https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85090
--- Comment #10 from Vladimir Makarov <vmakarov at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #5)
> I guess it depends on what exactly a normal subreg on lhs means.
> The documentation says:
> When used as an lvalue, 'subreg' is a word-based accessor.
> Storing to a 'subreg' modifies all the words of REG that
> overlap the 'subreg', but it leaves the other words of REG
> alone.
> but in this case, we really don't have a GPR, but rather a single much
> larger (512-bit) register. Does it still imply that for -m32 (subreg:SI
> (reg:V32HI) 0)
> sets just the low 32 bits of the large register and doesn't modify anything
> else,
> and for -m64 the same means set low 32 bits, have the 32 bits above it
> undefined and the rest of bits unmodified?
>
Yes, I guess so. For x86-64 "the 32 bits above" will be zero which might be
treated as undefined.
> Seems store_bit_field treats it that way, but perhaps IRA does not?
I believe IRA treats it in the same way.
LRA does not just emit
(insn 5658 4214 5812 2 (set (mem/c:DI (plus:DI (reg/f:DI 7 sp)
(const_int 264 [0x108])) [4 %sfp+-64 S8 A512])
(reg:DI 0 ax [4040])) "pr85090.c":13 85 {*movdi_internal}
(nil))
(insn 5812 5658 4219 2 (set (reg:DI 57 xmm20 [orig:422 i ] [422])
(mem/c:DI (plus:DI (reg/f:DI 7 sp)
(const_int 264 [0x108])) [4 %sfp+-64 S8 A512])) "pr85090.c":13
85 {*movdi_internal}
(nil))
It emits through emit_move_insn (instead of xmm20, a pseudo is used on some LRA
sub-passes):
(insn 5812 5658 4219 2 (set (subreg:DI (reg/v:V32HI 57 xmm20 [orig:422 i ]
[422]) 0)
(mem/c:DI (plus:DI (reg/f:DI 7 sp)
(const_int 264 [0x108])) [4 %sfp+-64 S8 A512])) "v.i":13 85
{*movdi_internal}
(expr_list:REG_DEAD (reg:TI 20 frame)
(nil)))
and this insn exists till the very LRA end where it is changed (through
alter_subreg):
(insn 5812 5658 4219 2 (set (reg:DI 57 xmm20 [orig:422 i ] [422])
(mem/c:DI (plus:DI (reg/f:DI 7 sp)
(const_int 264 [0x108])) [4 %sfp+-64 S8 A512])) "pr85090.c":13
85 {*movdi_internal}
(nil))
So LRA keeps subreg semantics. It cannot change movdi_internal into the right
vector insn sequence. I believe it is not LRA responsibility.
