On Mon, Nov 12, 2018 at 12:53:26PM +0000, Andrew Stubbs wrote:
> >>+/* Implement TARGET_LEGITIMATE_COMBINED_INSN.
> >>+
> >>+ Return false if the instruction is not appropriate as a combination
> >>of two
> >>+ or more instructions. */
> >>+
> >>+bool
> >>+gcn_legitimate_combined_insn (rtx_insn *insn)
> >>+{
> >>+ rtx pat = PATTERN (insn);
> >>+
> >>+ /* The combine pass tends to strip (use (exec)) patterns from insns.
> >>This
> >>+ means it basically switches everything to use the *_scalar form of
> >>the
> >>+ instructions, which is not helpful. So, this function disallows
> >>such
> >>+ combinations. Unfortunately, this also disallows combinations of
> >>genuine
> >>+ scalar-only patterns, but those only come from explicit expand code.
> >>+
> >>+ Possible solutions:
> >>+ - Invent TARGET_LEGITIMIZE_COMBINED_INSN.
> >>+ - Remove all (use (EXEC)) and rely on md_reorg with "exec"
> >>attribute.
> >>+ */
> >This seems a bit hokey. Why specifically is combine removing the USE?
>
> I don't understand combine fully enough to explain it now, although at
> the time I wrote this, and in a GCC 7 code base, I had followed the code
> through and observed what it was doing.
>
> Basically, if you have two patterns that do the same operation, but one
> has a "parallel" with an additional "use", then combine will tend to
> prefer the one without the "use". That doesn't stop the code working,
> but it makes a premature (accidental) decision about instruction
> selection that we'd prefer to leave to the register allocator.
>
> I don't recall if it did this to lone instructions, but it would
> certainly do so when combining two (or more) instructions, and IIRC
> there are typically plenty of simple moves around that can be easily
> combined.
If you don't want useless USEs deleted, use UNSPEC_VOLATILE instead?
Or actually use the register, i.e. as input to an actually needed
instruction.
If combine is changing an X and a USE to just that X if it can, combine
is doing a great job!
(combine cannot "combine" one instruction, fwiw; this sometime could be
useful (so just run simplification on every single instruction, see if
that makes a simpler valid instruction; and indeed a common case where it
can help is if the insn is a parallel and one of the arms of that isn't
needed).
Segher
