RFR: Fix conditional store in interpreter matrix barrier

[Aleksey Shipilev]

On 10/04/2017 07:44 PM, Roman Kennke wrote:
> We attempt to do a conditional store in the interpreter matrix barrier. We use testb(addr, 0) for
> this. testb and-s its operands and sets the condition flags. And-ing with 0 always yields 0, so the
> zero-flag will always be set, resulting in the matrix barrier always storing 1 into the matrix. This
> is not fatal, but it leads to unintentional false sharing and the like.
> 
> Fix:
> 
> diff --git a/src/cpu/x86/vm/macroAssembler_x86.cpp b/src/cpu/x86/vm/macroAssembler_x86.cpp
> --- a/src/cpu/x86/vm/macroAssembler_x86.cpp
> +++ b/src/cpu/x86/vm/macroAssembler_x86.cpp
> @@ -5493,7 +5493,7 @@
>    // Address is _matrix[to * stride + from]
>    movptr(rscratch1, (intptr_t) matrix_addr);
>    // Test if the element is already set.
> -  testb(Address(rscratch1, tmp, Address::times_1), 0);
> +  testb(Address(rscratch1, tmp, Address::times_1), 1);
>    jcc(Assembler::notZero, done);
>    // Store true, if not yet set.
>    movb(Address(rscratch1, tmp, Address::times_1), 1);
> 
> Test: hotspot_gc_shenandoah
> 
> ok?

Looks good. D'oh. Maybe we should use cmpb(..., 0) + jcc(notEqual, done) for clarity -- no strong
opinion here though.

-Aleksey