[jhc] foreign ptr bugs

John Meacham john at repetae.net
Fri Dec 31 00:42:54 CET 2010

On Thu, Dec 30, 2010 at 12:13 PM, Korcan Hussein <korcan_h at hotmail.com> wrote:
> I've never had to deal with aliasing bugs before fortunately but I did come across an article about getting warning messages from gcc for code not adhering to strict-aliasing rules which is -Wstrict-aliasing=2 apparently this warning is not enabled by -Wall by default, I've tried this with the test code I attached earlier and I do get many of these type of messages:
> "warning: dereferencing type-punned pointer will break strict-aliasing rules
> warning: initialization from incompatible pointer type
> warning: assignment from incompatible pointer type"
> There is also the attribute __may_alias__ for structs.

Yeah, I have played with those, almost all the warnings seem to be false
positives due to the way I handle pointers and their conversion to uintptr_t. I
am not sure there is going to be a good fix without non-trivial changes to the
code generator. I think compiling with -fno-strict-aliasing is the best
solution at the moment.

> I've also tried -fno-strict-aliasing without -fdebug and I do get different
> pointer values this time but the program does not work correctly, the image
> doesn't appear to get blitted to the screen buffer so I get a black window
> which means it is either a plain bug with generated code or another
> optimizations related to (strict-)aliasing messing things up or combination
> of both.

I am tempted to think that this is an independent bug. It doesn't feel like it
is related at least. Is there any way you can come up with a smaller test case,
like trying to dump the buffer to see if the bmp is even being loaded, or
seeing if you can plot something simple like a line? I don't have much
experience with the SDL libraries.


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