Hi, Richi.

>> So how can we resolve the issue when a non-VL operation like
>> .VEC_EXTRACT is used for _len support?

Do you mean non-VL extract last operation (I am sorry that not sure whether I understand your question correctly)? 
If yes, the answer is for RVV, we are reusing the same flow as ARM SVE (BIT_FILED_REF approach), see the example below:

https://godbolt.org/z/cqrWrY8q4 

#define EXTRACT_LAST(TYPE)          \
  TYPE __attribute__ ((noinline, noclone))  \
  test_##TYPE (TYPE *x, int n, TYPE value)  \
  {                     \
    TYPE last;                  \
    for (int j = 0; j < 64; ++j)            \
      {                     \
    last = x[j];                \
    x[j] = last * value;            \
      }                     \
    return last;                \
  }

#define TEST_ALL(T)             \
  T (uint8_t)                   \

TEST_ALL (EXTRACT_LAST)

  vect_cst__22 = {value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D), value_12(D)};
  vect_last_11.6_3 = MEM <vector(64) unsigned char> [(uint8_t *)x_10(D)];
  vect__4.7_23 = vect_last_11.6_3 * vect_cst__22;
  MEM <vector(64) unsigned char> [(uint8_t *)x_10(D)] = vect__4.7_23;
  _21 = BIT_FIELD_REF <vect_last_11.6_3, 8, 504>;

This approach works perfectly for both RVV and ARM SVE for non-VL and non-MASK EXTRACT_LAST operation.

>> So, why do we test for get_len_load_store_mode and not just for
>> VEC_EXTRACT?

Before answer this question, let me first elaborate how ARM SVE is doing with MASK EXTRACT_LAST.

Here is the example:
https://godbolt.org/z/8cTv1jqMb 

ARM SVE IR:

  <bb 4> [local count: 955630224]:
  # ivtmp_31 = PHI <ivtmp_32(4), 0(3)>

  # loop_mask_22 = PHI <next_mask_35(4), max_mask_34(3)> -----> For RVV, we want this to be loop_len = SELECT_VL;

  _7 = &MEM <vector([16,16]) unsigned char> [(uint8_t *)x_11(D) + ivtmp_31 * 1];
  vect_last_12.8_23 = .MASK_LOAD (_7, 8B, loop_mask_22);
  vect__4.9_27 = vect_last_12.8_23 * vect_cst__26;
  .MASK_STORE (_7, 8B, loop_mask_22, vect__4.9_27);
  ivtmp_32 = ivtmp_31 + POLY_INT_CST [16, 16];
  _1 = (unsigned int) ivtmp_32;

  next_mask_35 = .WHILE_ULT (_1, bnd.5_6, { 0, ... });

  if (next_mask_35 != { 0, ... })
    goto <bb 4>; [89.00%]
  else
    goto <bb 5>; [11.00%]

  <bb 5> [local count: 105119324]:

  _25 = .EXTRACT_LAST (loop_mask_22, vect_last_12.8_23); [tail call] ----> Use the last mask generated in BB 4, so for RVV, we are using the loop_len.

So this patch is trying to optimize the codegen with simulating same flow as ARM SVE but with replacing 'loop_mask_22' (This is generated in BB4) into 'loop_len'.

For ARM SVE, they only check whether target support EXTRACT_LAST pattern, this pattern is supported means:

1. Target is using loop MASK as the partial vector loop control.
2. extract_last optab is enabled in the backend.

So for RVV, we are also checking same conditions:

1. Target is using loop LEN as the partial vector loop control (I use get_len_load_store_mode to check whether target is using loop LEN as the partial vector loop control).
2. vec_extract optab is enabled in the backend.

An alternative approach is that we can adding EXTRACT_LAST_LEN internal FN, then we can only check this like ARM SVE only check EXTRACT_LAST.

>> can we double-check this on powerpc and s390?

Sure, I hope it can be beneficial to powerpc and s390.
And, I think Richard's comments are also very important so I am gonna wait for it.

