[PATCH] Handle overflow in dependence analysis lambda ops gracefully
Jakub Jelinek
jakub@redhat.com
Wed Jan 20 11:33:58 GMT 2021
On Wed, Jan 20, 2021 at 12:29:23PM +0100, Richard Biener wrote:
> +/* Compute the product of signed A and B and indicate in *OVERFLOW whether
> + that operation overflowed. */
> +
> +inline HOST_WIDE_INT
> +mul_hwi (HOST_WIDE_INT a, HOST_WIDE_INT b, bool *overflow)
> +{
> + unsigned HOST_WIDE_INT result = a * (unsigned HOST_WIDE_INT)b;
> + if (a != 0 && (HOST_WIDE_INT)result / a != b)
> + *overflow = true;
> + else
> + *overflow = false;
This isn't sufficiently bulletproof.
It should be
if ((a == -1 && b == HOST_WIDE_INT_MIN)
|| (a != 0 && (HOST_WIDE_INT)result / a != b))
*overflow = true;
else
*overflow = false;
or so.
And except that a == -1 && b == min checks my recent widening_mul changes
should match that to __builtin_mul_overflow, so it will not be perfect code,
but at least will not be unnecessarily slow on x86.
Jakub
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