[Bug c/51730] [4.7 Regression] autoconf 2.60 through 2.67 stdbool.h check fails with GCC 4.7

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51730

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-01-02 13:03:35 UTC ---
On Mon, 2 Jan 2012, jakub at gcc dot gnu.org wrote:

>          char digs[] = "0123456789";
>          int xlcbug = 1 / (&(digs + 5)[-2 + (_Bool) 1] == &digs[4] ? 1 : -1);
> check.  Until http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=172958
> GCC has been accepting this though, and I suppose we don't want to fold array
> refs that way when generating code.  Would it be possible to fold it that way
> (try harder) just when we know we are not going to generate code based on it
> (or when we know we'd error out otherwise)?  I know it sounds like an ugly

As I understand it, the point of that commit was that the conversion of 
all array references to pointer arithmetic (losing all information about 
signs of indices) was problematic.  But it should still be valid to fold a 
comparison that way: if the addresses being compared have the same base 
object and all offsets are constant integers, a final byte offset for each 
address can be computed mod the size of the address space and it's OK to 
fold based on comparing those offsets (if the actual, signed offsets 
involved overflow anywhere, that would have been execution-time undefined 
behavior).  That is, I think it would be better to fix this by improving 
the folding of address comparisons, rather than by changing how array 
references are expanded.