Thanks.


juzhe.zhong@rivai.ai
 
From: Richard Biener
Date: 2023-08-11 15:01
To: Ju-Zhe Zhong
CC: gcc-patches; richard.sandiford; linkw; krebbel
Subject: Re: [PATCH V3] VECT: Support loop len control on EXTRACT_LAST vectorization
On Fri, 11 Aug 2023, juzhe.zhong@rivai.ai wrote:
 
> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
> 
> Hi, Richard and Richi.
> 
> This patch add support live vectorization by VEC_EXTRACT for LEN loop control.
> 
> Consider this following case:
> 
> #include <stdint.h>
> 
> #define EXTRACT_LAST(TYPE) \
>   TYPE __attribute__ ((noinline, noclone)) \
>   test_##TYPE (TYPE *x, int n, TYPE value) \
>   { \
>     TYPE last; \
>     for (int j = 0; j < n; ++j) \
>       { \
> last = x[j]; \
> x[j] = last * value; \
>       } \
>     return last; \
>   }
> 
> #define TEST_ALL(T) \
>   T (uint8_t) \
> 
> TEST_ALL (EXTRACT_LAST)
> 
> ARM SVE IR:
> 
> Preheader:
>   max_mask_34 = .WHILE_ULT (0, bnd.5_6, { 0, ... });
> 
> Loop:
>   ...
>   # loop_mask_22 = PHI <next_mask_35(4), max_mask_34(3)>
>   ...
>   vect_last_12.8_23 = .MASK_LOAD (_7, 8B, loop_mask_22);
>   vect__4.9_27 = vect_last_12.8_23 * vect_cst__26;
>   .MASK_STORE (_7, 8B, loop_mask_22, vect__4.9_27);
>   ...
>   next_mask_35 = .WHILE_ULT (_1, bnd.5_6, { 0, ... });
>   ...
> 
> Epilogue:
>   _25 = .EXTRACT_LAST (loop_mask_22, vect_last_12.8_23);
> 
> For RVV since we prefer len in loop control, after this patch for RVV:
> 
> Loop:
>   ...
>   loop_len_22 = SELECT_VL;
>   vect_last_12.8_23 = .MASK_LOAD (_7, 8B, loop_len_22);
>   vect__4.9_27 = vect_last_12.8_23 * vect_cst__26;
>   .MASK_STORE (_7, 8B, loop_len_22, vect__4.9_27);
>   ...
> 
> Epilogue:
>   _25 = .VEC_EXTRACT (loop_len_22 + bias - 1, vect_last_12.8_23);
> 
> Details of this approach:
> 
> 1. Step 1 - Add 'vect_can_vectorize_extract_last_with_len_p'  to enable live vectorization
>             for LEN loop control.
>    
>    This function we check whether target support:
>     - Use LEN as the loop control.
>     - Support VEC_EXTRACT optab.
> 
> 2. Step 2 - Record LEN for loop control if 'vect_can_vectorize_extract_last_with_len_p' is true.
> 
> 3. Step 3 - Gerenate VEC_EXTRACT (v, LEN + BIAS - 1).
> 
> The only difference between mask and len is that len is using length generated by SELECT_VL and
> use VEC_EXTRACT pattern. The rest of the live vectorization is totally the same ARM SVE.
> 
> Bootstrap and Regression on X86 passed.
> 
> Tested on ARM QEMU.
> 
> Ok for trunk?
> 
> gcc/ChangeLog:
> 
> * tree-vect-loop.cc (vect_can_vectorize_extract_last_with_len_p): New function.
> (vectorizable_live_operation): Add loop len control.
> 
> ---
>  gcc/tree-vect-loop.cc | 76 +++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 70 insertions(+), 6 deletions(-)
> 
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index bf8d677b584..809b73b966c 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -8963,6 +8963,27 @@ vect_can_vectorize_without_simd_p (code_helper code)
>    && vect_can_vectorize_without_simd_p (tree_code (code)));
>  }
>  
> +/* Return true if target supports extract last vectorization with LEN.  */
> +
> +static bool
> +vect_can_vectorize_extract_last_with_len_p (tree vectype)
> +{
> +  /* Return false if target doesn't support LEN in loop control.  */
> +  machine_mode vmode;
> +  machine_mode vec_mode = TYPE_MODE (vectype);
> +  if (!VECTOR_MODE_P (vec_mode))
> +    return false;
> +  if (!get_len_load_store_mode (vec_mode, true).exists (&vmode)
> +      || !get_len_load_store_mode (vec_mode, false).exists (&vmode))
> +    return false;
 
So this "hidden" bit in the end decides whether to ...
 
