https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114991
Alex Coplan <acoplan at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2024-05-09
Status|UNCONFIRMED |NEW
CC| |acoplan at gcc dot gnu.org,
| |vmakarov at gcc dot gnu.org
Ever confirmed|0 |1
Keywords| |missed-optimization, ra
--- Comment #1 from Alex Coplan <acoplan at gcc dot gnu.org> ---
Confirmed. There is a lot to unpack here. Of course, the include isn't needed
in this testcase and the problem can be seen more clearly with a slightly
smaller array size:
typedef struct { int arr[16]; } S;
void g (S *);
void h (S);
void f(int x)
{
S s;
g (&s);
h (s);
}
In this case sizeof(S) = 64 so we should be able to do the copy with 2 LDPs + 2
STPs.
So just for clarity, the missed ldp/stp started when we turned off the early
ldp/stp formation in memcpy expansion, i.e. with
r14-9373-g19b23bf3c32df3cbb96b3d898a1d7142f7bea4a0 .
However, things already started to regress earlier for this testcase with
r14-4944-gf55cdce3f8dd8503e080e35be59c5f5390f6d95e i.e.
commit f55cdce3f8dd8503e080e35be59c5f5390f6d95e
Author: Vladimir N. Makarov <vmaka...@redhat.com>
Date: Thu Oct 26 14:50:40 2023
[RA]: Modfify cost calculation for dealing with equivalences
before that RA change we get:
f:
stp x29, x30, [sp, -144]!
mov x29, sp
add x0, sp, 80
bl g
ldp q29, q28, [sp, 80]
add x0, sp, 16
ldp q31, q30, [sp, 112]
stp q29, q28, [sp, 16]
stp q31, q30, [sp, 48]
bl h
ldp x29, x30, [sp], 144
ret
and afterwards we get:
f:
stp x29, x30, [sp, -160]!
mov x29, sp
str x19, [sp, 16]
add x19, sp, 96
mov x0, x19
bl g
add x0, sp, 32
ldp q29, q28, [x19]
ldp q31, q30, [x19, 32]
stp q29, q28, [x0]
stp q31, q30, [x0, 32]
bl h
ldr x19, [sp, 16]
ldp x29, x30, [sp], 160
ret
which is really not great as now we have a save/restore of x19 and the accesses
end up using different (non-sp) registers which I suspect doesn't help with the
ldp/stp formation (on trunk).
I will try to give a detailed analysis on what goes wrong with the ldp/stp
formation at the RTL level shortly (there are a lot of different issues), but I
think that RA change is a contributing factor.
