Re: [PATCH] RISC-V: Bugfix for rvv bool mode precision adjustment

[Richard Sandiford <richard.sandiford@arm.com>]

"Li, Pan2" <pan2.li@intel.com> writes:
> Thanks all for so much valuable and helpful materials.
>
> As I understand (Please help to correct me if any mistake.), for the VNx*BI (aka, 1, 2, 4, 8, 16, 32, 64),
> the precision and mode size need to be adjusted as below.
>
> Precision size [1, 2, 4, 8, 16, 32, 64]
> Mode size [1, 1, 1, 1, 2, 4, 8]
>
> Given that, if we ignore the self-test failure, only the adjust_precision part is able to fix the bug I mentioned.
> The genmode will first get the precision, and then leverage the mode_size = exact_div / 8 to generate.
> Meanwhile, it also provides the adjust_mode_size after the mode_size generation.
>
> The riscv parts has the mode_size_adjust already and the value of mode_size will be overridden by the adjustments.

Ah, OK!  In that case, would the following help:

Turn:

  mode_size[E_%smode] = exact_div (mode_precision[E_%smode], BITS_PER_UNIT);

into:

  if (!multiple_p (mode_precision[E_%smode], BITS_PER_UNIT,
		   &mode_size[E_%smode]))
    mode_size[E_%smode] = -1;

where -1 is an "obviously wrong" value.

Ports that might hit the -1 are then responsible for setting the size
later, via ADJUST_BYTESIZE.

After all the adjustments are complete, genmodes asserts that no size is
known_eq to -1.

That way, target-independent code doesn't need to guess what the
correct behaviour is.

Does the eventual value set by ADJUST_BYTESIZE equal the real number of
bytes loaded by vlm.v and stored by vstm.v (after the appropriate vsetvl)?
And does it equal the size of the corresponding LLVM machine type?
Or is the GCC size larger in some cases than the number of bytes
loaded and stored?

(You and Juzhe have probably answered that question before, sorry,
but I'm still not 100% sure of the answer.  Personally, I think I would
find the ISA behaviour easier to understand if the explanation doesn't
involve poly_ints.  It would be good to understand things "as the
architecture sees then" rather than in terms of GCC concepts.)

