Re: Ping : [PATCH] [gcc, combine] PR46164: Don't combine the insns if a volatile register is contained.

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

On Tue, Feb 17, 2015 at 11:39:34AM +0800, Terry Guo wrote:
> On Sun, Feb 15, 2015 at 7:35 PM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > Hi Terry,
> >
> > I still think this is stage1 material.
> >
> >> +  /* Don't combine if dest contains a user specified register and i3 contains
> >> +     ASM_OPERANDS, because the user specified register (same with dest) in i3
> >> +     would be replaced by the src of insn which might be different with
> >> +     the user's expectation.  */
> >
> > "Do not eliminate a register asm in an asm input" or similar?  Text
> > explaining why REG_USERVAR_P && HARD_REGISTER_P works here would be
> > good to have, too.

> diff --git a/gcc/combine.c b/gcc/combine.c
> index f779117..aeb2854 100644
> --- a/gcc/combine.c
> +++ b/gcc/combine.c
> @@ -1779,7 +1779,7 @@ can_combine_p (rtx_insn *insn, rtx_insn *i3, rtx_insn *pred ATTRIBUTE_UNUSED,
>  {
>    int i;
>    const_rtx set = 0;
> -  rtx src, dest;
> +  rtx src, dest, asm_op;
>    rtx_insn *p;
>  #ifdef AUTO_INC_DEC
>    rtx link;
> @@ -1914,6 +1914,14 @@ can_combine_p (rtx_insn *insn, rtx_insn *i3, rtx_insn *pred ATTRIBUTE_UNUSED,
>    set = expand_field_assignment (set);
>    src = SET_SRC (set), dest = SET_DEST (set);
>  
> +  /* Use REG_USERVAR_P and HARD_REGISTER_P to check whether DEST is a user
> +     specified register, and do not eliminate such register if it is in an
> +     asm input because we may end up with something different with user's
> +     expectation.  */

That doesn't explain why this will hit (almost) only on register asms.
The user's expectation doesn't matter that much either: GCC would violate
its own documentation / promises, that matters more ;-)

> +  if (REG_P (dest) && REG_USERVAR_P (dest) && HARD_REGISTER_P (dest)
> +      && ((asm_op = extract_asm_operands (PATTERN (i3))) != NULL))

You do not need the temporary variable, nor the != 0 or the extra parens;
just write

     && extract_asm_operands (PATTERN (i3))

Cheers,


Segher