SSA and implicit uses.

[Sanjiv Kumar Gupta]

--- Joe Buck <Joe.Buck@synopsys.COM> wrote:

> On Tue, Nov 02, 2004 at 06:40:35AM -0800, Sanjiv
> Kumar Gupta wrote:
> > For statements like;
> >   a = 0;
> >   a++;
> >   a++;
> > 
> > My compiler generates SSA form like below:
> >   r200 = 0;
> >   r201 = r200 + 1;
> >   r202 = r201 + 1;
> > 
> > now all of r200, r201 and r202 are independent
> live
> > ranges and may get assigned different hard regs.
> 
> Actually, the above will quickly be turned into
> 
>     r200 = 0;
>     r201 = 1;
>     r202 = 2;
> 
> and you'll end up not using *any* registers.  Of
> course, you
> could construct another example that would better
> illustrate
> your point.
> 
Okay. Consider initial value of 'a' as unknown.

The point is that, when a single var gets splitted
into mulitple live ranges (and different hard regs
consequently), the code generated is worse than
what could have been generated if the original
var was kept into a single hard reg.

--Sanjiv

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