Dropping of old loop optimizer

[Daniel Berlin]

On Thursday, February 27, 2003, at 12:13  PM, Pop Sébastian wrote:

> On Thu, Feb 27, 2003 at 05:39:17PM +0100, Zdenek Dvorak wrote:
>>
>> The problem I see is that rtl ssa is broken (or never worked?); but
> Last time I had a close look at rtl was about two years ago :-)
> Dan is the expert to ask about the status of SSA at rtl level (he 
> adapted
> the CCP code from rtl to tree-ssa).
The problem with RTL SSA is hard registers and subregs.
There are hard registers assigned in certain cases during initial RTL 
generation.
These can't be renamed, obviously.
You run into difficulties with subregs as well (in relation to liveness 
and renaming).

Thus, one has the following options:
1. Ignore them while creating SSA, and make sure optimizers don't touch 
them. (not clean)
2. Perform magic to save and restore the pieces of code using them. 
(not clean, not easy)
3. Don't generate hard registers until after SSA is done with (not all 
that easy, but could be done), and don't ever generate subregs (not 
really possible without severely rearchitecting RTL).

The reason we need subregs is because of how we do registers.
Most compilers do the reverse of what we do in relation to registers.
We have a single operator, like SET, and multiple register modes/sizes 
that can be operands.
So to get a full register copy, one simply does something like
(set (reg:SI 50) (reg:SI 52))

And to get a piece of a register, one needs a subregister operator.

like
(set (reg:SI 50) (subreg:SI (reg:DI 51) 0))

Most compilers have multiple operators (one for each size), and no 
register sizes.
So to get a full register copy, they do:
(set:DIDI (reg 50) (reg 51))
and to get a piece of a register, they do
(set:SIDI (reg 50) (reg 51))

Now consider trying to set a piece of a register

we would do:

(set (subreg:SI (reg:DI 51)) (reg:SI 50))

which is very hard to think about how to rename into SSA (because it's 
a partial assignment, and the subreg itself has no name)

while they would do:
(set:DISI (reg 51) (reg 50))

which is still a full def, and easy to rename.

Get it?


>
>> perhaps we could do this kind of optimizations just on trees (that
>> unfortunately also aren't ready for serious usage yet, right?)?
>>
> CCP (Conditional constant propagation) and
> DCE (Dead code elimination)
> seem to work fine on tree-SSA.
>

and PRE *will* work fine just as soon as Andrew finishes and posts the 
insertion code.
--Dan