Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Wed, Jul 5, 2023 at 8:44 AM Hao Liu OS via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Hi,
>>
>> If a loop is unrolled by n times during vectoriation, two steps are used to
>> calculate the induction variable:
>> - The small step for the unrolled ith-copy: vec_1 = vec_iv + (VF/n * Step)
>> - The large step for the whole loop: vec_loop = vec_iv + (VF * Step)
>>
>> This patch calculates an extra vec_n to replace vec_loop:
>> vec_n = vec_prev + (VF/n * S) = vec_iv + (VF/n * S) * n = vec_loop.
>>
>> So that we can save the large step register and related operations.
>
> OK. It would be nice to avoid the dead stmts created earlier though.
FWIW, I still don't think we should do this. Part of the point of
unrolling is to shorten loop-carried dependencies, whereas this patch
is going in the opposite direction.
Richard
>
> Thanks,
> Richard.
>
>> gcc/ChangeLog:
>>
>> PR tree-optimization/110449
>> * tree-vect-loop.cc (vectorizable_induction): use vec_n to replace
>> vec_loop for the unrolled loop.
>>
>> gcc/testsuite/ChangeLog:
>>
>> * gcc.target/aarch64/pr110449.c: New testcase.
>> ---
>> gcc/testsuite/gcc.target/aarch64/pr110449.c | 40 +++++++++++++++++++++
>> gcc/tree-vect-loop.cc | 21 +++++++++--
>> 2 files changed, 58 insertions(+), 3 deletions(-)
>> create mode 100644 gcc/testsuite/gcc.target/aarch64/pr110449.c
>>
>> diff --git a/gcc/testsuite/gcc.target/aarch64/pr110449.c
>> b/gcc/testsuite/gcc.target/aarch64/pr110449.c
>> new file mode 100644
>> index 00000000000..bb3b6dcfe08
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/aarch64/pr110449.c
>> @@ -0,0 +1,40 @@
>> +/* { dg-do compile } */
>> +/* { dg-options "-Ofast -mcpu=neoverse-n2 --param
>> aarch64-vect-unroll-limit=2" } */
>> +/* { dg-final { scan-assembler-not "8.0e\\+0" } } */
>> +
>> +/* Calcualte the vectorized induction with smaller step for an unrolled
>> loop.
>> +
>> + before (suggested_unroll_factor=2):
>> + fmov s30, 8.0e+0
>> + fmov s31, 4.0e+0
>> + dup v27.4s, v30.s[0]
>> + dup v28.4s, v31.s[0]
>> + .L6:
>> + mov v30.16b, v31.16b
>> + fadd v31.4s, v31.4s, v27.4s
>> + fadd v29.4s, v30.4s, v28.4s
>> + stp q30, q29, [x0]
>> + add x0, x0, 32
>> + cmp x1, x0
>> + bne .L6
>> +
>> + after:
>> + fmov s31, 4.0e+0
>> + dup v29.4s, v31.s[0]
>> + .L6:
>> + fadd v30.4s, v31.4s, v29.4s
>> + stp q31, q30, [x0]
>> + add x0, x0, 32
>> + fadd v31.4s, v29.4s, v30.4s
>> + cmp x0, x1
>> + bne .L6 */
>> +
>> +void
>> +foo2 (float *arr, float freq, float step)
>> +{
>> + for (int i = 0; i < 1024; i++)
>> + {
>> + arr[i] = freq;
>> + freq += step;
>> + }
>> +}
>> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
>> index 3b46c58a8d8..706ecbffd0c 100644
>> --- a/gcc/tree-vect-loop.cc
>> +++ b/gcc/tree-vect-loop.cc
>> @@ -10114,7 +10114,7 @@ vectorizable_induction (loop_vec_info loop_vinfo,
>> new_vec, step_vectype, NULL);
>>
>> vec_def = induc_def;
>> - for (i = 1; i < ncopies; i++)
>> + for (i = 1; i < ncopies + 1; i++)
>> {
>> /* vec_i = vec_prev + vec_step */
>> gimple_seq stmts = NULL;
>> @@ -10124,8 +10124,23 @@ vectorizable_induction (loop_vec_info loop_vinfo,
>> vec_def = gimple_convert (&stmts, vectype, vec_def);
>>
>> gsi_insert_seq_before (&si, stmts, GSI_SAME_STMT);
>> - new_stmt = SSA_NAME_DEF_STMT (vec_def);
>> - STMT_VINFO_VEC_STMTS (stmt_info).safe_push (new_stmt);
>> + if (i < ncopies)
>> + {
>> + new_stmt = SSA_NAME_DEF_STMT (vec_def);
>> + STMT_VINFO_VEC_STMTS (stmt_info).safe_push (new_stmt);
>> + }
>> + else
>> + {
>> + /* vec_1 = vec_iv + (VF/n * S)
>> + vec_2 = vec_1 + (VF/n * S)
>> + ...
>> + vec_n = vec_prev + (VF/n * S) = vec_iv + VF * S = vec_loop
>> +
>> + vec_n is used as vec_loop to save the large step register
>> and
>> + related operations. */
>> + add_phi_arg (induction_phi, vec_def, loop_latch_edge (iv_loop),
>> + UNKNOWN_LOCATION);
>> + }
>> }
>> }
>>
>> --
>> 2.34.1
