Hongtao Liu <crazy...@gmail.com> writes:
> On Mon, Jan 18, 2021 at 6:18 PM Richard Sandiford
> <richard.sandif...@arm.com> wrote:
>>
>> Hongtao Liu via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> > Hi:
>> > If SRC had been assigned a mode narrower than the copy, we can't link
>> > DEST into the chain even they have same
>> > hard_regno_nregs(i.e. HImode/SImode in i386 backend).
>>
>> In general, changes between modes within the same hard register are OK.
>> Could you explain in more detail what's going wrong?
>>
>
> cprop hardreg change
>
> (insn 457 499 460 33 (set (reg:SI 39 r11 [orig:86 _11 ] [86])
> (reg:SI 37 r9 [orig:86 _11 ] [86])) "test.c":29:36 75
> {*movsi_internal}
> (expr_list:REG_DEAD (reg:SI 37 r9 [orig:86 _11 ] [86])
> (nil)))
>
> to
>
> (insn 457 499 460 33 (set (reg:SI 39 r11 [orig:86 _11 ] [86])
> (reg:SI 22 xmm2 [orig:86 _11 ] [86])) "test.c":29:36 75
> {*movsi_internal}
> (expr_list:REG_DEAD (reg:SI 22 xmm2 [orig:86 _11 ] [86])
> (nil)))
>
> since (reg:SI 22 xmm2) and (reg:SI r9) are in the same value chain in
> which the oldest regno is k0.
>
> but with xmm2 defined as
>
> kmovw %k0, %edi # 69 [c=4 l=4] *movhi_internal/6----- kmovw move the
> lower 16bits to %edi, and clear the upper 16 bits.
> vmovd %edi, %xmm2 # 489 *movsi_internal --- vmovd move 32bits from
> %edi to %xmm2.
>
> (insn 69 68 70 12 (set (reg:HI 5 di [orig:96 _52 ] [96])
> (reg:HI 68 k0 [orig:82 var_6.0_1 ] [82])) "test.c":21:23 76
> {*movhi_internal}
> (nil))
>
> (insn 489 75 78 12 (set (reg:SI 22 xmm2 [297])
> (reg:SI 5 di [orig:96 _52 ] [96])) 75 {*movsi_internal}
> (nil))
The sequence is OK in itself, but insn 489 can't make any assumptions
about what's in the upper 16 bits of %edi. In other words, as far as
RTL semantics are concerned, insn 489 only leaves bits 0-15 of %xmm2
with defined values; the other bits are undefined.
If the target wants all 32 bits of %edi to be carried over to insn 489
then it needs to make insn 69 an SImode set instead of a HImode set.
So what cprop is doing is OK: it's changing the values of undefined
bits but not changing the definition of defined bits (from an RTL
point of view).
Thanks,
Richard
