RFA: Stop df from generating SUBREG REG_NOTES

[Richard Sandiford <rsandifo at nildram dot co dot uk>]

Ping!

Richard Sandiford <rsandifo@nildram.co.uk> writes:
> Richard Sandiford <rsandifo@nildram.co.uk> writes:
>> Jan Hubicka <jh@suse.cz> writes:
>>> The users of df_set_note all just take the registers from dataflow
>>> information and call the function to produce REG_DEAD/REG_UNUSED notes.
>>> This all is going to introduce invalid sharing for subregs in all cases
>>> and if we are going to produce the note with SUBREG, we simply must
>>> produce the copy. (If we are very very cautious, we probably can have
>>> some cache since we are probably recomputing those notes quite often,
>>> but I doubt it is worth the extra complexity and we can see it easilly
>>> from memory tester)
>>>
>>> Note that copy_rtx does nothing for REG, since registers are shared, so
>>> it should not be overly expensive and we simply must produce the
>>> duplicate for the case of SUBREG or any other unshared RTL expression.
>>>
>>> However I now wonder why do we want to see subregs there at all.
>>> Originally I assumed that we now do partial liveness for multiple word
>>> values, but it is not the case.  Perhaps the right thing to do is to
>>> simply strip away the SUBREG and keep only the REG itself?
>>> At least before dataflow, I don't think we produced SUBREGs in those
>>> notes, I might be wrong.
>>
>> FWIW, I agree.  Several places in gcc seem to assume that REG_DEAD
>> and REG_UNUSED notes are plain REGs, and access REGNO unconditionally.
>> It even looks like df_ref_record was written with that in mind;
>> there's an "oldreg" variable alongside "reg", presumably so that
>> "reg" could be turned into "SUBREG_REG (oldreg)" if necessary,
>> but the two variables stay the same throughout.
>>
>> This was causing a segmentation fault while building libjava
>> for mipsel-linux-gnu.  We had a REG_DEAD (subreg:DI (reg:DF $f0))
>> note, and some code was applying REGNO to that subreg.
>>
>> Also, there seems to be some redundancy in df_mw_hardreg.
>> It has both a mw_reg field and a loc field, but it looks like
>> all uses of loc can be replaced with uses of mw_reg.
>>
>> I'm testing the patch below.  I wouldn't normally post a patch
>> before it has finished testing, but since the question is being
>> actively debated, I thought I might as well.
>
> Of course, the problem with doing that is that I changed my mind
> soon afterwards.  The patch survived testing, but as discussed
> on #gcc yesterday, I no longer think using the inner REG is
> unconditionally the right thing to do.
>
> The SUBREG handling in df_ref_record assumes that we can ask the port
> for the range of registers that (subreg:M (reg:N R)) occupies, even when
> mode M is not valid for hard register R.  This assumption was inherited
> from flow, and I'm assuming that it is correct.  (The bug I'm fixing is
> a malformed register note rather than inaccurate liveness information.)
> The range of registers for M is obviously not always going to be the
> same as the range for N, even though it was in the MIPS case.
>
> The code that adds notes will try to use the original mw_reg if
> all of the referenced register is dead (for REG_DEAD) or unused
> (for REG_UNUSED).  It will otherwise create notes for each individual
> hard register.
>
> I think we should always take the latter approach -- creating individual
> notes for each hard register -- when the mw_reg is a SUBREG rather than
> a REG.  This ties in quite naturally with the DF_REF_PARTIAL setting;
> as far as I can tell, we always consider a reference to a SUBREG of a
> multiword hard register to be partial, even if the SUBREG appears to be
> the same width as the inner register.  (And if we decide to drop the
> DF_REF_PARTIAL in some cases, the approach taken in the patch below
> should still work for the cases that remain DF_REF_PARTIAL.)
>
> Kenny mentioned the old flow code on #gcc yesterday.  I think flow
> used to treat a _use_ of a SUBREG like a use of the whole SUBREG_REG.
