Re: A reload inheritance bug

[Bernd Schmidt <bernds_cb1 at t-online dot de>]

Mark Shinwell wrote:
> This code is guarded by
> 
>       /* I is nonneg if this reload used a register.
>      If rld[r].reg_rtx is 0, this is an optional reload
>      that we opted to ignore.  */
> 
>       if (i >= 0 && rld[r].reg_rtx != 0)
> 
> and in this [my original] case, i is negative (see my original patch).
> 
> I don't understand the "I is nonneg ..." comment above: the surrounding
> code seems to say that i is actually non-negative if the reload used a
> _spill_ register (rather than just any register).  Bernd, could you
> clarify the precise meaning of "spill register" in this context?  I've
> not yet managed to completely pin this down.

The term "spill register" is a bit of a historical artifact, and I think
we may be getting close to the root cause here (and it seems to be my
fault.)
In the early days of gcc, when a register was spilled, it was spilled
for the whole function and became a spill register that reload could use
freely.  That means that none of these spill registers would be selected
during find_reloads, as they don't appear in the insn anymore.  These
days, we are more selective and don't spill hard regs for the entire
function.
It appears that spill_reg_index is only set up for registers that go
through the choose_reload_regs process, not for the ones selected during
find_reloads.  That's probably a bad thing, as a reg_rtx chosen in
find_reloads could be used as a spill reg in a previous insn (as in your
example).


Bernd
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