Value array element size represented as log2

[John Rose]

I support this; I would even support burning a couple more bits to put in a
multiplier of 1/3/5/7.  This would reduce the fragmentation from 0-50-100%
to 0-13-25%.

On Oct 1, 2018, at 11:01 PM, Ioi Lam <ioi.lam at oracle.com> wrote:
> 
> (Not sure if this has been discussed before. I did a quick JBS search of "valueArrayKlass" but couldn't find anything).
> 
> For example, if my VT has 5 integers (20 bytes) it will take up 32 bytes inside the array. This seems kind of wasteful, especially when you have larger VT types.
> 
> See:
> 
> http://hg.openjdk.java.net/valhalla/valhalla/file/e0c13ab0b852/src/hotspot/share/oops/valueArrayKlass.cpp#l182
> 
> jint ValueArrayKlass::array_layout_helper(ValueKlass* vk) {
>   BasicType etype = T_VALUETYPE;
>   int atag  = _lh_array_tag_vt_value;
>   int esize = upper_log2(vk->raw_value_byte_size());
> 
> klass.hpp:
> 
>   // For arrays, layout helper is a negative number, containing four
>   // distinct bytes, as follows:
>   //    MSB:[tag, hsz, ebt, log2(esz)]:LSB
>   // where:
>   //    tag is 0x80 if the elements are oops, 0xC0 if non-oops
>   //    hsz is array header size in bytes (i.e., offset of first element)
>   //    ebt is the BasicType of the elements
>   //    esz is the element size in bytes
> 
>