Re: [PATCH][RFA][LRA] Don't try to break down subreg expressions if insn already matches

[pinskia at gmail dot com]

> On Feb 13, 2015, at 1:48 AM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:
> 
> Hi all,
> 
> In my tree added a pattern to the arm backend that's supposed to match:
> (set (reg:SI r0)
>     (subreg:SI
>       (plus:DI
>         (mult:DI (sign_extend:DI (reg:SI r1))
>                  (sign_extend:DI (reg:SI r2)))
>       (const_int 2147483648 [0x80000000])) 4))
> 
> That is, take two SImode regs, sign-extend to DImode, multiply in DImode,
> add a const_int and take the most significant SImode subreg.

Seems better to use shifts for the most significant simode and low part subreg after that. Isn't that what other targets do?

Thanks,
Andrew


> Combine matches it fine but I get an ICE in lra:
> 0xa665a5 crash_signal
>        $SRC/gcc/toplev.c:383
> 0x91ec1c lra_emit_add(rtx_def*, rtx_def*, rtx_def*)
>        $SRC/gcc/lra.c:418
> 0x91ef8a lra_emit_move(rtx_def*, rtx_def*)
>        $SRC/gcc/lra.c:528
> 0x9279b2 insert_move_for_subreg
>        $SRC/gcc/lra-constraints.c:1371
> 0x931bdb simplify_operand_subreg
>        $SRC/gcc/lra-constraints.c:1507
> 0x931bdb curr_insn_transform
>        $SRC/gcc/lra-constraints.c:3437
> 0x934793 lra_constraints(bool)
>        $SRC/gcc/lra-constraints.c:4451
> 0x9217d0 lra(_IO_FILE*)
>        $SRC/gcc/lra.c:2292
> 0x8e0dba do_reload
>        $SRC/gcc/ira.c:5418
> 0x8e0dba execute
>        $SRC/gcc/ira.c:5589
> 
> 
> I *think* the problem is that simplify_operand_subreg tries to break down
> this complex RTX involving the subreg into separate MULT/PLUS operations
> and breaking down the line.
> 
> If I apply the attached patch that stops trying to simplify the subreg
> expression if the whole thing matches something already my code compiles
> succesfully, and passes arm testing and x86_64 bootstrap.
> However, I'm concerned that calling recog_memoized is too heavy-handed,
> is there a better approach?
> Or is this even the right way to go about this?
> 
> I'm also attaching the patch to the arm backend that can be used to
> reproduce this ICE with the following C-code at -O2:
> 
> int
> smmulr (int a, int b)
> {
>  return ((long long)a * b + 0x80000000) >> 32;
> }
> 
> 
> Thanks,
> Kyrill
> 
> 2015-02-10  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> 
>    * lra-constraints.c (simplify_operand_subreg): Do not try to simplify
>    subreg if the pattern already matches.
> <lra-subreg.patch>
> <arm-smmulr.patch>