[PATCH] wide-int: Fix up wi::shifted_mask [PR106144]

[PATCH] wide-int: Fix up wi::shifted_mask [PR106144]

[Jakub Jelinek]

Hi!

As the following self-test testcase shows, wi::shifted_mask sometimes
doesn't create canonicalized wide_ints, which then fail to compare equal
to canonicalized wide_ints with the same value.
In particular, wi::mask (128, false, 128) gives { -1 } with len 1 and prec 128,
while wi::shifted_mask (0, 128, false, 128) gives { -1, -1 } with len 2
and prec 128.
The problem is that the code is written with the assumption that there are
3 bit blocks (or 2 if start is 0), but doesn't consider the possibility
where there are 2 bit blocks (or 1 if start is 0) where the highest block
isn't present.  In that case, there is the optional block of negate ? 0 : -1
elts, followed by just one elt (either one from the if (shift) or just
negate ? -1 : 0) and the rest is implicit sign-extension.
Only if end < prec there is 1 or more bits above it that have different bit
value and so we need to emit all the elts till end and then one more elt.

if (end == prec) would work too, because we have:
  if (width > prec - start)
    width = prec - start;
  unsigned int end = start + width;
so end is guaranteed to be end <= prec, dunno what is preferred.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2022-07-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/106144
	* wide-int.cc (wi::shifted_mask): If end >= prec, return right after
	emitting element for shift or if shift is 0 first element after start.
	(wide_int_cc_tests): Add tests for equivalency of wi::mask and
	wi::shifted_mask with 0 start.

--- gcc/wide-int.cc.jj	2022-01-11 23:11:23.592273263 +0100
+++ gcc/wide-int.cc	2022-06-30 20:41:25.506292687 +0200
@@ -842,6 +842,13 @@ wi::shifted_mask (HOST_WIDE_INT *val, un
 	val[i++] = negate ? block : ~block;
     }
 
+  if (end >= prec)
+    {
+      if (!shift)
+	val[i++] = negate ? 0 : -1;
+      return i;
+    }
+
   while (i < end / HOST_BITS_PER_WIDE_INT)
     /* 1111111 */
     val[i++] = negate ? 0 : -1;
@@ -2583,6 +2590,10 @@ wide_int_cc_tests ()
   run_all_wide_int_tests <widest_int> ();
   test_overflow ();
   test_round_for_mask ();
+  ASSERT_EQ (wi::mask (128, false, 128),
+	     wi::shifted_mask (0, 128, false, 128));
+  ASSERT_EQ (wi::mask (128, true, 128),
+	     wi::shifted_mask (0, 128, true, 128));
 }
 
 } // namespace selftest

	Jakub

Re: [PATCH] wide-int: Fix up wi::shifted_mask [PR106144]

[Richard Sandiford]

Jakub Jelinek <jakub@redhat.com> writes:
> Hi!
>
> As the following self-test testcase shows, wi::shifted_mask sometimes
> doesn't create canonicalized wide_ints, which then fail to compare equal
> to canonicalized wide_ints with the same value.
> In particular, wi::mask (128, false, 128) gives { -1 } with len 1 and prec 128,
> while wi::shifted_mask (0, 128, false, 128) gives { -1, -1 } with len 2
> and prec 128.
> The problem is that the code is written with the assumption that there are
> 3 bit blocks (or 2 if start is 0), but doesn't consider the possibility
> where there are 2 bit blocks (or 1 if start is 0) where the highest block
> isn't present.  In that case, there is the optional block of negate ? 0 : -1
> elts, followed by just one elt (either one from the if (shift) or just
> negate ? -1 : 0) and the rest is implicit sign-extension.
> Only if end < prec there is 1 or more bits above it that have different bit
> value and so we need to emit all the elts till end and then one more elt.
>
> if (end == prec) would work too, because we have:
>   if (width > prec - start)
>     width = prec - start;
>   unsigned int end = start + width;
> so end is guaranteed to be end <= prec, dunno what is preferred.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2022-07-01  Jakub Jelinek  <jakub@redhat.com>
>
> 	PR middle-end/106144
> 	* wide-int.cc (wi::shifted_mask): If end >= prec, return right after
> 	emitting element for shift or if shift is 0 first element after start.
> 	(wide_int_cc_tests): Add tests for equivalency of wi::mask and
> 	wi::shifted_mask with 0 start.

OK, thanks, but could you also remove the "end < prec" condition from:

  else if (end < prec)
    val[i++] = negate ? -1 : 0;

Richard

> --- gcc/wide-int.cc.jj	2022-01-11 23:11:23.592273263 +0100
> +++ gcc/wide-int.cc	2022-06-30 20:41:25.506292687 +0200
> @@ -842,6 +842,13 @@ wi::shifted_mask (HOST_WIDE_INT *val, un
>  	val[i++] = negate ? block : ~block;
>      }
>  
> +  if (end >= prec)
> +    {
> +      if (!shift)
> +	val[i++] = negate ? 0 : -1;
> +      return i;
> +    }
> +
>    while (i < end / HOST_BITS_PER_WIDE_INT)
>      /* 1111111 */
>      val[i++] = negate ? 0 : -1;
> @@ -2583,6 +2590,10 @@ wide_int_cc_tests ()
>    run_all_wide_int_tests <widest_int> ();
>    test_overflow ();
>    test_round_for_mask ();
> +  ASSERT_EQ (wi::mask (128, false, 128),
> +	     wi::shifted_mask (0, 128, false, 128));
> +  ASSERT_EQ (wi::mask (128, true, 128),
> +	     wi::shifted_mask (0, 128, true, 128));
>  }
>  
>  } // namespace selftest
>
> 	Jakub