Re: [Qemu-devel] x86: cvtsi2s{s,d} etc. array access

[Peter Maydell <peter.maydell@linaro.org>]

On 14 May 2012 22:05, Blue Swirl <blauwirbel@gmail.com> wrote:
> While working on the AREG0 patches, I noticed strange code in
> target-i386/translate.c.

> It's accessed like this (line 3537):
>            sse_op2 = sse_op_table3[(s->dflag == 2) * 2 + ((b >> 8) - 2)];
>
> b >> 8 can be only either 1 or 0.

I don't think this is true. At this point in the code we're inside
a "switch (b)" so we know that b is either 0x22a (cvtsi2ss) or
0x32a (cvtsi2sd). So "((b >> 8) - 2)" is 0 for cvtsi2ss and 1
for cvtsi2sd, giving us the lsbit of the array index, with
(s->dflag == 2) providing the next bit, so we end up with
indexes 0,1,2,3 in this table for these two insns in their
doubleword and quadword forms.

You could rewrite "((b >> 8) - 2)" as "((b >> 8) & 1)".

> The other access is as follows (line 3594):
>            sse_op2 = sse_op_table3[(s->dflag == 2) * 2 + ((b >> 8) - 2) + 4 +
>                                    (b & 1) * 4];
>
> This looks better because of + 4 but I think some array values are not
> accessible (max. 1 * 2 + (1 - 2) + 4 + 1 * 4 == 9).

Here we know b is 0x22c (cvttss2si) 0x32c (cvttsd2si) 0x22d (cvtss2si)
or 0x32d (cvtsd2si). ((b >> 8) - 2) distinguishes the 0x2XX and 0x3XX,
and (b & 1) the 0xXXc from 0xXXd. So the index is made up of (lsbit to
msbit) "0x2XX or 0x3XX?", "double or quad?", "0xXXC or 0xXXD?", and then
we add a constant offset of 4 because the entries start after the
4 entries for the cases we looked at earlier.

I think you could actually split sse_op_table3 into two separate
tables, one for each of these cases, which would be slightly
clearer IMHO.

-- PMM