Request for review (L): 6797305: Add LoadUB and LoadUI opcode class

[Christian Thalinger]

On Mon, 2009-03-02 at 09:34 -0800, John Rose wrote:
> On Mar 2, 2009, at 2:30 AM, Christian Thalinger wrote:
> 
> > - Similar to the one above for $mem, e.g. $mem$$Address$high, for long
> > memory accesses.  Currently I'm using:
> 
> Do be careful with the high/low terminology!  That had gotten us into  
> "endian" portability problems.
> 
> I suggest sticking to first/second, which unlike high/low  
> unambiguously refer to the lower address and the higher address.  Of  
> course, ADLC can make high/low be aliases for second/first on little  
> endian systems and first/second on big endian systems.  Even better  
> would be avoiding high/low, because in different contexts those words  
> can seem to refer to arithmetic significance or memory order, and only  
> on little-endian systems do those concepts (accidentally) align.  When  
> talking about arithmetic significance, msw/lsw is probably better than  
> high/low.
> 
> For register contents, we already use high/low in some places, and it  
> twists around our endian story, causing bugs when the system is  
> changed.  Those points are (loosely speaking) portability problems  
> embedded in the system due to early sloppiness, since HotSpot was  
> originally an x86-only system.  Let's make it better over time.
> 
> The "gold standard" would be to use a single terminology for word  
> pairs that is consistent across memory and register values.  Since  
> memory values are copied around in the JVM more than they are used for  
> 2s-complement arithmetic, first/second is far more helpful than msw/ 
> lsw.  And high/low is just ambiguous.

Thanks for pointing that out, and you're absolutely right.  CACAO
supported about 11 different architectures and I know what you mean by
"endian portability problems".

first/second is probably a bit more confusing when seen for the first
time than lsw/msw would be, but I'm fine with the former too.  Is there
already something like first/second around?

-- Christian