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RE: [RFC] Further LRA subreg handling issues

Matthew Fortune <> writes:
> Pseudo 300 is assigned to memory and then LRA produces a simple DImode
> load from the assigned stack slot. The only instruction to set pseudo
> 300 is:
> (insn 247 212 389 3 (set (reg:SI 300)
>         (ne:SI (subreg/s/u:SI (reg/v:DI 231 [ taken ]) 0)
>             (const_int 0 [0]))) "/home/mfortune/gcc/gcc/predict.c":2904
> 504 {*sne_zero_sisi}
>      (nil))
> Which leads to an SImode store to the stack slot:
> (insn 247 392 393 3 (set (reg:SI 4 $4 [300])
>         (ne:SI (reg:SI 20 $20 [orig:231 taken ] [231])
>             (const_int 0 [0]))) "/home/mfortune/gcc/gcc/predict.c":2904
> 504 {*sne_zero_sisi}
>      (nil))
> (insn 393 247 389 3 (set (mem/c:SI (plus:DI (reg/f:DI 29 $sp)
>                 (const_int 16 [0x10])) [403 %sfp+16 S4 A64])
>         (reg:SI 4 $4 [300])) "/home/mfortune/gcc/gcc/predict.c":2904 312
> {*movsi_internal}
>      (nil))
> ...
> (note 248 246 249 40 NOTE_INSN_DELETED)
> (note 249 248 256 40 NOTE_INSN_DELETED)
> (note 256 249 250 40 NOTE_INSN_DELETED)
> (insn 250 256 251 40 (set (reg:DI 6 $6)
>         (mem/c:DI (plus:DI (reg/f:DI 29 $sp)
>                 (const_int 16 [0x10])) [403 %sfp+16 S8 A64]))
> "/home/mfortune/gcc/gcc/predict.c":2904 310 {*movdi_64bit}
>      (nil))
> My assumption is that LRA is again expected to deal with this case and
> for insn
> 250 should be recognising that it must load 32-bits and rely on implicit
> LOAD_EXTEND_OP behaviour producing an acceptable 64-bit value. In this
> case it does not matter whether it is sign or zero extension and my
> assumption is that this construct would never appear if a specific sign
> or zero extension was required.
> I haven't got to looking at where the issue is this time but it seems
> different as this is a subreg in a simple move instruction where we
> already support the load/ store directly so no new reload instruction is
> required. I don't know if this implies that simple move patterns should
> reject subregs but that doesn't sound right either.
> Resolving this fixes at least one bug and potentially all bugs in the
> MIPS bootstrap as  manually modified the generated assembly code to use
> LW instead of LD for insn
> 250 and one of the buggy stage 3 objects is fixed.
> I'll keep thinking, any advice in the meantime is appreciated.

All I have been able to determine on this is that there is potentially
different behaviour for paradoxical subregs in LRA vs reload.  There is
this comment in reload.c:push_reload:

    If we have (SUBREG:M1 (MEM:M2 ...) ...) (or an inner REG that is still
     a pseudo and hence will become a MEM) with M1 wider than M2 and the
     register is a pseudo, also reload the inside expression.

To me this makes perfect sense as I believe the RTL is only saying that
there is an M2-mode object to access or at least only the M2-mode sized
bits are valid. There are comments to say there will always be sufficient
memory assigned for spill slots as they are sized to fit the largest
paradoxical subreg, I just don't know why that is useful/important.

However in lra-constraints.c:simplify_operand_subreg it quite happily
performs a reload using the outer mode in this case and only drops down to
the inner mode if the outer mode reload would be slower than the inner.

Presumably this is safe for non WORD_REGISTER_OPERATIONS targets as the
junk upper bits in registers will be ignored; On WORD_REGISTER_OPERATIONS
targets then the narrower-than-word mode load will take care of any
'magic' needed to set the upper bits to a safe value in register.

So my thinking is that at least WORD_REGISTER_OPERATIONS targets should
always reload the inner mode for the case mentioned above much like the same
is required for normal subregs. Does that seem reasonable? Have I
misunderstood the paradoxical subreg case entirely?

I've only done superficial testing of a change to this code so far but my
testcase starts working at least which is a start.


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