>> From mark_used_regs:
>
>     case SUBREG:
> #ifdef CANNOT_CHANGE_MODE_CLASS
>       if (flags & PROP_REG_INFO)
> 	record_subregs_of_mode (x);
> #endif
>
>       /* While we're here, optimize this case.  */
>       x = SUBREG_REG (x);
>       if (!REG_P (x))
> 	goto retry;
>       /* Fall through.  */
>
>     case REG:
>       /* See a register other than being set => mark it as needed.  */
>       mark_used_reg (pbi, x, cond, insn);
>       return;
>
> df is now trying to be more accurate by tracking individual hard
> registers in the SUBREG_REG.  The notes we add must clearly agree
> with the main df liveness information, so I don't think the old
> flow code sets any precedent here.  We're now tracking more
> registers individually, so adding more individual notes seems
> fair game.
>
> Flow used to track sets individually.  From mark_sets_1:
>
>     case SUBREG:
>       if (REG_P (SUBREG_REG (reg)))
> 	{
> 	  enum machine_mode outer_mode = GET_MODE (reg);
> 	  enum machine_mode inner_mode = GET_MODE (SUBREG_REG (reg));
>
> 	  /* Identify the range of registers affected.  This is moderately
> 	     tricky for hard registers.  See alter_subreg.  */
>
> 	  regno_last = regno_first = REGNO (SUBREG_REG (reg));
> 	  if (regno_first < FIRST_PSEUDO_REGISTER)
> 	    {
> 	      regno_first += subreg_regno_offset (regno_first, inner_mode,
> 						  SUBREG_BYTE (reg),
> 						  outer_mode);
> 	      regno_last = (regno_first
> 			    + hard_regno_nregs[regno_first][outer_mode] - 1);
>
> 	      /* Since we've just adjusted the register number ranges, make
> 		 sure REG matches.  Otherwise some_was_live will be clear
> 		 when it shouldn't have been, and we'll create incorrect
> 		 REG_UNUSED notes.  */
> 	      reg = gen_rtx_REG (outer_mode, regno_first);
> 	    }
>
> If every register in [regno_first, regno_last] was unused, flow would
> add a REG_UNUSED note for the REG created above.  If only some of those
> registers were unused, flow would add notes for the individual unused
> registers.  As I said on IRC yesterday, I'm not convinced the first
> behaviour was a good idea.  That REG might well be invalid (otherwise
> why have a SUBREG in the first place?) and it's better not to have
> invalid registers floating around.
>
> Bootstrapped & regression-tested on x86_64-linux-gnu.  Also tested by
> building mipsel-linux-gnu (which failed before the patch).  OK to install?
>
> Richard
>
>
> gcc/
> 	* df.h (df_mw_hardreg): Remove "loc" field.
> 	* df-scan.c (df_ref_record): Don't set it.  Remove redundant
> 	local variable.
> 	* df-problems.c (df_whole_mw_reg_unused_p): New function,
> 	split out from df_set_unused_notes_for_mw.  Return false for
> 	partial references.  Assert that mw_reg is a REG when returning true.
> 	(df_set_unused_notes_for_mw): Use it.  Use mw_reg instead of *loc.
> 	(df_whole_mw_reg_dead_p): New function, split out from
> 	df_set_dead_notes_for_mw.  Return false for partial references.
> 	Assert that mw_reg is a REG when returning true.
> 	(df_set_dead_notes_for_mw): Use it.  Use mw_reg instead of *loc.
> 	Remove redundant bitmap check.
