Perform gc-state checks with LoadB to fit C2 matchers

[Roman Kennke]

Am 12.06.2018 um 17:52 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/c2-match-gc-state/webrev.01/
> 
> Since we picked up the fix for matchers [1], all our precious LoadUB for gc-state loads now mismatch
> again. We can replace them with LoadB to match the other two matchers from upstream. Patch converts
> all LoadUBs for gc_state to LoadB, asserts the shape is correct, and blocks LoadB->LoadUB conversion
> in AddINode::Ideal.
> 
> Sample WB check before:
> 
>    ...
>        0x00007fa89ccf0254: movzbl 0x20(%r15),%r11d
>        0x00007fa89ccf0259: test   %r11d,%r11d
>   ╭    0x00007fa89ccf025c: jne    0x00007fa89ccf0278
>   │    ...
>   ↘    0x00007fa89ccf0278: movzbl 0x20(%r15),%r11d
>        0x00007fa89ccf027d: test   $0x14,%r11d
>        0x00007fa89ccf0284: jne    0x00007fa89ccf02d2
>        ...
> 
> Sample WB check after:
> 
>        0x00007f3940de31b4: cmpb   $0x0,0x20(%r15)      ; heap-stable check
>     ╭  0x00007f3940de31b9: jne    0x00007f3940de31dd
>     │   ...
>     ↘  0x00007f3940de31dd: testb  $0x14,0x20(%r15)     ; wb slowpath, evac-enabled check
>        0x00007f3940de31e2: jne    0x00007f3940de33ce
> 
> 
> Testing: tier3_gc_shenandoah, eyeballing assembly
> 
> [1] https://bugs.openjdk.java.net/browse/JDK-8204479
> 

Patch is ok.

Why don't we have matchers for LoadUB anymore?

Roman