GCC won`t emit my instriction

Jim Wilson jimw@sifive.com
Mon Jun 10 22:20:00 GMT 2019


On Sun, Jun 9, 2019 at 11:15 PM Alex Hill via gcc-help
<gcc-help@gcc.gnu.org> wrote:
> I found some decision, i used flags -fexceptions -fnon-call-exceptions and it emits max insn, now i try to understand why this happens.
> Here is the max it emits:
> (define_insn "smax<GPR:mode>3"
>     [(set (match_operand:GPR 0 "register_operand" "=r")
>         (smax:GPR (match_operand:GPR 1 "register_operand" " r")
>         (match_operand:GPR 2 "register_operand" " r")))]
> ""
> "max\t%0,%1,%2"
> [(set_attr "type" "move")
> (set_attr "mode" "<MODE>")])

There is some code in noce_try_minmax in ifcvt.c that will try to
recognize an if statement in RTL that performs min/max and try to
convert it to a direct min/max operation.  However, in general, you
are more likely to get a min/max operation if you recognize it when
parsing, and then carry it all of the way through the optimizer.  To
make that work, you need min/max named patterns, that emit min/max
operations in RTL.  Like what you have immediately above.  If trying
to understand how this works, look at the debugging dumps you can get
with -fdump-tree-all and -fdump-rtl-all when compiling a trivial
testcase.  And maybe compare with another CPU target that already has
working min/max support.

Jim



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