>
> Index: gcc/df.h
> ===================================================================
> --- gcc/df.h	(revision 126108)
> +++ gcc/df.h	(working copy)
> @@ -312,7 +312,6 @@ struct dataflow
>  struct df_mw_hardreg
>  {
>    rtx mw_reg;                   /* The multiword hardreg.  */ 
> -  rtx *loc;			/* The location of the reg.  */
>    enum df_ref_type type;        /* Used to see if the ref is read or write.  */
>    enum df_ref_flags flags;	/* Various flags.  */
>    unsigned int start_regno;     /* First word of the multi word subreg.  */
> Index: gcc/df-scan.c
> ===================================================================
> --- gcc/df-scan.c	(revision 126108)
> +++ gcc/df-scan.c	(working copy)
> @@ -2627,7 +2627,6 @@ df_ref_record (struct df_collection_rec 
>  	       enum df_ref_type ref_type, 
>  	       enum df_ref_flags ref_flags) 
>  {
> -  rtx oldreg = reg;
>    unsigned int regno;
>  
>    gcc_assert (REG_P (reg) || GET_CODE (reg) == SUBREG);
> @@ -2658,7 +2657,7 @@ df_ref_record (struct df_collection_rec 
>  	{
>  	  /* Sets to a subreg of a multiword register are partial. 
>  	     Sets to a non-subreg of a multiword register are not.  */
> -	  if (GET_CODE (oldreg) == SUBREG)
> +	  if (GET_CODE (reg) == SUBREG)
>  	    ref_flags |= DF_REF_PARTIAL;
>  	  ref_flags |= DF_REF_MW_HARDREG;
>  
> @@ -2666,7 +2665,6 @@ df_ref_record (struct df_collection_rec 
>  	  hardreg->type = ref_type;
>  	  hardreg->flags = ref_flags;
>  	  hardreg->mw_reg = reg;
> -	  hardreg->loc = loc;
>  	  hardreg->start_regno = regno;
>  	  hardreg->end_regno = endregno - 1;
>  	  hardreg->mw_order = df->ref_order++;
> Index: gcc/df-problems.c
> ===================================================================
> --- gcc/df-problems.c	(revision 126109)
> +++ gcc/df-problems.c	(working copy)
> @@ -3717,6 +3717,32 @@ df_set_note (enum reg_note note_type, rt
>    return old;
>  }
>  
> +/* A subroutine of df_set_unused_notes_for_mw, with a selection of its
> +   arguments.  Return true if the register value described by MWS's
> +   mw_reg is known to be completely unused, and if mw_reg can therefore
> +   be used in a REG_UNUSED note.  */
> +
> +static bool
> +df_whole_mw_reg_unused_p (struct df_mw_hardreg *mws,
> +			  bitmap live, bitmap artificial_uses)
> +{
> +  unsigned int r;
> +
> +  /* If MWS describes a partial reference, create REG_UNUSED notes for
> +     individual hard registers.  */
> +  if (mws->flags & DF_REF_PARTIAL)
> +    return false;
> +
> +  /* Likewise if some part of the register is used.  */
> +  for (r = mws->start_regno; r <= mws->end_regno; r++)
> +    if (bitmap_bit_p (live, r)
> +	|| bitmap_bit_p (artificial_uses, r))
> +      return false;
> +
> +  gcc_assert (REG_P (mws->mw_reg));
> +  return true;
> +}
> +
>  /* Set the REG_UNUSED notes for the multiword hardreg defs in INSN
>     based on the bits in LIVE.  Do not generate notes for registers in
>     artificial uses.  DO_NOT_GEN is updated so that REG_DEAD notes are
> @@ -3729,7 +3755,6 @@ df_set_unused_notes_for_mw (rtx insn, rt
>  			    bitmap live, bitmap do_not_gen, 
>  			    bitmap artificial_uses)
>  {
> -  bool all_dead = true;
