wide int knowledge/bug?

wide int knowledge/bug?

[Andrew MacLeod]

I was looking at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97596

and the ranger is constructing some 128 bit constants and calling 
wide_int_to_tree to turn them into trees.

In particular, it starts with the value

p r.lower_bound(0)
{<wide_int_storage> = {val = {-65535, 9223372036854775807, 
140737488257608}, len = 2, precision = 128}, static is_sign_extended = true}

p r.lower_bound(0).dump()
[0x7fffffffffffffff,0xffffffffffff0001], precision = 128


  and proceeds to call

wide_int new_lb = wi::set_bit (r.lower_bound (0), 127)

and creates the value:

p new_lb
{<wide_int_storage> = {val = {-65535, -1, 0}, len = 2, precision = 128}, 
static is_sign_extended = true}

p new_lb.dump()
[0xffffffffffffffff,0xffffffffffff0001], precision = 128


but when we try to call wide_int_to_tree() on this value, it dies in the 
second assert (this is the trap in the PR) :

  /* Verify that everything is canonical.  */
   int l = pcst.get_len ();
   if (l > 1)
     {
       if (pcst.elt (l - 1) == 0)
         gcc_checking_assert (pcst.elt (l - 2) < 0);
       if (pcst.elt (l - 1) == HOST_WIDE_INT_M1)
         gcc_checking_assert (pcst.elt (l - 2) >= 0);               
<<<---  here.
     }

If i look at the value, its:

  p pcst.elt (l - 2)
$18 = -65535

which is clearly not >= 0... it seems to be treating it as a signed 
value?   So did the set_bit routine do something wrong when creating the 
new value?
I cant really read this, but something seems amok...

any ideas?

Andrew

Re: wide int knowledge/bug?

[Jakub Jelinek]

On Tue, Oct 27, 2020 at 01:18:03PM -0400, Andrew MacLeod via Gcc wrote:
> I was looking at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97596
> 
> and the ranger is constructing some 128 bit constants and calling
> wide_int_to_tree to turn them into trees.
> 
> In particular, it starts with the value
> 
> p r.lower_bound(0)
> {<wide_int_storage> = {val = {-65535, 9223372036854775807, 140737488257608},
> len = 2, precision = 128}, static is_sign_extended = true}
> 
> p r.lower_bound(0).dump()
> [0x7fffffffffffffff,0xffffffffffff0001], precision = 128
> 
> 
>  and proceeds to call
> 
> wide_int new_lb = wi::set_bit (r.lower_bound (0), 127)
> 
> and creates the value:
> 
> p new_lb
> {<wide_int_storage> = {val = {-65535, -1, 0}, len = 2, precision = 128},
> static is_sign_extended = true}

This is non-canonical and so invalid, if the low HWI has the MSB set
and the high HWI is -1, it should have been just
val = {-65535}, len = 1, precision = 128}

I guess the bug is that wi::set_bit_large doesn't call canonize.
So perhaps totally untested:

--- gcc/wide-int.cc.jj	2020-10-19 18:42:41.134426398 +0200
+++ gcc/wide-int.cc	2020-10-27 18:33:38.546703763 +0100
@@ -702,8 +702,11 @@ wi::set_bit_large (HOST_WIDE_INT *val, c
       /* If the bit we just set is at the msb of the block, make sure
 	 that any higher bits are zeros.  */
       if (bit + 1 < precision && subbit == HOST_BITS_PER_WIDE_INT - 1)
-	val[len++] = 0;
-      return len;
+	{
+	  val[len++] = 0;
+	  return len;
+	}
+      return canonize (val, len, precision);
     }
   else
     {



	Jakub

Re: wide int knowledge/bug?

