Re: Re: [PATCH] RISC-V: optimize stack manipulation in save-restore

["Fei Gao" <gaofei@eswincomputing.com>]

Hi Palmer and all, 

I have split the patches and triggerred a new thread.
https://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg297206.html

Could you please review at your convenience?

Thanks & BR, 
Fei

On 2022-12-01 11:07  Fei Gao <gaofei@eswincomputing.com> wrote:
>
>On 2022-12-01 06:50  Palmer Dabbelt <palmer@dabbelt.com> wrote:
>>
>>On Wed, 30 Nov 2022 00:37:17 PST (-0800), gaofei@eswincomputing.com wrote:
>>> The stack that save-restore reserves is not well accumulated in stack allocation and deallocation.
>>> This patch allows less instructions to be used in stack allocation and deallocation if save-restore enabled,
>>> and also a much clear logic for save-restore stack manipulation.
>>>
>>> before patch:
>>> bar:
>>> call	t0,__riscv_save_4
>>> addi	sp,sp,-64
>>> ...
>>> li	t0,-12288
>>> addi	t0,t0,-1968 # optimized out after patch
>>> add	sp,sp,t0 # prologue
>>> ...
>>> li	t0,12288 # epilogue
>>> addi	t0,t0,2000 # optimized out after patch
>>> add	sp,sp,t0
>>> ...
>>> addi	sp,sp,32
>>> tail	__riscv_restore_4
>>>
>>> after patch:
>>> bar:
>>> call	t0,__riscv_save_4
>>> addi	sp,sp,-2032
>>> ...
>>> li	t0,-12288
>>> add	sp,sp,t0 # prologue
>>> ...
>>> li	t0,12288 # epilogue
>>> add	sp,sp,t0
>>> ...
>>> addi	sp,sp,2032
>>> tail	__riscv_restore_4
>>>
>>> gcc/ChangeLog:
>>>
>>>         * config/riscv/riscv.cc (riscv_first_stack_step): add a new function parameter remaining_size.
>>>         (riscv_compute_frame_info): adapt new riscv_first_stack_step interface.
>>>         (riscv_expand_prologue): consider save-restore in stack allocation.
>>>         (riscv_expand_epilogue): consider save-restore in stack deallocation.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>>         * gcc.target/riscv/stack_save_restore.c: New test.
>>> ---
>>>  gcc/config/riscv/riscv.cc                     | 58 ++++++++++---------
>>>  .../gcc.target/riscv/stack_save_restore.c     | 40 +++++++++++++
>>>  2 files changed, 70 insertions(+), 28 deletions(-)
>>>  create mode 100644 gcc/testsuite/gcc.target/riscv/stack_save_restore.c
>>
>>I guess with the RISC-V backend still being open for things as big as
>>the V port we should probably be taking code like this as well?  I
>>wouldn't be opposed to making an exception for the V code and holding
>>everything else back, though.
>>
>>> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
>>> index 05bdba5ab4d..9e92e729a5f 100644
>>> --- a/gcc/config/riscv/riscv.cc
>>> +++ b/gcc/config/riscv/riscv.cc
>>> @@ -4634,7 +4634,7 @@ riscv_save_libcall_count (unsigned mask)
>>>     They decrease stack_pointer_rtx but leave frame_pointer_rtx and
>>>     hard_frame_pointer_rtx unchanged.  */
>>>
>>> -static HOST_WIDE_INT riscv_first_stack_step (struct riscv_frame_info *frame);
>>> +static HOST_WIDE_INT riscv_first_stack_step (struct riscv_frame_info *frame, poly_int64 remaining_size);
>>>
>>>  /* Handle stack align for poly_int.  */
>>>  static poly_int64
>>> @@ -4663,7 +4663,7 @@ riscv_compute_frame_info (void)
>>>       save/restore t0.  We check for this before clearing the frame struct.  */
>>>    if (cfun->machine->interrupt_handler_p)
>>>      {
>>> -      HOST_WIDE_INT step1 = riscv_first_stack_step (frame);
>>> +      HOST_WIDE_INT step1 = riscv_first_stack_step (frame, frame->total_size);
>>>        if (! POLY_SMALL_OPERAND_P ((frame->total_size - step1)))
>>>  interrupt_save_prologue_temp = true;
>>>      }
>>> @@ -4913,31 +4913,31 @@ riscv_restore_reg (rtx reg, rtx mem)
