On 12/24/23 05:24, Roger Sayle wrote:
What's exceedingly weird is T_N_T_M_P (DImode, SImode) isn't
actually a truncation! The output precision is first, the input
precision is second. The docs explicitly state the output precision
should be smaller than the input precision (which makes sense for truncation).
That's where I'd start with trying to untangle this mess.
Thanks (both) for correcting my misunderstanding.
At the very least might I suggest that we introduce a new
TRULY_NOOP_EXTENSION_MODES_P target hook that MIPS can use for this
purpose? It'd help reduce confusion, and keep the
documentation/function naming correct.
Yes. It is good for me.
T_N_T_M_P is a really confusion naming.
Ignore my suggestion for a new target hook. GCC already has one.
You shouldn't be using TRULY_NOOP_TRUNCATION_MODES_P
with incorrectly ordered arguments. The correct target hook is
TARGET_MODE_REP_EXTENDED, which the MIPS backend correctly
defines via mips_mode_rep_extended.
It's MIPS definition of (and interpretation of) mips_truly_noop_truncation
that's suspect.
My latest theory is that these sign extensions should be:
(set (reg:DI) (sign_extend:DI (truncate:SI (reg:DI))))
and not
(set (reg:DI) (sign_extend:DI (subreg:SI (reg:DI))))
In isolation these are the same. I think the fact that the MIPS backend
wipes out the sign extension turning the result into a NOP is what makes
them different.
Of course that's kind of the point behind the TRULY_NOOP_TRUNCATION
macro. That's what allows the MIPS target to wipe out the sign extension.
ISTM this might be worth noting in the docs for TRULY_NOOP_TRUNCATION.
Jeff
