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Date: Wed, 04 Aug 2010 12:58:00 -0000
From: Richard Guenther <rguenther@suse.de>
To: Arnaud Charlet <charlet@adacore.com>
Cc: gcc-patches@gcc.gnu.org, Eric Botcazou <botcazou@adacore.com>
Subject: Re: [PATCH] Keep lattice abstract in the SSA propagator, substitue lattice values at defs
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On Wed, 4 Aug 2010, Richard Guenther wrote:

> On Wed, 4 Aug 2010, Arnaud Charlet wrote:
> 
> > > > > Bootstrapped and tested on x86_64-unknown-linux-gnu.  c52102b
> > > > > and c52102d now fail, because they rely on undefined signed integer
> > > > > overflow.
> > > > 
> > > > Ada ACATS tests do not rely on signed integer overflow: Ada has built-in
> > > > overflow checks as part of the language, but this capability requires the
> > > > -gnato switch.
> > > > 
> > > > If these tests are failing, it means the -gnato switch is missing on these
> > > > tests.
> > > > 
> > > > Can you please fix that by updating
> > > > gcc/testsuite/ada/acats/overflow.lst ?
> > > > Thanks in advance.
> > > 
> > > Adding -gnato doesn't fix the testcases (checked c52102b).
> > 
> > Well, then there's something fishy here that needs to be investigated, can you
> > expand on why these tests "rely on undefined signed integer overflow" ?
> 
> I looked at the VRP dump and it derives perfectly valid assumptions.
> From
> 
>   <various bounds checking concludes ident_int_2_12 is [1, 4]
>    and ident_int_1_10 is [1, 4]>
> 
> <bb 15>:
>   D.2465_155 = ident_int_2_12 + 1;
>   L27b_156 = D.2465_155 - ident_int_1_10;
>   L29b_159 = L27b_156 + 1;
> 
> <bb 16>:
>   # L29b_363 = PHI <L29b_159(15), 1(14)>
> ...
> 
>   and we arrive with [-3, 9] for prephitmp.53_302 and [-1, 5] for L29b_363
> 
>   if (prephitmp.53_302 > L29b_363)
>     goto <bb 27>;
> 
> <bb 27>:
> 
>   thus we can conclude that prephitmp.53_302 is [0, 9] here.
>   The following statement then produces a range of [2147483647, overflow].
>   So we happily substitute a constant value of 2147483647 for D.2544_253.
> 
>   D.2544_253 = prephitmp.53_302 + 2147483647;
>   D.2553_254 = (size_type) D.2544_253;
> ...
>   D.2554_257 = D.2553_254 + 1;

Btw, is it really necessary to use INT_MIN as minimum index value for
arrays?

    typedef character c52102b__B_2__T40b[-2147483647:(integer) L29b > 1 ? 
(size_type) (((integer) L29b + -1) + -2147483648) : -2147483648];

    S38b[-2147483647 ...]{lb: -2147483648 sz: 1} = ...

not that it is invalid, but the above situation might be triggered
by this.

Richard.