[PATCH] -Warray-bounds TLC

[Richard Biener]

On April 17, 2015 8:01:42 PM GMT+02:00, Steve Ellcey <sellcey@imgtec.com> wrote:
>On Thu, 2015-04-16 at 13:55 +0200, Richard Biener wrote:
>> The following applies the patch produced earlier this year, applying
>> TLC to array bound warnings and catching a few more cases.
>> 
>> Bootstrapped and tested on x86_64-unknown-linux-gnu, applied.
>> 
>> Richard.
>> 
>> 2015-04-16  Richard Biener  <rguenther@suse.de>
>> 
>> 	PR tree-optimization/64277
>> 	* tree-vrp.c (check_array_ref): Fix anti-range handling,
>> 	simplify upper bound handling.
>> 	(search_for_addr_array): Simplify.
>> 	(check_array_bounds): Handle ADDR_EXPRs here.
>> 	(check_all_array_refs): Simplify.
>
>This caused an interesting build failure of glibc when using the latest
>GCC.  Here is a cut down case from elf/dl-open.c:
>
>	extern void foo(void);
>	struct s { int n; } v[1];
>	int bar (int i)
>	{
>  		if ((i != 0 && i != -1 && i != -2) && (v[i].n == 0))
>    			foo ();
>  		return 0;
>	}
>	int baz (int i)
>	{
>  		if ((i != 0 && i != -2) && (v[i].n == 0))
>    			foo ();
>  		return 0;
>	}
>
>When compiled with -O2 -Wall -Werror, I now get an error about v[i]
>being out-of-bounds in bar.  But I do not get this error in baz (where
>we don't check for -1.  In reality, in glibc, we know that i can only
>be
>0, -1, or -2.  GCC of course doesn't know that.  Does this
>error/warning
>seem right? 

Yes.  Once you reach the array reference it will be out of bounds.

 The difference in behavior between bar and baz seems odd.
>

Yeah, I suppose VRP gets conservative in a way that's not helpful for consistency of this warning.  ~[0,0] and ~[-2,-2] likely meet as VARYING and the warning code doesn't look at equivalences.

Richard.

>Steve Ellcey
>sellcey@imgtec.com