On Jun 18, 2007, at 10:29 PM, Chris Lattner wrote:
>
> Hi Dan,
>
> I'm sorry to be such a pain, but this seems like a step backward.
> We've gone from having an explicit flag in the .td files to having
> another magic table in the .cpp file (somewhat amusing because
> 'duplicable' just made the opposite transition).
>
> How about this proposal (Obviously feel free to pick better names for
> these things):
>
> 1. Reintroduce the 'isremat-able' flag, set it to true for all the
> instructions that are *potentially* rematerializable.
> 2. Add a virtual target hook that can override the flag:
> "TII::isReallyRematerializable(Machineinstr*)".
> 3. Introduce a new non-virtual method:
> bool TII::isRematerializable(Machineinstr *MI) {
> return MI->flags->isrematable && isReallyRematerializable(MI);
> }
>
> This achieves two things:
>
> 1. Just looking at the .td file, you can tell which instructions are
> candidates for remat.
> 2. The isRematerializable predicate is faster for instructions that
> are not remat-able.
> 3. The isReallyRematerializable only needs to be implemented by
> targets with instructions that are remat-able only in some cases
> (like the x86 instructions).
I okay'd Dan patch after considering the trade-offs. To me this gets
rid of the duplicate instructions so it's worth it. Not to mention I
had already considered the "trivial rematerialization" scheme to be
temporary.
If we are really concerned about the speed, then I agree the hybrid
approach is the best. Sorry about the confusion.
Evan
>
> To me, #1 is the killer feature. In general, I'd like to move away
> from having tables (either explicit, like the one in
> X86RegisterInfo::foldMemoryOperand, or just big switch stmts) to
> having properties on .td file entries. That makes it much more clear
> what is going on when inspecting the .td files.
>
> I'm sorry I didn't look at your patch when you asked for comments,
> but does this proposal sound sane?
>
> -Chris
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