On Tue, May 31, 2011 at 07:23:46PM +0000, Christian Weisgerber wrote:
> Marc Espie <es...@nerim.net> wrote:
>
> > Not surprisingly, a lot of software that claims to be 64 bits-ready isn't.
> > This touches all web navigators, most jit engines, and probably lots more
> > of software (our ports tree version of gnu-grep, for instance).
>
> I don't think a lot suffers from it, but some prominent cases do.
> Three problems have been mentioned:
>
> (1) Truncation of pointers to 32 bits. Our malloc(3) has returned
> addresses >4 GB for some time now on amd64 (and before that on
> other archs like alpha), so I don't expect any new fallout
> there. I seem to remember that we had a rash of ports fixes
> back when this first happened on amd64.
>
> (2) Tagged pointers. A tagged pointer is when you "know" that not
> all the bits in a pointer are used to generate an address and
> you squeeze some other data into the "spare" bits. This blocks
> newer versions of Firefox on sparc64. Mozilla's new JavaScript
> engine uses tagged pointers and those "unused" address bits on
> x86 are actually used on sparc64.
>
> (3) The expectation that, no matter what their absolute address, the
> relative offsets between all your pieces of data fit into 32
> bits, i.e., all data is within a 4 GB window. That sounds like
> a bizarre requirement, but apparently some JIT engines are
> "optimized" to rely on this. These are the cases that break
> with new vmmap.
The smart programmers "solve" number (3) by allocating 2G of memory in
advance to store their jit compiled code, so their code can use 32 bit
relative offsets. They say, hey, it's only virtual memory, so it
doesn't take much resources. Often that is true and it seems a smart
idea, but it has the consequence that you lose randomization and
protected memory with page size granularity. Or you are forced to do
all the memory mangement on your own, basically rewriting the memory
management part of the OS in your browser. Suddenly the smart idea
does not sound so smart anymore.
-Otto
>
> But, hey, 64-bit desktop machines have only been around since 1993
> or so, and I guess some of the Mozilla programmers weren't born yet
> when we watched oh-so-clever tagged pointer use blow up at, say,
> the Motorola 68000 to 68020 transition some 25 years ago.
>
> --
> Christian "naddy" Weisgerber na...@mips.inka.de