> +  /* Target need to support VEC_EXTRACT to extract the last active element.  */
> +  return convert_optab_handler (vec_extract_optab,
> + vec_mode,
> + TYPE_MODE (TREE_TYPE (vectype)))
> + != CODE_FOR_nothing;
> +}
> +
>  /* Create vector init for vectorized iv.  */
>  static tree
>  vect_create_nonlinear_iv_init (gimple_seq* stmts, tree init_expr,
> @@ -10279,7 +10300,8 @@ vectorizable_live_operation (vec_info *vinfo, stmt_vec_info stmt_info,
>        if (loop_vinfo && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo))
>  {
>    if (!direct_internal_fn_supported_p (IFN_EXTRACT_LAST, vectype,
> -        OPTIMIZE_FOR_SPEED))
> +        OPTIMIZE_FOR_SPEED)
> +       && !vect_can_vectorize_extract_last_with_len_p (vectype))
>      {
>        if (dump_enabled_p ())
>  dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> @@ -10308,9 +10330,14 @@ vectorizable_live_operation (vec_info *vinfo, stmt_vec_info stmt_info,
>    else
>      {
>        gcc_assert (ncopies == 1 && !slp_node);
> -       vect_record_loop_mask (loop_vinfo,
> -      &LOOP_VINFO_MASKS (loop_vinfo),
> -      1, vectype, NULL);
> +       if (vect_can_vectorize_extract_last_with_len_p (vectype))
> + vect_record_loop_len (loop_vinfo,
> +       &LOOP_VINFO_LENS (loop_vinfo),
> +       1, vectype, 1);
 
.. record a loop_len here.  I think powerpc at least has .VEC_EXTRACT as 
well but of course .VEC_EXTRACT support itself doesn't have anything to
do with 'len' support.
 
x86 has .VEC_SET but not yet .VEC_EXTRACT, if it gets .VEC_EXTRACT
its partial vector support still wants masks, not lens (and once
we record both we fail).
 
So how can we resolve the issue when a non-VL operation like
.VEC_EXTRACT is used for _len support?
 
Note x86 doens't yet support IFN_EXTRACT_LAST either.
 
So, why do we test for get_len_load_store_mode and not just for
VEC_EXTRACT?
 
> +       else
> + vect_record_loop_mask (loop_vinfo,
> +        &LOOP_VINFO_MASKS (loop_vinfo),
> +        1, vectype, NULL);
>      }
>  }
>        /* ???  Enable for loop costing as well.  */
> @@ -10336,7 +10363,9 @@ vectorizable_live_operation (vec_info *vinfo, stmt_vec_info stmt_info,
>    gimple *vec_stmt;
>    if (slp_node)
>      {
> -      gcc_assert (!loop_vinfo || !LOOP_VINFO_FULLY_MASKED_P (loop_vinfo));
> +      gcc_assert (!loop_vinfo
> +   || (!LOOP_VINFO_FULLY_MASKED_P (loop_vinfo)
> +       && !LOOP_VINFO_FULLY_WITH_LENGTH_P (loop_vinfo)));
>  
>        /* Get the correct slp vectorized stmt.  */
>        vec_lhs = SLP_TREE_VEC_DEFS (slp_node)[vec_entry];
> @@ -10380,7 +10409,42 @@ vectorizable_live_operation (vec_info *vinfo, stmt_vec_info stmt_info,
>  
>        gimple_seq stmts = NULL;
>        tree new_tree;
> -      if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
> +      if (LOOP_VINFO_FULLY_WITH_LENGTH_P (loop_vinfo))
> + {
> +   /* Emit:
> +
> +        SCALAR_RES = VEC_EXTRACT <VEC_LHS, LEN + BIAS - 1>
> +
> +      where VEC_LHS is the vectorized live-out result and MASK is
> +      the loop mask for the final iteration.  */
> +   gcc_assert (ncopies == 1 && !slp_node);
> +   gimple_seq tem = NULL;
> +   gimple_stmt_iterator gsi = gsi_last (tem);
> +   tree len
> +     = vect_get_loop_len (loop_vinfo, &gsi,
> + &LOOP_VINFO_LENS (loop_vinfo),
> + 1, vectype, 0, 0);
> +
> +   /* BIAS - 1.  */
> +   signed char biasval = LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS (loop_vinfo);
> +   tree bias_minus_one
> +     = int_const_binop (MINUS_EXPR,
> +        build_int_cst (TREE_TYPE (len), biasval),
> +        build_one_cst (TREE_TYPE (len)));
> +
> +   /* LAST_INDEX = LEN + (BIAS - 1).  */
> +   tree last_index = gimple_build (&stmts, PLUS_EXPR, TREE_TYPE (len),
> +   len, bias_minus_one);
> +
> +   /* SCALAR_RES = VEC_EXTRACT <VEC_LHS, LEN + BIAS - 1>.  */
> +   tree scalar_res
> +     = gimple_build (&stmts, CFN_VEC_EXTRACT, TREE_TYPE (vectype),
> +     vec_lhs_phi, last_index);
> +
 
can we double-check this on powerpc and s390?
 
Thanks,
Richard.
 
> +   /* Convert the extracted vector element to the scalar type.  */
> +   new_tree = gimple_convert (&stmts, lhs_type, scalar_res);
> + }
> +      else if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
>  {
>    /* Emit:
>