RFR: 8169901: AArch64: CompareAndExchange intrinsics clobber address register

Andrew Haley aph at redhat.com
Thu Nov 17 17:50:40 UTC 2016

On 17/11/16 17:44, Roman Kennke wrote:
> the various compareAndExchange intrinsics in aarch64.ad potentially
> clobber the address register:
> ;; cmpxchg {
>           0x0000007f88b5919c: ldxr w0, [x0]
>           0x0000007f88b591a0: cmp w0, w2
>           0x0000007f88b591a4: b.ne 0x0000007f88b591b0
>           0x0000007f88b591a8: stlxr w8, w3, [x0]
>           0x0000007f88b591ac: cbnz w8, 0x0000007f88b5919c
>          ;; } cmpxchg 
> This is very bad because it's subsequently used (within the same
> instruction) for the store-exclusive. It results in occasional
> segfaults.
> The fix is to declare the result register TEMP_DEF, this ensures that
> it's distinct from address, newval and oldval. This also removes the
> need to pin the input registers, which, I presume, was an attempt to
> resolve this exact problem, but did not consider the address.
> http://cr.openjdk.java.net/~rkennke/aarch64-cae/webrev.00/
> bug: https://bugs.openjdk.java.net/browse/JDK-8169901
> Tested by running jcstress (which segfaulted without this change).

Looks OK.  I guess you must have just struck lucky; I did a clean jcstress
run an hour or so ago.

This comment is wrong, please adjust:

   1 dnl Copyright (c) 2014, Red Hat Inc. All rights reserved.  21 dnl
  22 dnl
  23 dnl Process this file with m4 aarch64_ad.m4 to generate the arithmetic
  24 dnl and shift patterns patterns used in aarch64.ad.

I've never had any luck with C2 using TEMP_DEF in htis way, but if it
works for you I guess it's OK.



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