Am 08.06.24 um 11:32 schrieb Mikael Pettersson via Gcc:
On Thu, Jun 6, 2024 at 8:59 PM Dimitar Dimitrov <dimi...@dinux.eu> wrote:
Have you tried defining TARGET_LEGITIMIZE_ADDRESS for your target? From
a quick search I see that the iq2000 and rx backends are rewriting some
PLUS expression addresses with insn sequence to calculate the address.
I have partial success.
The key was to define both TARGET_LEGITIMATE_ADDRESS_P and an
addptr<Pmode>3 insn.
I had tried TARGET_LEGITIMATE_ADDRESS_P before, together with various
combinations of TARGET_LEGITIMIZE_ADDRESS and
LEGITIMIZE_RELOAD_ADDRESS, but they all threw gcc into reload loops.
My add<Pmode>3 insn clobbers the CC register. The docs say to define
addptr<Pmode>3 in this case, and that eliminated the reload loops.
The issue now is that the machine cannot perform an add without
clobbering the CC register, so I'll have to hide that somehow. When
emitting the asm code, can one check if the CC register is LIVE-OUT
from the insn? If it isn't I shouldn't have to generate code to
preserve it.
/Mikael
There is a different approach like liken by AVR (and maybe some
more targets):
Don't introduce CC until after reload, i.e. keep cbranch insns
and only split them to compare + branch after reload in the
first post reload split pass.
It's some effort because the number of insns is effectively
doubled: One pre-reload version of the insn without CC,
and a postreload version with CC. On AVR, most insns don't
set CCmode is a usable way, so that works, though not as
well like the killed cc0 representation.
Then I am not sure whether TARGET_LEGITIMIZE_ADDRESS works in
all situations, in particular when it comes to accessing frame
locations. It might be required to fake-supply reg+offset
addressing, and then split that post-reload.
An example is the Reduced Tiny cores (-mavrtiny) of the AVR
port that only support POST_INC or byte addressing. Splitting
locations after reload improved code quality a lot.
But it also revealed an unfixed bug in DSE:
DSE does not track frame pointer changes and eliminates
living stores. So be prepared.
Johann
