RFR: 8338912: CDS: Segmented roots array [v5]

[Ioi Lam]

On Tue, 10 Sep 2024 08:12:19 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> > Diff of the map files confirms this:
> > ```
> > - heap_root_segments.roots_count:         2628
> > - heap_root_segments.seg_max_size_elems:  1048576
> > - heap_root_segments.seg_max_size_bytes:  131070
> > ....
> > 131c131
> > < 0x0000000000000300:   0010000000000a44 0000020a0001fffe 0000000000000002 000000000000f022   
> > ---
> > > 0x0000000000000300:   0010000000000a44 000001e20001fffe 0000000000000002 000000000000f022   
> > 
> >     2628 = 0x00000a44
> >  1048576 = 0x00100000
> >   131080 = 0x00001ff2
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > The 0x000001e2 is garbage.
> 
> Whoa, that looks fragile! It feels safer to memset(0) the entire header then? Whack-a-mole-ing the alignment paddings every time we add a field is not convenient.

memset the header first is not going to help. The random bits are added by this call:


void set_heap_root_segments(HeapRootSegments segments) { _heap_root_segments = segments; }


`segments` is passed in by value on the stack. Apparently VC++ will not zero-out the unused bits when pushing `segments` onto the stack, but will insist copying the unused bits when doing the assignment.

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PR Comment: https://git.openjdk.org/jdk/pull/20858#issuecomment-2340083735