>>>     without adding extra instructions.  */
>>>
>>>  static HOST_WIDE_INT
>>> -riscv_first_stack_step (struct riscv_frame_info *frame)
>>> +riscv_first_stack_step (struct riscv_frame_info *frame, poly_int64 remaining_size)
>>>  {
>>> -  HOST_WIDE_INT frame_total_constant_size;
>>> -  if (!frame->total_size.is_constant ())
>>> -    frame_total_constant_size
>>> -      = riscv_stack_align (frame->total_size.coeffs[0])
>>> -	- riscv_stack_align (frame->total_size.coeffs[1]);
>>> +  HOST_WIDE_INT remaining_const_size;
>>> +  if (!remaining_size.is_constant ())
>>> +    remaining_const_size
>>> +      = riscv_stack_align (remaining_size.coeffs[0])
>>> +	- riscv_stack_align (remaining_size.coeffs[1]);
>>
>>The alignment looks off here, at least in the email.  Worth fixing it up
>>if you're touching the lines anyway.
>
>Sure, i will change RISCV_STACK_ALIGN into riscv_stack_align.
>
>>
>>>    else
>>> -    frame_total_constant_size = frame->total_size.to_constant ();
>>> +    remaining_const_size = remaining_size.to_constant ();
>>>
>>> -  if (SMALL_OPERAND (frame_total_constant_size))
>>> -    return frame_total_constant_size;
>>> +  if (SMALL_OPERAND (remaining_const_size))
>>> +    return remaining_const_size;
>>>
>>>    HOST_WIDE_INT min_first_step =
>>> -    RISCV_STACK_ALIGN ((frame->total_size - frame->frame_pointer_offset).to_constant());
>>> +    RISCV_STACK_ALIGN ((remaining_size - frame->frame_pointer_offset).to_constant());
>>>    HOST_WIDE_INT max_first_step = IMM_REACH / 2 - PREFERRED_STACK_BOUNDARY / 8;
>>> -  HOST_WIDE_INT min_second_step = frame_total_constant_size - max_first_step;
>>> +  HOST_WIDE_INT min_second_step = remaining_const_size - max_first_step;
>>>    gcc_assert (min_first_step <= max_first_step);
>>>
>>>    /* As an optimization, use the least-significant bits of the total frame
>>>       size, so that the second adjustment step is just LUI + ADD.  */
>>>    if (!SMALL_OPERAND (min_second_step)
>>> -      && frame_total_constant_size % IMM_REACH < IMM_REACH / 2
>>> -      && frame_total_constant_size % IMM_REACH >= min_first_step)
>>> -    return frame_total_constant_size % IMM_REACH;
>>> +      && remaining_const_size % IMM_REACH < IMM_REACH / 2
>>> +      && remaining_const_size % IMM_REACH >= min_first_step)
>>> +    return remaining_const_size % IMM_REACH;
>>
>>Looks like this entire frame->total_size -> remaining_size conversion
>>could be done as an independent patch that would change no
>>functionality, that's always a nice way to do things as it makes the
>>code easier to read.
>
>Sure, i will split this patch.
>
>>
>>I spent a bit poking around here and nothing wrong is jumping out, but
>>trying to keep all these offset differences in my head is a bit tricky. 
>>If you have the time to refactor this to be easier to read that'd be
>>great, otherwise hopefully I (or someone else) will have the time to
>>take a look -- probably not today on my end, though, as I've got some
>>Linux backlog to look at.
>
>I'll try it. Also i propose to add step0 for pre-allocated stack cases like save-restore lib call,
>and the future Zcmp in Zc* extension if needed.
>So we have a clear logic of manipulate stack step by step.
>
>>
>>Thanks!
>>
>>>    if (TARGET_RVC)
>>>      {
>>> @@ -5037,9 +5037,7 @@ riscv_expand_prologue (void)
>>>    /* Save the registers.  */
>>>    if ((frame->mask | frame->fmask) != 0)
>>>      {
>>> -      HOST_WIDE_INT step1 = riscv_first_stack_step (frame);
>>> -      if (size.is_constant ())
>>> -	step1 = MIN (size.to_constant(), step1);
