Re: Bootstrap failure on trunk: x86_64-linux-gnu

[Laurent GUERBY <laurent at guerby dot net>]

On Tue, 2006-02-28 at 18:59 -0500, Daniel Jacobowitz wrote:
> On Tue, Feb 28, 2006 at 03:40:37PM -0700, Jeffrey A Law wrote:
> > Here's a great example from uintp.adb (which is the cause of the
> > testsuite hang FWIW)
> > 
> > We have a loop with the following termination code in uintp.num_bits
> > 
> >   # BLOCK 8
> >   # PRED: 5 [100.0%]  (fallthru,exec) 6 (fallthru,dfs_back,exec)
> >   # num_2 = PHI <num_49(5), num_10(6)>;
> >   # bits_1 = PHI <bits_50(5), bits_13(6)>;
> > <L7>:;
> >   num.265_5 = (types__TintB) num_2;
> >   if (num.265_5 <= 0) goto <L5>; else goto <L4>;
> >   # SUCC: 7 (true,exec) 6 (false,exec)
> 
> ...
> 
> > Sooooo, why am I bringing this up?  Because num can actually have
> > the value 0x80000000 at runtime, which is out of its type's
> > MIN/MAX range.  And what do you think happens?  Well, given the
> > (valid) simplification of the loop test and the way "num" is
> > assigned within the loop (num = num / 2), the loop never terminates.
> 
> I've been following this entire thread, and I think there's a serious
> disconnect between the parties - it's unfortunate that none of the
> tree-ssa folks involved know Ada as I suspect that would straighten
> it out in a hurry.  This is a perfect example.  Now that we have some
> concrete code that's causing a problem, let's take a look at it
> (bear in mind, I don't know Ada either):
> 
>    function Num_Bits (Input : Uint) return Nat is
>       Bits : Nat;
>       Num  : Nat;
> 
>    begin
>       if UI_Is_In_Int_Range (Input) then
>          Num := abs (UI_To_Int (Input));
>          Bits := 0;
> 
>       else
>          Bits := Base_Bits * (Uints.Table (Input).Length - 1);
>          Num  := abs (Udigits.Table (Uints.Table (Input).Loc));
>       end if;
> 
>       while Types.">" (Num, 0) loop
>          Num := Num / 2;
>          Bits := Bits + 1;
>       end loop;
> 
>       return Bits;
>    end Num_Bits;

The specifications in uintp.ads says:

   function Num_Bits (Input : Uint) return Nat;
   --  Approximate number of binary bits in given universal integer.
   --  This function is used for capacity checks, and it can be one
   --  bit off without affecting its usage.

So this is a hint function, anything not hanging should do, like

if UI_Is_In_Int_Range (Input) then
   return Nat'Size;
else
...

> I'm going to assume that UI_Is_In_Int_Range isn't true for 0x80000000.
> The other case is still assigning 0x80000000 to Nat somehow.  I'd be
> amazed if that's really valid Ada!  Could someone who knows the
> language comment, please?

Ada checks are off when compiling the FE, in full Ada you
should get a Constraint_Error on the abs line on int'first.

You should get an exception it if you put a 

   pragma Unsuppress (All_Checks);

before begin.

So this is likely to be a FE Ada coding bug and not a FE/ME/BE interface
issue, thanks for spotting this!

Laurent