https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121240
Bug ID: 121240
Summary: missing support for section anchors to FP or vector
constants
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tnfchris at gcc dot gnu.org
Target Milestone: ---
Couldn't find an existing ticket for this. but the following example
const double b[4] = {0.2435334343f, 0.2233535343f, 0.4232433f, 0.34343434f};
typedef double v2df __attribute__ ((vector_size (16)));
typedef double v2df __attribute__ ((vector_size (16)));
v2df f (v2df c1, v2df c2)
{
v2df a1 = *(v2df *)&b[0];
v2df a2 = *(v2df *)&b[2];
return (a1 * c1) + (a2 * c2);
}
at -O3 on AArch64 generates:
f:
adrp x0, .LC0
ldr q30, [x0, #:lo12:.LC0]
adrp x0, .LC1
ldr q31, [x0, #:lo12:.LC1]
fmul v1.2d, v1.2d, v30.2d
fmla v1.2d, v0.2d, v31.2d
mov v0.16b, v1.16b
ret
.global b
.section .rodata.cst16,"aM",@progbits,16
.align 4
.LC0:
.word 536870912
.word 1071322731
.word 0
.word 1070987988
.align 4
.LC1:
.word -2147483648
.word 1070541850
.word 1073741824
.word 1070372569
where we generate two different labels instead of using
an anchor + offset.
This means on functions using lots of FP or vector constants we
do quite a lot of unneeded address computations and we miss pairwise
loads because all the constants have different bases.
I wonder whether this can be implemented by extending force_const_mem to
support "global" constant pools and re-using the target hooks from
use_anchored_address
The expected code to come out of this is the same as
v2df f (v2df c1, v2df c2)
{
double *l = &b[0];
asm volatile ("" : "+r"(l));
v2df a1 = *(v2df *)l;
v2df a2 = *(v2df *)(l+2);
return (a1 * c1) + (a2 * c2);
}
which generates:
f:
adrp x0, .LANCHOR0
add x0, x0, :lo12:.LANCHOR0
ldp q31, q30, [x0]
fmul v1.2d, v1.2d, v30.2d
fmla v1.2d, v31.2d, v0.2d
mov v0.16b, v1.16b
ret
.global b
.section .rodata
.align 4
.set .LANCHOR0,. + 0
.type b, %object
.size b, 32
b:
.word -2147483648
.word 1070541850
.word 1073741824
.word 1070372569
.word 536870912
.word 1071322731
.word 0
.word 1070987988
