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From: "stammark at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/109697] New: arm: lack of MVE instruction costing
 causing worse codegen on a vec_duplicate
Date: Tue, 02 May 2023 12:11:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109697

            Bug ID: 109697
           Summary: arm: lack of MVE instruction costing causing worse
                    codegen on a vec_duplicate
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stammark at gcc dot gnu.org
  Target Milestone: ---

Hi all,

In the arm backend, for MVE targets we previously had this bug on the vcmp
patterns: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107987

The fix is fine, but it resulted in some failing tests:
        * gcc.target/arm/mve/intrinsics/vcmpcsq_n_u16.c
        * gcc.target/arm/mve/intrinsics/vcmpcsq_n_u32.c
        * gcc.target/arm/mve/intrinsics/vcmpcsq_n_u8.c
        * gcc.target/arm/mve/intrinsics/vcmpeqq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpeqq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmpeqq_n_u16.c
        * gcc.target/arm/mve/intrinsics/vcmpeqq_n_u32.c
        * gcc.target/arm/mve/intrinsics/vcmpeqq_n_u8.c
        * gcc.target/arm/mve/intrinsics/vcmpgeq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpgeq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmpgtq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpgtq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmphiq_n_u16.c
        * gcc.target/arm/mve/intrinsics/vcmphiq_n_u32.c
        * gcc.target/arm/mve/intrinsics/vcmphiq_n_u8.c
        * gcc.target/arm/mve/intrinsics/vcmpleq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpleq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmpltq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpltq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmpneq_n_f16.c
        * gcc.target/arm/mve/intrinsics/vcmpneq_n_f32.c
        * gcc.target/arm/mve/intrinsics/vcmpneq_n_u16.c
        * gcc.target/arm/mve/intrinsics/vcmpneq_n_u32.c
        * gcc.target/arm/mve/intrinsics/vcmpneq_n_u8.c
(after Andrea improved these tests in GCC13)

The testcases that are failing are the ones that compare against a scalar
immediate (e.g. "vcmpeqq (a, 1.1)"), because the compiler prefers to do:
```
        vldr.64 d6, .L5
        vldr.64 d7, .L5+8
        vcmp.f16        eq, q0, q3
```
When previously we would much more simply:
```
        movs    r3, #1
        vcmp.u16        cs, q0, r3
```

The underlying reason for this change is a known deficiency of the MVE
implementation: the lack of proper instruction costing.
The compiler falls back to calculating costs based on the operands and the =
new
vec_duplicate in the patterns (mve_vcmp<mve_cmp_op>q_n_<mode>, etc) gets gi=
ven
a cost of 32 (when instead it should know that the vec duplicate is free and
this is all just one instruction...), so the "literal load + vector-vector
compare" wins out against the "put the immediate in a GP reg + vector-scalar
compare".
For now, I plan on simply XFAIL-ing the tests.=