>>> +      HOST_WIDE_INT step1 = riscv_first_stack_step (frame, size);
>>>
>>>        insn = gen_add3_insn (stack_pointer_rtx,
>>>      stack_pointer_rtx,
>>> @@ -5142,6 +5140,8 @@ riscv_expand_epilogue (int style)
>>>    HOST_WIDE_INT step2 = 0;
>>>    bool use_restore_libcall = ((style == NORMAL_RETURN)
>>>        && riscv_use_save_libcall (frame));
>>> +  unsigned libcall_size = use_restore_libcall ?
>>> +                            frame->save_libcall_adjustment : 0;
>>>    rtx ra = gen_rtx_REG (Pmode, RETURN_ADDR_REGNUM);
>>>    rtx insn;
>>>
>>> @@ -5212,13 +5212,18 @@ riscv_expand_epilogue (int style)
>>>        REG_NOTES (insn) = dwarf;
>>>      }
>>>
>>> +  if (use_restore_libcall)
>>> +    frame->mask = 0; /* Temporarily fib for GPRs.  */
>>> +
>>>    /* If we need to restore registers, deallocate as much stack as
>>>       possible in the second step without going out of range.  */
>>>    if ((frame->mask | frame->fmask) != 0)
>>> -    {
>>> -      step2 = riscv_first_stack_step (frame);
>>> -      step1 -= step2;
>>> -    }
>>> +    step2 = riscv_first_stack_step (frame, frame->total_size - libcall_size);
>>> +
>>> +  if (use_restore_libcall)
>>> +    frame->mask = mask; /* Undo the above fib.  */
>>> +
>>> +  step1 -= step2 + libcall_size;
>>>
>>>    /* Set TARGET to BASE + STEP1.  */
>>>    if (known_gt (step1, 0))
>>> @@ -5272,15 +5277,12 @@ riscv_expand_epilogue (int style)
>>>      frame->mask = 0; /* Temporarily fib that we need not save GPRs.  */
>>>
>>>    /* Restore the registers.  */
>>> -  riscv_for_each_saved_reg (frame->total_size - step2, riscv_restore_reg,
>>> +  riscv_for_each_saved_reg (frame->total_size - step2 - libcall_size,
>>> +                            riscv_restore_reg,
>>>      true, style == EXCEPTION_RETURN);
>>>
>>>    if (use_restore_libcall)
>>> -    {
>>>        frame->mask = mask; /* Undo the above fib.  */
>>> -      gcc_assert (step2 >= frame->save_libcall_adjustment);
>>> -      step2 -= frame->save_libcall_adjustment;
>>> -    }
>>>
>>>    if (need_barrier_p)
>>>      riscv_emit_stack_tie ();
>>> diff --git a/gcc/testsuite/gcc.target/riscv/stack_save_restore.c b/gcc/testsuite/gcc.target/riscv/stack_save_restore.c
>>> new file mode 100644
>>> index 00000000000..4695ef9469a
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.target/riscv/stack_save_restore.c
>>> @@ -0,0 +1,40 @@
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-march=rv32imafc -mabi=ilp32f -msave-restore -O2 -fno-schedule-insns -fno-schedule-insns2 -fno-unroll-loops -fno-peel-loops" } */
>>> +/* { dg-final { check-function-bodies "**" "" } } */
>>> +
>>> +char my_getchar();
>>> +float getf();
>>> +
>>> +/*
>>> +**bar:
>>> +**	call	t0,__riscv_save_4
>>> +**	addi	sp,sp,-2032
>>> +**	...
>>> +**	li	t0,-12288
>>> +**	add	sp,sp,t0
>>> +**	...
>>> +**	li	t0,12288
>>> +**	add	sp,sp,t0
>>> +**	...
>>> +**	addi	sp,sp,2032
>>> +**	tail	__riscv_restore_4
>>> +*/
>>
>>The test needs to actually check this, it can't just be manual.
>
>I didn't get your point.
>The { dg-final { check-function-bodies "**" "" } } matches the output with the expected result above automatically. 
>Please let me know your idea. 
>
>Thanks & BR, 
>Fei
>
>>
>>> +int bar()
>>> +{
>>> +  float volatile farray[3568];
>>> +
>>> +  float sum = 0;
>>> +  float f1 = getf();
>>> +  float f2 = getf();
>>> +  float f3 = getf();
>>> +  float f4 = getf();
>>> +
>>> +  for (int i = 0; i < 3568; i++)
>>> +  {
>>> +    farray[i] = my_getchar() * 1.2;
>>> +    sum += farray[i];
>>> +  }
>>> +
>>> +  return sum + f1 + f2 + f3 + f4;
>>> +}
>>> +