Thanks,
Richard

> Unfortunately, the early stage mode_size generation leveraged exact_div, which doesn't honor precision size < 8
> with the adjustment and fails on exact_div assertions.
>
> Besides the precision adjustment, I am not sure if we can narrow down the problem to.
>
>
>   1.  Defined the real size of both the precision and mode size to align the riscv ISA.
>   2.  Besides, make the general mode_size = precision_size / 8 is able to take care of both the exact_div and the dividend less than the divisor (like 1/8 or 2/8) cases.
>
> Could you please share your professional suggestions about this? Thank you all again and have a nice day!
>
> Pan
>
> From: juzhe.zhong@rivai.ai <juzhe.zhong@rivai.ai>
> Sent: Wednesday, March 1, 2023 10:19 PM
> To: rguenther <rguenther@suse.de>
> Cc: richard.sandiford <richard.sandiford@arm.com>; gcc-patches <gcc-patches@gcc.gnu.org>; Pan Li <incarnation.p.lee@outlook.com>; Li, Pan2 <pan2.li@intel.com>; kito.cheng <kito.cheng@sifive.com>
> Subject: Re: Re: [PATCH] RISC-V: Bugfix for rvv bool mode precision adjustment
>
>>> So given the above I think that modeling the size as being the same
>>> but with accurate precision would work.  It's then only the size of the
>>> padding in bytes we cannot represent with poly-int which should be fine.
>
>>> Correct?
> Yes.
>
>>> Btw, is storing a VNx1BI and then loading a VNx2BI from the same
>>> memory address well-defined?  That is, how is the padding handled
>>> by the machine load/store instructions?
>
> storing VNx1BI is storing the data from addr 0 ~ 1/8 poly (1,1) and keep addr 1/8  poly (1,1) ~ 2/8  poly (1,1) memory data unchange.
> load VNx2BI will load 0 ~ 2/8  poly (1,1), note that 0 ~ 1/8 poly (1,1) is the date that we store above, 1/8  poly (1,1) ~ 2/8  poly (1,1)  is the orignal memory data.
> You can see here for this case (LLVM):
> https://godbolt.org/z/P9e1adrd3
> foo:                                    # @foo
>         vsetvli a2, zero, e8, mf8, ta, ma
>         vsm.v   v0, (a0)
>         vsetvli a2, zero, e8, mf4, ta, ma
>         vlm.v   v8, (a0)
>         vsm.v   v8, (a1)
>         ret
>
> We can also doing like this in GCC as long as we can differentiate VNx1BI and VNx2BI, and GCC do not eliminate statement according precision even though
> they have same bytesize.
>
> First we emit vsetvl e8mf8 +vsm for VNx1BI
> Then we emit vsetvl e8mf8 + vlm for VNx2BI
>
> Thanks.
> ________________________________
> juzhe.zhong@rivai.ai<mailto:juzhe.zhong@rivai.ai>
>
> From: Richard Biener<mailto:rguenther@suse.de>
> Date: 2023-03-01 22:03
> To: juzhe.zhong<mailto:juzhe.zhong@rivai.ai>
> CC: richard.sandiford<mailto:richard.sandiford@arm.com>; gcc-patches<mailto:gcc-patches@gcc.gnu.org>; Pan Li<mailto:incarnation.p.lee@outlook.com>; pan2.li<mailto:pan2.li@intel.com>; kito.cheng<mailto:kito.cheng@sifive.com>
> Subject: Re: Re: [PATCH] RISC-V: Bugfix for rvv bool mode precision adjustment
> On Wed, 1 Mar 2023, Richard Biener wrote:
>
>> On Wed, 1 Mar 2023, juzhe.zhong@rivai.ai<mailto:juzhe.zhong@rivai.ai> wrote:
>>
>> > Let's me first introduce RVV load/store basics  and stack allocation.
>> > For scalable vector memory allocation, we allocate memory according to machine vector-length.
>> > To get this CPU vector-length value (runtime invariant but compile time unknown), we have an instruction call csrr vlenb.
>> > For example, csrr a5,vlenb (store CPU a single register vector-length value (describe as bytesize) in a5 register).
>> > A single register size in bytes (GET_MODE_SIZE) is poly value (8,8) bytes. That means csrr a5,vlenb, a5 has the value of size poly (8,8) bytes.
>> >
>> > Now, our problem is that VNx1BI, VNx2BI, VNx4BI, VNx8BI has the same bytesize poly (1,1). So their storage consumes the same size.
>> > Meaning when we want to allocate a memory storge or stack for register spillings, we should first csrr a5, vlenb, then slli a5,a5,3 (means a5 = a5/8)
>> > Then, a5 has the bytesize value of poly (1,1). All VNx1BI, VNx2BI, VNx4BI, VNx8BI are doing the same process as I described above. They all consume
>> > the same memory storage size since we can't model them accurately according to precision or you bitsize.
>> >
>> > They consume the same storage (I am agree it's better to model them more accurately in case of memory storage comsuming).
>> >
>> > Well, even though they are consuming same size memory storage, I can make their memory accessing behavior (load/store) accurately by
>> > emiting  the accurate RVV instruction for them according to RVV ISA.
>> >
>> > VNx1BI,VNx2BI, VNx4BI, VNx8BI are consuming same memory storage with size  poly (1,1)
>> > The instruction for these modes as follows:
>> > VNx1BI: vsevl e8mf8 + vlm,  loading 1/8 of poly (1,1) storage.
>> > VNx2BI: vsevl e8mf8 + vlm,  loading 1/4 of poly (1,1) storage.
>> > VNx4BI: vsevl e8mf8 + vlm,  loading 1/2 of poly (1,1) storage.
>> > VNx8BI: vsevl e8mf8 + vlm,  loading 1 of poly (1,1) storage.
>> >
>> > So base on these, It's fine that we don't model VNx1BI,VNx2BI, VNx4BI, VNx8BI accurately according to precision or bitsize.
>> > This implementation is fine even though their memory storage is not accurate.
>> >
>> > However, the problem is that since they have the same bytesize, GCC will think they are the same and do some incorrect statement elimination:
>> >
>> > (Note: Load same memory base)
>> > load v0 VNx1BI from base0
>> > load v1 VNx2BI from base0
>> > load v2 VNx4BI from base0
>> > load v3 VNx8BI from base0
>> >
>> > store v0 base1
>> > store v1 base2
>> > store v2 base3
>> > store v3 base4
>> >
>> > This program sequence, in GCC, it will eliminate the last 3 load instructions.
>> >
>> > Then it will become:
>> >
>> > load v0 VNx1BI from base0 ===> vsetvl e8mf8 + vlm (only load 1/8 of poly size (1,1) memory data)
>> >
>> > store v0 base1
>> > store v0 base2
>> > store v0 base3
>> > store v0 base4
>> >
>> > This is what we want to fix. I think as long as we can have the way to differentiate VNx1BI,VNx2BI, VNx4BI, VNx8BI
>> > and GCC will not do th incorrect elimination for RVV.
>> >
>> > I think it can work fine  even though these 4 modes consume inaccurate memory storage size
>> > but accurate data memory access load store behavior.
>>
>> So given the above I think that modeling the size as being the same
>> but with accurate precision would work.  It's then only the size of the
>> padding in bytes we cannot represent with poly-int which should be fine.
>>
>> Correct?
>
> Btw, is storing a VNx1BI and then loading a VNx2BI from the same
> memory address well-defined?  That is, how is the padding handled
> by the machine load/store instructions?
>
> Richard.