>    unsigned int r;
>    
>  #ifdef REG_DEAD_DEBUGGING
> @@ -3737,18 +3762,11 @@ df_set_unused_notes_for_mw (rtx insn, rt
>      fprintf (dump_file, "mw_set_unused looking at mws[%d..%d]\n", 
>  	     mws->start_regno, mws->end_regno);
>  #endif
> -  for (r=mws->start_regno; r <= mws->end_regno; r++)
> -    if ((bitmap_bit_p (live, r))
> -	|| bitmap_bit_p (artificial_uses, r))
> -      {
> -	all_dead = false;
> -	break;
> -      }
> -  
> -  if (all_dead)
> +
> +  if (df_whole_mw_reg_unused_p (mws, live, artificial_uses))
>      {
>        unsigned int regno = mws->start_regno;
> -      old = df_set_note (REG_UNUSED, insn, old, *(mws->loc));
> +      old = df_set_note (REG_UNUSED, insn, old, mws->mw_reg);
>  
>  #ifdef REG_DEAD_DEBUGGING
>        df_print_note ("adding 1: ", insn, REG_NOTES (insn));
> @@ -3773,6 +3791,34 @@ df_set_unused_notes_for_mw (rtx insn, rt
>  }
>  
>  
> +/* A subroutine of df_set_dead_notes_for_mw, with a selection of its
> +   arguments.  Return true if the register value described by MWS's
> +   mw_reg is known to be completely dead, and if mw_reg can therefore
> +   be used in a REG_DEAD note.  */
> +
> +static bool
> +df_whole_mw_reg_dead_p (struct df_mw_hardreg *mws,
> +			bitmap live, bitmap artificial_uses,
> +			bitmap do_not_gen)
> +{
> +  unsigned int r;
> +
> +  /* If MWS describes a partial reference, create REG_DEAD notes for
> +     individual hard registers.  */
> +  if (mws->flags & DF_REF_PARTIAL)
> +    return false;
> +
> +  /* Likewise if some part of the register is not dead.  */
> +  for (r = mws->start_regno; r <= mws->end_regno; r++)
> +    if (bitmap_bit_p (live, r)
> +	|| bitmap_bit_p (artificial_uses, r)
> +	|| bitmap_bit_p (do_not_gen, r))
> +      return false;
> +
> +  gcc_assert (REG_P (mws->mw_reg));
> +  return true;
> +}
> +
>  /* Set the REG_DEAD notes for the multiword hardreg use in INSN based
>     on the bits in LIVE.  DO_NOT_GEN is used to keep REG_DEAD notes
>     from being set if the instruction both reads and writes the
> @@ -3783,7 +3829,6 @@ df_set_dead_notes_for_mw (rtx insn, rtx 
>  			  bitmap live, bitmap do_not_gen,
>  			  bitmap artificial_uses)
>  {
> -  bool all_dead = true;
>    unsigned int r;
>    
>  #ifdef REG_DEAD_DEBUGGING
> @@ -3799,25 +3844,13 @@ df_set_dead_notes_for_mw (rtx insn, rtx 
>      }
>  #endif
>  
> -  for (r = mws->start_regno; r <= mws->end_regno; r++)
> -    if ((bitmap_bit_p (live, r))
> -	|| bitmap_bit_p (artificial_uses, r)
> -	|| bitmap_bit_p (do_not_gen, r))
> -      {
> -	all_dead = false;
> -	break;
> -      }
> -  
> -  if (all_dead)
> +  if (df_whole_mw_reg_dead_p (mws, live, artificial_uses, do_not_gen))
>      {
> -      if (!bitmap_bit_p (do_not_gen, mws->start_regno))
> -	{
> -	  /* Add a dead note for the entire multi word register.  */
> -	  old = df_set_note (REG_DEAD, insn, old, *(mws->loc));
> +      /* Add a dead note for the entire multi word register.  */
> +      old = df_set_note (REG_DEAD, insn, old, mws->mw_reg);
>  #ifdef REG_DEAD_DEBUGGING
> -	  df_print_note ("adding 1: ", insn, REG_NOTES (insn));
> +      df_print_note ("adding 1: ", insn, REG_NOTES (insn));
>  #endif
> -	}
>      }
>    else
>      {