[9] RFR (S): 8152179: C2: Folding of unsigned loads is broken w/ @Stable

[John Rose]

Reviewed.

On Mar 23, 2016, at 9:56 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> http://cr.openjdk.java.net/~vlivanov/8152179/webrev.00/
> https://bugs.openjdk.java.net/browse/JDK-8152179
> 
> Byte and short loads with small masks are converted to unsigned loads.
> Constant folding logic for array elements doesn't respect unsigned loads and replace them with signed values.
> 
> It usually happens when a small mask is applied to the value produced by a signed load (see AndINode::Ideal [1]):
> 
>  byteArr [i] & 0x000F;  // AndI (LoadB 0x000F) => AndI (LoadUB 0xF)
>  shortArr[i] & 0x000F;  // AndI (LoadS 0x000F) => AndI (LoadUS 0xF)
> 
>  byteArr [i] & 0x00FF;  // AndI (LoadB 0x00FF) => LoadUB
>  shortArr[i] & 0xFFFF;  // AndI (LoadS 0xFFFF) => LoadUS
> 
> Field loads aren't affected since they are constant folded during parsing.
> 
> Testing: regression tests, hotspot/compiler/stable, JPRT
> 
> Thanks!
> 
> Best regards,
> Vladimir Ivanov
> 
> PS: while experimenting with the fix, I found that constant folding of @Stable loads doesn't always happen. Filed 8152590 [2].
> 
> [1] http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/file/d9d0a63499ce/src/share/vm/opto/mulnode.cpp#l483
> 
> [2] https://bugs.openjdk.java.net/browse/JDK-8152590