Re: setmemsi, movmemsi and post_inc

[Segher Boessenkool <segher@kernel.crashing.org>]

On Fri, Mar 26, 2021 at 05:15:30PM -0500, Segher Boessenkool wrote:
> On Thu, Mar 25, 2021 at 08:44:07AM -0600, Jeff Law via Gcc-help wrote:
> > I'm also not aware of a target where an autoinc happens in contexts 
> > other than in a MEM.
> 
> See arm's "lazy_load_multiple_insn" for example.  Well it looks like
> this is the only example in the current compiler :-)

And I'm not sure how it is supposed to work even?  Without an enclosing
mem, it isn't clear to increment by how much?

In that arm example there is another parallel arm that *does* have a
mem with that same operand as address, but that looks to tricky to
work reliably.

rtl.def says

/* These unary operations are used to represent incrementation
   and decrementation as they occur in memory addresses.
   The amount of increment or decrement are not represented
   because they can be understood from the machine-mode of the
   containing MEM.  These operations exist in only two cases:
   1. pushes onto the stack.
   2. created automatically by the auto-inc-dec pass.  */
DEF_RTL_EXPR(PRE_DEC, "pre_dec", "e", RTX_AUTOINC)
DEF_RTL_EXPR(PRE_INC, "pre_inc", "e", RTX_AUTOINC)
DEF_RTL_EXPR(POST_DEC, "post_dec", "e", RTX_AUTOINC)
DEF_RTL_EXPR(POST_INC, "post_inc", "e", RTX_AUTOINC)


Segher