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ARM SVE issues with non "standard" vector lengths

From: Laurent Desnogues
Subject: ARM SVE issues with non "standard" vector lengths
Date: Thu, 23 Apr 2020 15:59:46 +0200


I found the following issues in SVE while playing with a vector length
of 640-bit.

1. sve_uzp_p

I think the comment about VL not being a power of 2 should be that VL
is not a multiple of 512-bit elements with VL > 512 (not sure how to
phrase that properly).

        if (oprsz & 15) {
            d[i] = compress_bits(n[2 * i] >> odd, esz);
Here n[2 * i + 1] should be taken into account.

            for (i = 0; i < oprsz_16; i++) {
                l = m[2 * i + 0];
                h = m[2 * i + 1];
                l = compress_bits(l >> odd, esz);
                h = compress_bits(h >> odd, esz);
                tmp_m.p[i] = l + (h << 32);
            tmp_m.p[i] = compress_bits(m[2 * i] >> odd, esz);
Here m[2 * i + 1] should be taken into account.

2. sve_zip_p

This generates extraneous data in the higher part of the result.

I hit this when I got a wrong result on an instruction that ends up
using sve_cntp which counts all bits set in each 64-bit chunk. There
might be some other instructions beyond ZIP that generate extra data
that would break sve_cntp.  So perhaps it'd be easier to fix sve_cmtp
(and hope that it's the only function that uses bits beyond vector

I hope I got all of this correctly I'm not familiar with that
implementation of SVE :)



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