cmd/compile: optimize right shifts of uint32 on riscv

cmd/compile: optimize right shifts of uint32 on riscv

The compiler is currently zero extending 32 bit unsigned integers to
64 bits before right shifting them using a 64 bit shift instruction.
There's no need to do this as RISC-V has instructions for right
shifting 32 bit unsigned values (srlw and srliw) which zero extend
the result of the shift to 64 bits.  Change the compiler so that
it uses srlw and srliw for 32 bit unsigned shifts reducing in most
cases the number of instructions needed to perform the shift.

Here are some examples of code sequences that are changed by this
patch:

uint32(a) >> 2

  before:

    sll     x5,x10,0x20
    srl     x10,x5,0x22

  after:

    srlw    x10,x10,0x2

uint32(a) >> int(b)

  before:

    sll     x5,x10,0x20
    srl     x5,x5,0x20
    srl     x5,x5,x11
    sltiu   x6,x11,64
    neg     x6,x6
    and     x10,x5,x6

  after:

    srlw    x5,x10,x11
    sltiu   x6,x11,32
    neg     x6,x6
    and     x10,x5,x6

bits.RotateLeft32(uint32(a), 1)

  before:

    sll     x5,x10,0x1
    sll     x6,x10,0x20
    srl     x7,x6,0x3f
    or      x5,x5,x7

  after:

   sll     x5,x10,0x1
   srlw    x6,x10,0x1f
   or      x10,x5,x6

bits.RotateLeft32(uint32(a), int(b))

  before:
    and     x6,x11,31
    sll     x7,x10,x6
    sll     x8,x10,0x20
    srl     x8,x8,0x20
    add     x6,x6,-32
    neg     x6,x6
    srl     x9,x8,x6
    sltiu   x6,x6,64
    neg     x6,x6
    and     x6,x9,x6
    or      x6,x6,x7

  after:

    and     x5,x11,31
    sll     x6,x10,x5
    add     x5,x5,-32
    neg     x5,x5
    srlw    x7,x10,x5
    sltiu   x5,x5,32
    neg     x5,x5
    and     x5,x7,x5
    or      x10,x6,x5

The one regression observed is the following case, an unbounded right
shift of a uint32 where the value we're shifting by is known to be
< 64 but > 31.  As this is an unusual case this commit does not
optimize for it, although the existing code does.

uint32(a) >> (b & 63)

  before:

    sll     x5,x10,0x20
    srl     x5,x5,0x20
    and     x6,x11,63
    srl     x10,x5,x6

  after

    and     x5,x11,63
    srlw    x6,x10,x5
    sltiu   x5,x5,32
    neg     x5,x5
    and     x10,x6,x5

Here we have one extra instruction.

Some benchmark highlights, generated on a VisionFive2 8GB running
Ubuntu 23.04.

pkg: math/bits
LeadingZeros32-4    18.64n ± 0%     17.32n ± 0%   -7.11% (p=0.000 n=10)
LeadingZeros64-4    15.47n ± 0%     15.51n ± 0%   +0.26% (p=0.027 n=10)
TrailingZeros16-4   18.48n ± 0%     17.68n ± 0%   -4.33% (p=0.000 n=10)
TrailingZeros32-4   16.87n ± 0%     16.07n ± 0%   -4.74% (p=0.000 n=10)
TrailingZeros64-4   15.26n ± 0%     15.27n ± 0%   +0.07% (p=0.043 n=10)
OnesCount32-4       20.08n ± 0%     19.29n ± 0%   -3.96% (p=0.000 n=10)
RotateLeft-4        8.864n ± 0%     8.838n ± 0%   -0.30% (p=0.006 n=10)
RotateLeft32-4      8.837n ± 0%     8.032n ± 0%   -9.11% (p=0.000 n=10)
Reverse32-4         29.77n ± 0%     26.52n ± 0%  -10.93% (p=0.000 n=10)
ReverseBytes32-4    9.640n ± 0%     8.838n ± 0%   -8.32% (p=0.000 n=10)
Sub32-4             8.835n ± 0%     8.035n ± 0%   -9.06% (p=0.000 n=10)
geomean             11.50n          11.33n        -1.45%

pkg: crypto/md5
Hash8Bytes-4             1.486µ ± 0%   1.426µ ± 0%  -4.04% (p=0.000 n=10)
Hash64-4                 2.079µ ± 0%   1.968µ ± 0%  -5.36% (p=0.000 n=10)
Hash128-4                2.720µ ± 0%   2.557µ ± 0%  -5.99% (p=0.000 n=10)
Hash256-4                3.996µ ± 0%   3.733µ ± 0%  -6.58% (p=0.000 n=10)
Hash512-4                6.541µ ± 0%   6.072µ ± 0%  -7.18% (p=0.000 n=10)
Hash1K-4                 11.64µ ± 0%   10.75µ ± 0%  -7.58% (p=0.000 n=10)
Hash8K-4                 82.95µ ± 0%   76.32µ ± 0%  -7.99% (p=0.000 n=10)
Hash1M-4                10.436m ± 0%   9.591m ± 0%  -8.10% (p=0.000 n=10)
Hash8M-4                 83.50m ± 0%   76.73m ± 0%  -8.10% (p=0.000 n=10)
Hash8BytesUnaligned-4    1.494µ ± 0%   1.434µ ± 0%  -4.02% (p=0.000 n=10)
Hash1KUnaligned-4        11.64µ ± 0%   10.76µ ± 0%  -7.52% (p=0.000 n=10)
Hash8KUnaligned-4        83.01µ ± 0%   76.32µ ± 0%  -8.07% (p=0.000 n=10)
geomean                  28.32µ        26.42µ       -6.72%

Change-Id: I20483a6668cca1b53fe83944bee3706aadcf8693
Reviewed-on: https://go-review.googlesource.com/c/go/+/528975
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>