f60226fd72
df_lr_bb_local_compute has:
FOR_EACH_INSN_INFO_DEF (def, insn_info)
/* If the def is to only part of the reg, it does
not kill the other defs that reach here. */
if (!(DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL)))
However, as noted in the comment in the patch and below, almost all
partial definitions have an associated use. This means that the
confluence function:
IN = (OUT & ~DEF) | USE
is unaffected by whether partial definitions are in DEF or not.
Even though the choice doesn't matter for the LR problem itself,
it's IMO much more convenient for consumers if DEF contains all the
definitions in the block. The only pre-RTL-SSA code that tries to
consume DEF directly is shrink-wrap.c, which already has to work
around the incompleteness of the information:
/* DF_LR_BB_INFO (bb)->def does not comprise the DF_REF_PARTIAL and
DF_REF_CONDITIONAL defs. So if DF_LIVE doesn't exist, i.e.
at -O1, just give up searching NEXT_BLOCK. */
I hit the same problem when trying to fix the RTL-SSA part of PR98863.
This patch treats partial definitions as both a def and a use,
just like the df_ref records almost always do.
To show that partial definitions almost always have uses:
DF_REF_CONDITIONAL:
Added by:
case COND_EXEC:
df_defs_record (collection_rec, COND_EXEC_CODE (x),
bb, insn_info, DF_REF_CONDITIONAL);
break;
Later, df_get_conditional_uses creates uses for all DF_REF_CONDITIONAL
definitions.
DF_REF_PARTIAL:
In total, there are 4 locations at which we add partial definitions.
Case 1:
if (GET_CODE (dst) == STRICT_LOW_PART)
{
flags |= DF_REF_READ_WRITE | DF_REF_PARTIAL | DF_REF_STRICT_LOW_PART;
loc = &XEXP (dst, 0);
dst = *loc;
}
Corresponding use:
case STRICT_LOW_PART:
{
rtx *temp = &XEXP (dst, 0);
/* A strict_low_part uses the whole REG and not just the
SUBREG. */
dst = XEXP (dst, 0);
df_uses_record (collection_rec,
(GET_CODE (dst) == SUBREG) ? &SUBREG_REG (dst) : temp,
DF_REF_REG_USE, bb, insn_info,
DF_REF_READ_WRITE | DF_REF_STRICT_LOW_PART);
}
break;
Case 2:
if (GET_CODE (dst) == ZERO_EXTRACT)
{
flags |= DF_REF_READ_WRITE | DF_REF_PARTIAL | DF_REF_ZERO_EXTRACT;
loc = &XEXP (dst, 0);
dst = *loc;
}
Corresponding use:
case ZERO_EXTRACT:
{
df_uses_record (collection_rec, &XEXP (dst, 1),
DF_REF_REG_USE, bb, insn_info, flags);
df_uses_record (collection_rec, &XEXP (dst, 2),
DF_REF_REG_USE, bb, insn_info, flags);
if (GET_CODE (XEXP (dst,0)) == MEM)
df_uses_record (collection_rec, &XEXP (dst, 0),
DF_REF_REG_USE, bb, insn_info,
flags);
else
df_uses_record (collection_rec, &XEXP (dst, 0),
DF_REF_REG_USE, bb, insn_info,
DF_REF_READ_WRITE | DF_REF_ZERO_EXTRACT);
----------------------------^^^^^^^^^^^^^^^^^
}
break;
Case 3:
else if (GET_CODE (dst) == SUBREG && REG_P (SUBREG_REG (dst)))
{
if (read_modify_subreg_p (dst))
flags |= DF_REF_READ_WRITE | DF_REF_PARTIAL;
flags |= DF_REF_SUBREG;
df_ref_record (DF_REF_REGULAR, collection_rec,
dst, loc, bb, insn_info, DF_REF_REG_DEF, flags);
}
Corresponding use:
case SUBREG:
if (read_modify_subreg_p (dst))
{
df_uses_record (collection_rec, &SUBREG_REG (dst),
DF_REF_REG_USE, bb, insn_info,
flags | DF_REF_READ_WRITE | DF_REF_SUBREG);
break;
}
Case 4:
/* If this is a multiword hardreg, we create some extra
datastructures that will enable us to easily build REG_DEAD
and REG_UNUSED notes. */
if (collection_rec
&& (endregno != regno + 1) && insn_info)
{
/* Sets to a subreg of a multiword register are partial.
Sets to a non-subreg of a multiword register are not. */
if (GET_CODE (reg) == SUBREG)
ref_flags |= DF_REF_PARTIAL;
ref_flags |= DF_REF_MW_HARDREG;
Corresponding use:
None. However, this case should be rare to non-existent on most
targets, and the current handling seems suspect. See the comment
in the patch for more details.
gcc/
* df-problems.c (df_lr_bb_local_compute): Treat partial definitions
as read-modify operations.
gcc/testsuite/
* gcc.dg/rtl/aarch64/multi-subreg-1.c: New test.
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