Thanks for your response.
On 2022/08/03 16:52, Richard Sandiford wrote:
> Takayuki 'January June' Suwa via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> Emitting "(clobber (reg X))" before "(set (subreg (reg X)) (...))" keeps
>> data flow consistent, but it also increases register allocation pressure
>> and thus often creates many unwanted register-to-register moves that
>> cannot be optimized away.
>
> There are two things here:
>
> - If emit_move_complex_parts emits a clobber of a hard register,
> then that's probably a bug/misfeature. The point of the clobber is
> to indicate that the register has no useful contents. That's useful
> for wide pseudos that are written to in parts, since it avoids the
> need to track the liveness of each part of the pseudo individually.
> But it shouldn't be necessary for hard registers, since subregs of
> hard registers are simplified to hard registers wherever possible
> (which on most targets is "always").
>
> So I think the emit_move_complex_parts clobber should be restricted
> to !HARD_REGISTER_P, like the lower-subreg clobber is. If that helps
> (if only partly) then it would be worth doing as its own patch.
>
> - I think it'd be worth looking into more detail why a clobber makes
> a difference to register pressure. A clobber of a pseudo register R
> shouldn't make R conflict with things that are live at the point of
> the clobber.
I agree with its worth.
In fact, aside from other ports, on the xtensa one, RA in code with frequent
D[FC]mode pseudos is terribly bad.
For example, in __muldc3 on libgcc2, the size of the stack frame reserved will
almost double depending on whether or not this patch is applied.
>
>> It seems just analogous to partial register
>> stall which is a famous problem on processors that do register renaming.
>>
>> In my opinion, when the register to be clobbered is a composite of hard
>> ones, we should clobber the individual elements separetely, otherwise
>> clear the entire to zero prior to use as the "init-regs" pass does (like
>> partial register stall workarounds on x86 CPUs). Such redundant zero
>> constant assignments will be removed later in the "cprop_hardreg" pass.
>
> I don't think we should rely on the zero being optimised away later.
>
> Emitting the zero also makes it harder for the register allocator
> to elide the move. For example, if we have:
>
> (set (subreg:SI (reg:DI P) 0) (reg:SI R0))
> (set (subreg:SI (reg:DI P) 4) (reg:SI R1))
>
> then there is at least a chance that the RA could assign hard registers
> R0:R1 to P, which would turn the moves into nops. If we emit:
>
> (set (reg:DI P) (const_int 0))
>
> beforehand then that becomes impossible, since R0 and R1 would then
> conflict with P.
Ah, surely, as you pointed out for targets where "(reg: DI)" corresponds to one
hard register.
>
> TBH I'm surprised we still run init_regs for LRA. I thought there was
> a plan to stop doing that, but perhaps I misremember.
Sorry I am not sure about the status of LRA... because the xtensa port is still
using reload.
As conclusion, trying to tweak the common code side may have been a bit
premature.
I'll consider if I can deal with those issues on the side of the
target-specific code.
>
> Thanks,
> Richard
>
>> This patch may give better output code quality for the reasons above,
>> especially on architectures that don't have DFmode hard registers
>> (On architectures with such hard registers, this patch changes virtually
>> nothing).
>>
>> For example (Espressif ESP8266, Xtensa without FP hard regs):
>>
>> /* example */
>> double _Complex conjugate(double _Complex z) {
>> __imag__(z) *= -1;
>> return z;
>> }
>>
>> ;; before
>> conjugate:
>> movi.n a6, -1
>> slli a6, a6, 31
>> mov.n a8, a2
>> mov.n a9, a3
>> mov.n a7, a4
>> xor a6, a5, a6
>> mov.n a2, a8
>> mov.n a3, a9
>> mov.n a4, a7
>> mov.n a5, a6
>> ret.n
>>
>> ;; after
>> conjugate:
>> movi.n a6, -1
>> slli a6, a6, 31
>> xor a6, a5, a6
>> mov.n a5, a6
>> ret.n
>>
>> gcc/ChangeLog:
>>
>> * lower-subreg.cc (resolve_simple_move):
>> Add zero clear of the entire register immediately after
>> the clobber.
>> * expr.cc (emit_move_complex_parts):
>> Change to clobber the real and imaginary parts separately
>> instead of the whole complex register if possible.
>> ---
>> gcc/expr.cc | 26 ++++++++++++++++++++------
>> gcc/lower-subreg.cc | 7 ++++++-
>> 2 files changed, 26 insertions(+), 7 deletions(-)
>>
>> diff --git a/gcc/expr.cc b/gcc/expr.cc
>> index 80bb1b8a4c5..9732e8fd4e5 100644
>> --- a/gcc/expr.cc
>> +++ b/gcc/expr.cc
>> @@ -3775,15 +3775,29 @@ emit_move_complex_push (machine_mode mode, rtx x,
>> rtx y)
>> rtx_insn *
>> emit_move_complex_parts (rtx x, rtx y)
>> {
>> - /* Show the output dies here. This is necessary for SUBREGs
>> - of pseudos since we cannot track their lifetimes correctly;
>> - hard regs shouldn't appear here except as return values. */
>> - if (!reload_completed && !reload_in_progress
>> - && REG_P (x) && !reg_overlap_mentioned_p (x, y))
>> - emit_clobber (x);
>> + rtx_insn *re_insn, *im_insn;
>>
>> write_complex_part (x, read_complex_part (y, false), false, true);
>> + re_insn = get_last_insn ();
>> write_complex_part (x, read_complex_part (y, true), true, false);
>> + im_insn = get_last_insn ();
>> +
>> + /* Show the output dies here. This is necessary for SUBREGs
>> + of pseudos since we cannot track their lifetimes correctly. */
>> + if (can_create_pseudo_p ()
>> + && REG_P (x) && ! reg_overlap_mentioned_p (x, y))
>> + {
>> + /* Hard regs shouldn't appear here except as return values. */
>> + if (HARD_REGISTER_P (x) && REG_NREGS (x) % 2 == 0)
>> + {
>> + emit_insn_before (gen_clobber (SET_DEST (PATTERN (re_insn))),
>> + re_insn);
>> + emit_insn_before (gen_clobber (SET_DEST (PATTERN (im_insn))),
>> + im_insn);
>> + }
>> + else
>> + emit_insn_before (gen_clobber (x), re_insn);
>> + }
>>
>> return get_last_insn ();
>> }
>> diff --git a/gcc/lower-subreg.cc b/gcc/lower-subreg.cc
>> index 03e9326c663..4ff0a7d1556 100644
>> --- a/gcc/lower-subreg.cc
>> +++ b/gcc/lower-subreg.cc
>> @@ -1086,7 +1086,12 @@ resolve_simple_move (rtx set, rtx_insn *insn)
>> unsigned int i;
>>
>> if (REG_P (dest) && !HARD_REGISTER_NUM_P (REGNO (dest)))
>> - emit_clobber (dest);
>> + {
>> + emit_clobber (dest);
>> + /* We clear the entire of dest with zero after the clobber,
>> + similar to the "init-regs" pass. */
>> + emit_move_insn (dest, CONST0_RTX (GET_MODE (dest)));
>> + }
>>
>> for (i = 0; i < words; ++i)
>> {
