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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/49263] SH Target: underutilized "TST #imm, R0"
 instruction
Date: Sun, 28 May 2023 10:48:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D49263

--- Comment #44 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Alexander Klepikov from comment #43)
>=20
> Well, not really. Look what's happening during expand pass when 'ashrsi3'=
 is
> expanding. Function 'expand_ashiftrt' is called and what it does at the e=
nd
> - it explicitly emits 3 insns:
> [...]

>=20
> By the way, right shift for integers expands to only one 'lshiftrt' insn =
and
> that's why it can be catched and converted to 'tst'.
>=20

Yeah, I might have dropped the ball on the right shift patterns back then a=
nd
only reworked the left shift patterns to do that.=20


>=20
> As far as I understand these insns could be catched later by a peephole a=
nd
> converted to 'tstsi_t' insn like it is done for other much simple insn
> sequences.

It's the combine RTL pass and split1 RTL pass that does most of this work h=
ere.
 Peephole pass in GCC is too simplistic for this.


>=20
> Thank you for your time and detailed explanations! I agree with you on all
> points. Software cannot be perfect and it's OK for GCC not to be super
> optimized, so this part better sholud be left intact.

We can't have it perfect, but we can try ;)

>=20
> By the way, I tried to link library to my project and I figured out that
> linker is smart enough to link only necessary library functions even with=
out
> LTO. So increase in size is about 100 or 200 bytes, that is acceptable.
> Thank you very much for help!

You're welcome.

Yes, to strip out unused library functions it doesn't need LTO.  But using =
LTO
for embedded/MCU  firmware, I find it can reduce the code size by about 20%=
.=20
For example, it can also inline small library functions in your code (if the
library was also compiled with LTO).=