RFR(S)[13]: AArch64: float point register corruption in ZBarrierSetAssembler::load_at

[Andrew Haley]

On 6/24/19 10:06 AM, Stuart Monteith wrote:
> Ok, r19 might have been a bad choice, but I'll work something out.
> rscratch1/2 might not be safe to use

In the interpreter most of the lower-numbered registers are free. I'd
start at r10 and work up.

> "load_at" is called by "access_load_at". If you look at
> templateTable_aarch64.cpp you see 25 instances of "noreg" being passed
> as tmp, 5 matches in methodHandles_aarch64.cpp. You'll find a similar
> story in arm and x86 and for store_at.

Fair point. It's horrid. There's a great deal of bad practice copied
from x86, but you don't need to do this sort of thing in new code.

> I'd like to suggest I address the use of rscratch1/2 and the passing
> of noreg separately, the latter is a larger issue that should be
> handled in the original bug.  I can open a bug to handle the use of
> noreg, if that is appropriate.

Sure.

I'm working on a patch which handles rscratch1 and 2 so that they are
declared at the point of use, not globally, and the compiler enforces
their allocation. However, it's a complicated job so won't happen
soon.  It also opens a can of worms.

In new code, please reserve these registers for short-term scratch use
rather than as parameters or longer-term storage. That way I won't
have to fix things up later.

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