[aarch64-port-dev ] 8219993: AArch64: Compiled CI stubs are unsafely modified

[Andrew Haley]

According to the Arm manual, except in a few limited cases it is not
safe to modify code while it is being executed. We mostly get this
right, but our CompiledStaticCall stubs are patched at runtime by
rewriting immediate loads. We can fix this by moving the rewritable
data fields into memory so that they are loaded by means of load(mem)
rather than mov(immediate data).

After this patch, CI stubs look like:

      dmb ld
      ldr rmethod, L1
      ldr rscratch1, L2
      br [rscratch1]
L1:   .data 0
L2:   .data 0

When we modify such a stub we rewrite the two data fields to the
method metadata and the interpreter entry point, execute a memory
fence, then rewrite the compiled jump to point to the stub.

http://cr.openjdk.java.net/~aph/8219006.1/

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