[aarch64] assert(is_power_of_2(x)) failed: x must be a power of 2: 0xffffffff80000000

[Andrew Haley]

Argh.

Sorry, this is wrong: Martin Doerr is quite right to point out that
converting from int to unsigned long sign extends first, as per
4.7, Integral conversions. Apologies.

On 2/26/20 2:30 PM, Andrew Haley wrote:

> We could make it so that is_power_of_2 failed to compile if passed a
> signed type. However, existing practice in HotSpot is that all of the
> log2 functions cast signed types to unsigned types, for example:
> 
> inline int log2_int(int x) {
>   STATIC_ASSERT(sizeof(int) <= sizeof(uintptr_t));
>   return log2_intptr((uintptr_t)x);
> }
> 
> inline int log2_jint(jint x) {
>   STATIC_ASSERT(sizeof(jint) <= sizeof(uintptr_t));
>   return log2_intptr((uintptr_t)x);
> }
> 
> Therefore, I believe that it would be appropriate to follow existing
> practice in is_power_of_2, like so:
> 
> template <typename T>
> bool is_power_of_2(T val) {
>   STATIC_ASSERT(sizeof(T) <= sizeof(julong));
>   julong x = val;
>   return (x != 0) && ((x & (x - 1)) == 0);
> }
> 
> // Log2 of a power of 2
> template <typename T>
> inline int exact_log2(T val) {
>   STATIC_ASSERT(sizeof(T) <= sizeof(julong));
>   julong x = val;
>   assert(is_power_of_2(x), "x must be a power of 2: " JULONG_FORMAT, x);
>   return log2_long(x);
> }


-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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