcc may be screwing this up.

It's probably best just to test the size of off_t.

	-Daniel
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