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

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

Richard Biener <rguenther@suse.de> writes:
> On Tue, 25 Apr 2023, Richard Sandiford wrote:
>> juzhe.zhong@rivai.ai writes:
>> > diff --git a/gcc/tree-ssa-loop-manip.cc b/gcc/tree-ssa-loop-manip.cc
>> > index a52277abdbf..54845a62298 100644
>> > --- a/gcc/tree-ssa-loop-manip.cc
>> > +++ b/gcc/tree-ssa-loop-manip.cc
>> > @@ -59,14 +59,14 @@ static bitmap_obstack loop_renamer_obstack;
>> >  void
>> >  create_iv (tree base, tree step, tree var, class loop *loop,
>> >  	   gimple_stmt_iterator *incr_pos, bool after,
>> > -	   tree *var_before, tree *var_after)
>> > +	   tree *var_before, tree *var_after, enum tree_code code)
>> 
>> The comment needs to be updated to describe the new interface.
>> 
>> This is personal preference, but: I think the interface would be
>> clearer if the code argument came between the base and step,
>> so that the order matches a SCEV.  The code could no longer be
>> a default argument, and so all callers would need to be updated,
>> but IMO that's OK.  Not sure what others think though.
>
> Just a quick comment - I think decrementing IVs are already supported,
> you just have to make 'step' negative (or large positive).  If you
> really think using MINUS_EXPR is better or even required for
> VLA step please add an assert that 'code' is either PLUS_EXPR or
> MINUS_EXPR.
>
> Note that for INTEGER_CST step we rewrite x - CST to x + -CST
> during folding.

Yeah.  I think the problem in this case is that the step is variable.
So if we only supported PLUS_EXPRs, we'd need a separate NEGATE_EXPR
stmt (which presumably would be folded in later).

Thanks,
Richard