On Thu, Nov 8, 2012 at 9:04 PM, Jakub Jelinek wrote:
> Hi!
>
> With -fwrapv the range test optimization miscompiles the following testcase
> (both inter-bb version in 4.8+ in first function and the pure fold-const.c
> one since 3.4). The problem is that with -fwrapv and signed type
> -x in [-,b] doesn't imply x in [-b,-] and -x in [a,-] doesn't imply
> x in [-,-a], as -INT_MIN is INT_MIN and thus if INT_MIN was in the range,
> after negation it needs to be in the range too (and similarly if it wasn't,
> it can't be there). Turns out that if we make sure both low and high
> are non-NULL (i.e. replace - with INT_MIN resp. INT_MAX), normalize:
> will do the right thing that it does also for unsigned operation
> - if it detects the high bound is lower than low bound, it adjusts the
> range by inverting it.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk (what
> about 4.7 where bar is miscompiled too)?
>
> Perhaps for PLUS_EXPR/MINUS_EXPR and signed -fwrapv type we could do the
> same instead of return NULL_TREE; that we do right now (added by Kazu
> for PR23518).
Ok.
Thanks,
Richard.
> 2012-11-08 Jakub Jelinek
>
> PR tree-optimization/55236
> * fold-const.c (make_range_step) : For -fwrapv
> and signed ARG0_TYPE, force low and high to be non-NULL.
>
> * gcc.dg/pr55236.c: New test.
>
> --- gcc/fold-const.c.jj 2012-11-07 11:24:34.0 +0100
> +++ gcc/fold-const.c2012-11-08 16:54:38.978339040 +0100
> @@ -3880,6 +3880,17 @@ make_range_step (location_t loc, enum tr
>return arg0;
>
> case NEGATE_EXPR:
> + /* If flag_wrapv and ARG0_TYPE is signed, make sure
> +low and high are non-NULL, then normalize will DTRT. */
> + if (!TYPE_UNSIGNED (arg0_type)
> + && !TYPE_OVERFLOW_UNDEFINED (arg0_type))
> + {
> + if (low == NULL_TREE)
> + low = TYPE_MIN_VALUE (arg0_type);
> + if (high == NULL_TREE)
> + high = TYPE_MAX_VALUE (arg0_type);
> + }
> +
>/* (-x) IN [a,b] -> x in [-b, -a] */
>n_low = range_binop (MINUS_EXPR, exp_type,
>build_int_cst (exp_type, 0),
> --- gcc/testsuite/gcc.dg/pr55236.c.jj 2012-11-08 16:40:49.171781137 +0100
> +++ gcc/testsuite/gcc.dg/pr55236.c 2012-11-08 16:41:20.044578919 +0100
> @@ -0,0 +1,31 @@
> +/* PR tree-optimization/55236 */
> +/* { dg-do run } */
> +/* { dg-options "-O2 -fwrapv" } */
> +
> +extern void abort ();
> +
> +__attribute__((noinline, noclone)) void
> +foo (int i)
> +{
> + if (i > 0)
> +abort ();
> + i = -i;
> + if (i < 0)
> +return;
> + abort ();
> +}
> +
> +__attribute__((noinline, noclone)) void
> +bar (int i)
> +{
> + if (i > 0 || (-i) >= 0)
> +abort ();
> +}
> +
> +int
> +main ()
> +{
> + foo (-__INT_MAX__ - 1);
> + bar (-__INT_MAX__ - 1);
> + return 0;
> +}
>
> Jakub
