Re: [PATCH] Fix ICE in ia64 speculation support

[Maxim Kuvyrkov <maxim@codesourcery.com>]

Paolo Bonzini wrote:
> 
>> OK, back to the immediate problem: speculative load appears to be trap 
>> risky to the rtl analyzer due to use of (unspec) in its pattern.  
>> UNSPEC is placed into speculative loads to distinguish them from 
>> regular ones.  What else, besides UNSPEC, can we use to make insn emit 
>> different asm, but still have the same RTL meaning?  I can think of 
>> only one alternative: use a parallel with a nop, e.g., (parallel [(set 
>> (reg) (mem)) (const_int 0)]).  I think this is uglier than unspec, 
>> plus rtl analyzers favor parallel less than unspec.  I appreciate any 
>> advice here.
> 
> It seems to me that only UNSPEC_VOLATILE is counted as possibly 
> trapping.

The case is (set (reg) (UNSPEC:<fp_mode> (mem))), which is trapping 
because it is a floating point operation (it doesn't even get to 
analyzing the mem).

> The problem is than the UNSPEC recurs inside the MEM and that 
> one is marked as trapping.  You could add a target hook like 
> unspec_may_trap_p, with a patch like this:

This is how the same problem is solved on sel-sched-branch.  I agree 
that it won't hurt to teach may_trap_p to ask target about UNSPECs.

> 
> Index: rtlanal.c
> ===================================================================
> --- rtlanal.c   (revision 126191)
> +++ rtlanal.c   (working copy)
> @@ -2206,8 +2206,11 @@ may_trap_p_1 (rtx x, unsigned flags)
>      case SCRATCH:
>        return 0;
> 
> -    case ASM_INPUT:
> +    case UNSPEC:
>      case UNSPEC_VOLATILE:
> +      return targetm.unspec_may_trap_p (x, flags);
> +
> +    case ASM_INPUT:
>      case TRAP_IF:
>        return 1;
> 
> 
> and a default implementation of
> 
>   int j;
>   if (GET_CODE (x) == UNSPEC_VOLATILE)
>     return 1;
> 
>   for (j = 0; j < XVECLEN (x, 0); j++)
>     if (may_trap_p_1 (XVECEXP (x, 0, j), flags))
>       return 1;
> 
> The ia64 back-end could special case the unspec like this:
> 
>   if (XINT (x, 1) == ...)
>     return 0;
> 
>   return default_unspec_may_trap_p (x);
> 
> (Hmm, requires making may_trap_p_1 public, it's static now).

Not if default_unspec_may_trap_p () is defined in rtlanal.c .


Thanks,

Maxim