[Bug libc/34334] New: [RISC-V] Vector memset performance issues on Spacemit X100 (K3)

https://sourceware.org/bugzilla/show_bug.cgi?id=34334

            Bug ID: 34334
           Summary: [RISC-V] Vector memset performance issues on Spacemit
                    X100 (K3)
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: anton at ozlabs dot org
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Here are some results for memset on the Spacemit X100. We did suspect that
smaller lmul levels might be preferable on OoO CPUs, and the X100 data confirms
that. The current lmul=8 vector implementation has some quite large regressions
to the scalar implementation for small (< 128B) lengths, and an lmul=2 version
largely fixes that.

On top of this, I did notice some sensitivity to the compiled code. My initial
suspicion is some fetch alignment issues, but I'm not convinced I understand it
completely. From build to build the numbers bounce around a bit even with
explicit alignment of the memset routine. Perhaps the alignment of the
benchmark code itself is significant.

In the data below:

__memset_vector  - Current lmul=8 loop
m2               - lmul=2 loop
m2_balign32      - 32 byte aligned function
m2_balign32_loop - 32 byte aligned loop

I'm likely overaligning the loops at 32 bytes, but it's a starting point.

             generic_memset        __memset_vector                    m2       
   m2_balign32      m2_balign32_loop      __memset_generic
==========================================================================================================================================
     length=1:         6.47        14.12 (-118.2%)        9.10 (-40.72%)       
3.66 ( 43.48%)        4.10 ( 36.56%)        8.67 (-34.04%)
     length=2:         9.61        14.55 (-51.38%)        9.10 (  5.29%)       
3.65 ( 62.06%)        4.10 ( 57.35%)       11.38 (-18.36%)
     length=4:         8.19        14.10 (-72.07%)        9.10 (-11.05%)       
3.64 ( 55.56%)        4.10 ( 49.97%)       13.19 (-61.02%)
     length=8:         6.83        14.10 (-106.4%)        9.10 (-33.21%)       
3.64 ( 46.69%)        4.10 ( 39.99%)       10.92 (-59.86%)
    length=16:         8.65        14.10 (-62.96%)        9.10 ( -5.17%)       
3.65 ( 57.85%)        4.10 ( 52.62%)       13.19 (-52.50%)
    length=32:        11.38        14.10 (-23.92%)        9.10 ( 20.03%)       
3.64 ( 67.99%)        4.10 ( 63.97%)       14.56 (-27.94%)
    length=64:         8.19        14.10 (-72.07%)        9.10 (-11.05%)       
3.65 ( 55.49%)        4.10 ( 49.97%)       11.83 (-44.38%)
   length=128:        12.29        14.10 (-14.73%)       10.62 ( 13.58%)       
6.38 ( 48.10%)        7.29 ( 40.69%)       14.86 (-20.94%)
   length=256:        16.26        14.10 ( 13.29%)       14.10 ( 13.26%)       
8.35 ( 48.64%)        7.75 ( 52.33%)       20.89 (-28.50%)
   length=512:        29.92        21.85 ( 26.98%)       21.84 ( 27.01%)      
18.46 ( 38.29%)       18.46 ( 38.30%)       35.48 (-18.60%)
  length=1024:        90.66        89.31 (  1.49%)       89.40 (  1.40%)      
89.81 (  0.94%)       88.92 (  1.92%)       90.26 (  0.44%)
  length=2048:       116.83       102.34 ( 12.40%)      102.87 ( 11.95%)     
102.36 ( 12.38%)      102.37 ( 12.38%)      122.75 ( -5.06%)
  length=4096:       233.98       219.48 (  6.20%)      220.03 (  5.96%)     
219.48 (  6.20%)      220.11 (  5.93%)      239.11 ( -2.19%)
  length=8192:       467.34       462.42 (  1.05%)      462.23 (  1.09%)     
462.20 (  1.10%)      462.12 (  1.12%)      472.60 ( -1.13%)
 length=16384:       959.76       947.15 (  1.31%)      946.98 (  1.33%)     
940.38 (  2.02%)      800.38 ( 16.61%)     1013.21 ( -5.57%)
 length=32768:      1904.87      1708.09 ( 10.33%)     1703.92 ( 10.55%)    
1702.82 ( 10.61%)     1702.48 ( 10.62%)     1874.48 (  1.60%)
 length=65536:      3786.95      3484.48 (  7.99%)     3384.22 ( 10.63%)    
3364.85 ( 11.15%)     3339.17 ( 11.82%)     3739.33 (  1.26%)
length=131072:      7720.54      7153.91 (  7.34%)     7102.45 (  8.01%)    
7022.39 (  9.04%)     6962.38 (  9.82%)     7954.83 ( -3.03%)

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