On Tue, Jun 20, 2017 at 11:57:59AM +0100, Wilco Dijkstra wrote:
> Improve the dup pattern to prefer vector registers. When doing a dup
> after a load, the register allocator thinks the costs are identical
> and chooses an integer load. However a dup from an integer register
> includes an int->fp transfer which is not modelled. Adding a '?' to
> the integer variant means the cost is increased slightly so we prefer
> using a vector register. This improves the following example:
>
> #include <arm_neon.h>
> void f(unsigned *a, uint32x4_t *b)
> {
> b[0] = vdupq_n_u32(a[1]);
> b[1] = vdupq_n_u32(a[2]);
> }
>
> Before:
> ldr w2, [x0, 4]
> dup v0.4s, w2
> str q0, [x1]
> ldr w0, [x0, 8]
> dup v0.4s, w0
> str q0, [x1, 16]
> ret
>
> After:
> ldr s0, [x0, 4]
> dup v0.4s, v0.s[0]
> str q0, [x1]
> ldr s0, [x0, 8]
> dup v0.4s, v0.s[0]
> str q0, [x1, 16]
> ret
>
> Passes regress & bootstrap, OK for commit?
>
> ChangeLog:
> 2017-06-20 Wilco Dijkstra <wdijk...@arm.com>
>
> * config/aarch64/aarch64-simd.md (aarch64_simd_dup):
> Swap alternatives, make integer dup more expensive.
Have you tested this in cases where an integer dup is definitely the right
thing to do?
e.g. in
#include <arm_neon.h>
void f(unsigned a, unsigned b, uint32x4_t *c)
{
c[0] = vdupq_n_u32(a);
c[1] = vdupq_n_u32(b);
}
And similar cases? If these still look good, then the patch is OK - though
I'm still very nervous about the register allocator cost model!
Thanks,
James
