On 17/05/18 09:46, Kyrill Tkachov wrote:


On 15/05/18 18:56, Richard Sandiford wrote:
Kyrill  Tkachov <kyrylo.tkac...@foss.arm.com> writes:
Hi all,

We've a deficiency in our vec_set family of patterns.  We don't
support directly loading a vector lane using LD1 for V2DImode and all
the vector floating-point modes.  We do do it correctly for the other
integer vector modes (V4SI, V8HI etc) though.

The alternatives on the relative floating-point patterns only allow a
register-to-register INS instruction.  That means if we want to load a
value into a vector lane we must first load it into a scalar register
and then perform an INS, which is wasteful.

There is also an explicit V2DI vec_set expander dangling around for no
reason that I can see. It seems to do the exact same things as the
other vec_set expanders. This patch removes that.  It now unifies all
vec_set expansions into a single "vec_set<mode>" define_expand using
the catch-all VALL_F16 iterator.

I decided to leave two aarch64_simd_vec_set<mode> define_insns. One
for the integer vector modes (that now include V2DI) and one for the
floating-point vector modes. That is so that we can avoid specifying
"w,r" alternatives for floating-point modes in case the
register-allocator gets confused and starts gratuitously moving
registers between the two banks.  So the floating-point pattern only
two alternatives, one for SIMD-to-SIMD INS and one for LD1.
Did you see any cases in which this was necessary?  In some ways it
seems to run counter to Wilco's recent patches, which tended to remove
the * markers from the "unnatural" register class and trust the register
allocator to make a sensible decision.

I think our default position should be trust the allocator here.
If the consumers all require "w" registers then the RA will surely
try to use "w" registers if at all possible.  But if the consumers
don't care then it seems reasonable to offer both, since in those
cases it doesn't really make much difference whether the payload
happens to be SF or SI (say).

There are also cases in which the consumer could actively require
an integer register.  E.g. some code uses unions to bitcast floats
to ints and then do bitwise arithmetic on them.


Thanks, that makes sense. Honestly, it's been a few months since I worked on 
this patch.
I believe my reluctance to specify that alternative was that it would mean 
merging the integer and
floating-point patterns into one (like the attached version) which would put the "w, 
r" alternative
first for the floating-point case. I guess we should be able to trust the 
allocator to pick
the sensible  alternative though.


With some help from Wilco I can see how this approach will give us suboptimal 
code though.
If we modify the example from my original post to be:
v4sf
foo_v4sf (float *a, float *b, float *c, float *d)
{
    v4sf res = { *a, b[2], *c, *d };
    return res;
}

The b[2] load will load into a GP register then do an expensive INS into the 
SIMD register
instead of loading into an FP S-register and then doing a SIMD-to-SIMD INS.
The only way I can get it to use the FP load then is to mark the "w, r" 
alternative with a '?'

Kyrill


This version is then made even simpler due to all the vec_set patterns being 
merged into one.
Bootstrapped and tested on aarch64-none-linux-gnu.

Is this ok for trunk?

Thanks,
Kyrill

2018-05-17  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    * config/aarch64/aarch64-simd.md (vec_set<mode>): Use VALL_F16 mode
    iterator.  Delete separate integer-mode vec_set<mode> expander.
    (aarch64_simd_vec_setv2di): Delete.
    (vec_setv2di): Delete.
    (aarch64_simd_vec_set<mode>): Delete all other patterns with that name.
    Use VALL_F16 mode iterator.  Add LD1 alternative and use vwcore for
    the "w, r" alternative.

2018-05-17  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    * gcc.target/aarch64/vect-init-ld1.c: New test.

With this patch we avoid loading values into scalar registers and then
doing an explicit INS on them to move them into the desired vector
lanes. For example for:

typedef float v4sf __attribute__ ((vector_size (16)));
typedef long long v2di __attribute__ ((vector_size (16)));

v2di
foo_v2di (long long *a, long long *b)
{
    v2di res = { *a, *b };
    return res;
}

v4sf
foo_v4sf (float *a, float *b, float *c, float *d)
{
    v4sf res = { *a, *b, *c, *d };
    return res;
}

we currently generate:

foo_v2di:
          ldr     d0, [x0]
          ldr     x0, [x1]
          ins     v0.d[1], x0
          ret

foo_v4sf:
          ldr     s0, [x0]
          ldr     s3, [x1]
          ldr     s2, [x2]
          ldr     s1, [x3]
          ins     v0.s[1], v3.s[0]
          ins     v0.s[2], v2.s[0]
          ins     v0.s[3], v1.s[0]
          ret

but with this patch we generate the much cleaner:
foo_v2di:
          ldr     d0, [x0]
          ld1     {v0.d}[1], [x1]
          ret

foo_v4sf:
          ldr     s0, [x0]
          ld1     {v0.s}[1], [x1]
          ld1     {v0.s}[2], [x2]
          ld1     {v0.s}[3], [x3]
          ret
Nice!  The original reason for:

   /* FIXME: At the moment the cost model seems to underestimate the
      cost of using elementwise accesses.  This check preserves the
      traditional behavior until that can be fixed.  */
   if (*memory_access_type == VMAT_ELEMENTWISE
       && !STMT_VINFO_STRIDED_P (stmt_info)
       && !(stmt == GROUP_FIRST_ELEMENT (stmt_info)
       && !GROUP_NEXT_ELEMENT (stmt_info)
       && !pow2p_hwi (GROUP_SIZE (stmt_info))))
     {
       if (dump_enabled_p ())
    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
             "not falling back to elementwise accesses\n");
       return false;
     }

was that we seemed to be too optimistic about how cheap it was to
construct a vector from scalars.  Maybe this patch brings the code
closer to the cost (for AArch64 only of course).

FWIW, the patch looks good to me bar the GPR/FPR split.

Thanks,
Richard


Reply via email to