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Re: Question about constraints and reload


On Sat, 25 Sep 2004, Steven Bosscher wrote:

> On Saturday 25 September 2004 16:56, Ulrich Weigand wrote:
> > Steven Bosscher wrote:
> > >What is this necessary for, is there any reason to rely on reload to
> > >turn a constant into a MEM?
> >
> > Certainly!  Reload will, as one of its options, force a constant to
> > the literal pool, thereby turning the operand into a MEM.  The s390
> > port relies extensively on that feature, because we can only use a
> > limited set of immediate operands, and these differ widely between
> > different instructions.  Thus the best way to handle constants is
> > to allow them more or less freely before reload, and have reload
> > choose to use either an actual immediate operand where available,
> > reload the constant into a register, or else push it into the
> > literal pool.
> 
> Maybe it's the *easiest* way for your port and other ports to handle
> constants this way, but it may not be the "best way" ;-)  I suppose
> you would want to keep constants around mostly for optimization, but
> do we still need that, if the tree optimizers do most of the work?
>
> (bits removed for brevity)
>
> Perhaps we can experiment with putting non-legitimate constants into the
> constant pool and turn them into MEMs somewhere *before* greg and reload.
> Perhaps even already somewhere late in the tree optimizers (turn them 
> into CONST_DECLs), or otherwise in some pre-regalloc pass.

This fits nicely into the midRTL/targetRTL paradigm.

The register allocator will still need to generate literal pool entries
for accessing deep stacks on machines with limited displacements on
indirect addressing modes, however.

> Gr.
> Steven

Toshi



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