Re: [PATCH V4] VECT: Add decrement IV iteration loop control by variable amount support

[Richard Sandiford <richard.sandiford@arm.com>]

钟居哲 <juzhe.zhong@rivai.ai> writes:
> I am sorry that I am still confused about that.
>
> Is this what you want ?
>
>   bool use_minus_p = TREE_CODE (step) == INTEGER_CST && ((TYPE_UNSIGNED (TREE_TYPE (step)) && tree_int_cst_lt (step1, step))
>                      || (!TYPE_UNSIGNED (TREE_TYPE (step)) && !tree_expr_nonnegative_warnv_p (step, &ovf) && may_negate_without_overflow_p (step)));
>
>   /* For easier readability of the created code, produce MINUS_EXPRs
>      when suitable.  */
>   if (TREE_CODE (step) == INTEGER_CST)
>     {
>       if (TYPE_UNSIGNED (TREE_TYPE (step)))
> {
>   step1 = fold_build1 (NEGATE_EXPR, TREE_TYPE (step), step);
>   if (tree_int_cst_lt (step1, step))
>     {
>       incr_op = MINUS_EXPR; /* Remove it.  */
>       step = step1;
>     }
> }
>       else
> {
>   bool ovf;
>
>   if (!tree_expr_nonnegative_warnv_p (step, &ovf)
>       && may_negate_without_overflow_p (step))
>     {
>       incr_op = MINUS_EXPR; /* Remove it.  */
>       step = fold_build1 (NEGATE_EXPR, TREE_TYPE (step), step);
>     }
> }
>     }
>   if (POINTER_TYPE_P (TREE_TYPE (base)))
>     {
>       if (TREE_CODE (base) == ADDR_EXPR)
> mark_addressable (TREE_OPERAND (base, 0));
>       step = convert_to_ptrofftype (step);
>       if (incr_op == MINUS_EXPR) /* Change it into if (use_minus_p)  */
> step = fold_build1 (NEGATE_EXPR, TREE_TYPE (step), step);
>       incr_op = POINTER_PLUS_EXPR; /* Remove it.  */
>     }
>   /* Gimplify the step if necessary.  We put the computations in front of the
>      loop (i.e. the step should be loop invariant).  */
>   step = force_gimple_operand (step, &stmts, true, NULL_TREE);
>   if (stmts)
>     gsi_insert_seq_on_edge_immediate (pe, stmts);
>   
>   if (POINTER_TYPE_P (TREE_TYPE (base)))
>     stmt = gimple_build_assign (va, POINTER_PLUS_EXPR, vb, step);
>   else if (use_minus_p)
>     stmt = gimple_build_assign (va, MINUS_EXPR, vb, step);
>   else
>     stmt = gimple_build_assign (va, incr_op, vb, step);
> ...
>
> Since I have no idea to make stmts flips between PLUS_EXPR and MINUS_EXPR.

No, I meant:

- Rename the "code" argument to "incr_op".

- Remove "tree_code incr_op = code;".

- Replace both instances of:

     incr_op = MINUS_EXPR;

  with:

     incr_op = (incr_op == PLUS_EXPR ? MINUS_EXPR : PLUS_EXPR);

The point is that the current code (rightly) assumes that incr_op
always starts out as PLUS_EXPR, i.e. that STEP starts out applying
positively.  Making STEP apply in the opposite direction is then as
simple as changing incr_op to MINUS_EXPR.  But the new interface
allows STEP to start out applying positively or negatively, and so
this code needs to cope with both cases.

Thanks,
Richard