Re: Q about Ada and value ranges in types

[Diego Novillo <dnovillo@redhat.com>]

On Mon, May 02, 2005 at 09:46:59PM -0400, Richard Kenner wrote:

> You're not showing where this comes from, so it's hard to say.  However
> D.1480 is created by the gimplifier, not the Ada front end.  There could
> easily be a typing problem in the tree there (e.g., that of the subtraction),
> but I can't tell for sure.
> 
Yeah, I didn't show all of it, sorry.  My patch to address this
problem includes a more detailed description
(http://gcc.gnu.org/ml/gcc-patches/2005-05/msg00127.html).

Florian Weimer suggested that instead of marking the range as
varying, we could check the super-type to see if it has a wider
range.  That is true in this case; the parent type is
types__Tname_idB which has range [-2147483648, 2147483647].  But
I'm not sure if that would be true in general.

>     If the Ada language allows that kind of runtime check, then my
>     fix to VRP will be different. 
> 
> I don't see it as a language issue: I'd argue that the tree in statement 2
> is invalid given the typing.  That should be true for any language.
> 
Dunno.  All the operands in the snippet I showed are of the exact
same type (types__name_id___XDLU_300000000__399999999).  I'm not
really sure where this type is coming from, but it's relatively
easy to reproduce.

Configure a compiler for target i386-pc-linux-gnu (or any other
i386 variant, not sure if it occurs elsewhere) and compile
ada/sem_intr.adb with:

$ ./xgcc -B./ -c -g -O2 -gnatpg -gnata -I- -I. -Iada -I<src>/gcc/ada <src>/gcc/ada/sem_intr.adb -o ada/sem_intr.o -v -save-temps

Launch gdb and set a bkpt at tree-vrp.c:552 (extract_range_from_assert).
You should get to this ASSERT_EXPR:

ASSERT_EXPR <D.1480_32, D.1480_32 <= 1>

which is in the following context:

-----------------------------------------------------------------------------
;; basic block 44, loop depth 0, count 0
;; prev block 43, next block 84
;; pred:       43 (true,exec)
;; succ:       84 (true,exec) 45 (false,exec)
<L51>:;
D.1478_28 = sinfo__etype (e_5);
nam_30 = sinfo__chars (e_5);
D.1480_32 = nam_30 - 300000361;
if (D.1480_32 <= 1) goto <L112>; else goto <L52>;

;; basic block 84, loop depth 0, count 0
;; prev block 44, next block 45
;; pred:       44 (true,exec)
;; succ:       50 [100.0%]  (fallthru)
<L112>:;
D.1480_94 = ASSERT_EXPR <D.1480_32, D.1480_32 <= 1>;
goto <bb 50> (<L57>);
-----------------------------------------------------------------------------

So, after calling sinfo__chars() and subtracting 300000361, the
FE is emitting that range check.  AFAICT, the call to
sinfo__chars(e_5) comes from ada/sem_intr.adb:148

	     Nam : constant Name_Id   := Chars (E);

and 'if (D.1480_32 <= 1)' is at line 155:
     
     	     if Nam = Name_Op_Add


Thanks.  Diego.