From: Richard Sandiford <richard@codesourcery.com>
To: gcc-patches@gcc.gnu.org
Subject: Re: RFA: Stop df from generating SUBREG REG_NOTES
Date: Wed, 25 Jul 2007 22:50:00 -0000 [thread overview]
Message-ID: <87k5sodu51.fsf@firetop.home> (raw)
In-Reply-To: <874pk8eiea.fsf_-_@firetop.home> (Richard Sandiford's message of "Fri\, 13 Jul 2007 10\:56\:45 +0100")
Ping^2
Cross builds of mips-linux-gnu have been broken since the
dataflow merge, so it would be nice to get this sorted.
Richard
Richard Sandiford <rsandifo@nildram.co.uk> writes:
> 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
>> {
next prev parent reply other threads:[~2007-07-25 21:57 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-06-26 3:43 RTL sharing tester (for testing) Jan Hubicka
2007-06-27 15:55 ` Kenneth Zadeck
2007-06-27 18:17 ` Diego Novillo
2007-06-27 21:43 ` Jan Hubicka
2007-06-28 2:02 ` Kaz Kojima
2007-06-28 4:24 ` Jan Hubicka
2007-06-28 8:11 ` Paolo Bonzini
2007-06-28 17:16 ` Kenneth Zadeck
2007-06-28 18:26 ` Jan Hubicka
2007-06-28 19:57 ` Kenneth Zadeck
2007-06-28 20:27 ` Richard Sandiford
2007-06-29 8:29 ` Richard Sandiford
2007-06-29 12:11 ` Kenneth Zadeck
2007-06-29 13:35 ` Paolo Bonzini
2007-06-29 13:37 ` Bernd Schmidt
2007-06-29 13:39 ` Jan Hubicka
2007-06-29 16:55 ` Richard Sandiford
2007-07-13 10:15 ` RFA: Stop df from generating SUBREG REG_NOTES Richard Sandiford
2007-07-25 22:50 ` Richard Sandiford [this message]
2007-07-27 0:01 ` Ian Lance Taylor
2007-06-28 20:19 ` RTL sharing tester (for testing) Paolo Bonzini
2007-06-27 23:06 ` Mark Mitchell
2007-06-27 23:06 ` Jan Hubicka
2007-06-27 23:12 ` Eric Christopher
2007-06-28 0:48 ` Jan Hubicka
2007-06-28 22:24 ` Eric Christopher
2007-06-29 23:14 ` Graham Stott
2007-06-30 0:17 ` Jan Hubicka
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