Reserving an extra slot on the stack

[Andrew Haley]

On 25/01/18 14:59, David Lloyd wrote:
> For an experiment I'm working on, I'd like to modify the JVM to
> reserve one reference-sized slot (which eventually should be visible
> to GC) on every stack frame at a fixed well-known offset (such that I
> can reach it via Unsafe from Java-space).  But it appears that the
> stack layout code is pretty deep in per-CPU areas.  Is there a generic
> mechanism that I've missed?  Like some way to make the compiler act as
> if there were one extra local variable or something like that?

Each compiler does its own stack layout.  The only thing that's fixed
is the calling convention.  To make this work you'd need to change C1,
C2, and the template interpreter.  As it happens, the interpreter has
an unused slot on most architectures, so you can have that for free.
Do you actually need to test your experiment with C1 and C2?

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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