[Richard Sandiford]

Jakub Jelinek via Gcc <gcc@gcc.gnu.org> writes:
> On Tue, Oct 27, 2020 at 01:18:03PM -0400, Andrew MacLeod via Gcc wrote:
>> I was looking at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97596
>> 
>> and the ranger is constructing some 128 bit constants and calling
>> wide_int_to_tree to turn them into trees.
>> 
>> In particular, it starts with the value
>> 
>> p r.lower_bound(0)
>> {<wide_int_storage> = {val = {-65535, 9223372036854775807, 140737488257608},
>> len = 2, precision = 128}, static is_sign_extended = true}
>> 
>> p r.lower_bound(0).dump()
>> [0x7fffffffffffffff,0xffffffffffff0001], precision = 128
>> 
>> 
>>  and proceeds to call
>> 
>> wide_int new_lb = wi::set_bit (r.lower_bound (0), 127)
>> 
>> and creates the value:
>> 
>> p new_lb
>> {<wide_int_storage> = {val = {-65535, -1, 0}, len = 2, precision = 128},
>> static is_sign_extended = true}
>
> This is non-canonical and so invalid, if the low HWI has the MSB set
> and the high HWI is -1, it should have been just
> val = {-65535}, len = 1, precision = 128}
>
> I guess the bug is that wi::set_bit_large doesn't call canonize.

Yeah, looks like a micro-optimisation gone wrong.

> So perhaps totally untested:
>
> --- gcc/wide-int.cc.jj	2020-10-19 18:42:41.134426398 +0200
> +++ gcc/wide-int.cc	2020-10-27 18:33:38.546703763 +0100
> @@ -702,8 +702,11 @@ wi::set_bit_large (HOST_WIDE_INT *val, c
>        /* If the bit we just set is at the msb of the block, make sure
>  	 that any higher bits are zeros.  */
>        if (bit + 1 < precision && subbit == HOST_BITS_PER_WIDE_INT - 1)
> -	val[len++] = 0;
> -      return len;
> +	{
> +	  val[len++] = 0;
> +	  return len;
> +	}
> +      return canonize (val, len, precision);
>      }
>    else
>      {

LGTM, thanks.

Richard

Re: wide int knowledge/bug?

[Andrew MacLeod]

On 10/27/20 1:46 PM, Richard Sandiford wrote:
> Jakub Jelinek via Gcc <gcc@gcc.gnu.org> writes:
>> On Tue, Oct 27, 2020 at 01:18:03PM -0400, Andrew MacLeod via Gcc wrote:
>>> I was looking at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97596
>>>
>>> and the ranger is constructing some 128 bit constants and calling
>>> wide_int_to_tree to turn them into trees.
>>>
>>> In particular, it starts with the value
>>>
>>> p r.lower_bound(0)
>>> {<wide_int_storage> = {val = {-65535, 9223372036854775807, 140737488257608},
>>> len = 2, precision = 128}, static is_sign_extended = true}
>>>
>>> p r.lower_bound(0).dump()
>>> [0x7fffffffffffffff,0xffffffffffff0001], precision = 128
>>>
>>>
>>>  and proceeds to call
>>>
>>> wide_int new_lb = wi::set_bit (r.lower_bound (0), 127)
>>>
>>> and creates the value:
>>>
>>> p new_lb
>>> {<wide_int_storage> = {val = {-65535, -1, 0}, len = 2, precision = 128},
>>> static is_sign_extended = true}
>> This is non-canonical and so invalid, if the low HWI has the MSB set
>> and the high HWI is -1, it should have been just
>> val = {-65535}, len = 1, precision = 128}
>>
>> I guess the bug is that wi::set_bit_large doesn't call canonize.
> Yeah, looks like a micro-optimisation gone wrong.
>
>> So perhaps totally untested:
>>
>> --- gcc/wide-int.cc.jj	2020-10-19 18:42:41.134426398 +0200
>> +++ gcc/wide-int.cc	2020-10-27 18:33:38.546703763 +0100
>> @@ -702,8 +702,11 @@ wi::set_bit_large (HOST_WIDE_INT *val, c
>>         /* If the bit we just set is at the msb of the block, make sure
>>   	 that any higher bits are zeros.  */
>>         if (bit + 1 < precision && subbit == HOST_BITS_PER_WIDE_INT - 1)
>> -	val[len++] = 0;
>> -      return len;
>> +	{
>> +	  val[len++] = 0;
>> +	  return len;
>> +	}
>> +      return canonize (val, len, precision);
>>       }
>>     else
>>       {
> LGTM, thanks.
>
> Richard
>
Seems to resolve my problem.

want me to run this thru the tests and apply it to this PR, or is that 
already underway? :-)

Andrew