Re: [PATCH v2 00/19] target/i386: decoder changes for 8.2

[Richard Henderson]

On 10/19/23 03:46, Paolo Bonzini wrote:
> This includes:
>
> - implementing SHA and CMPccXADD instruction extensions
>
> - introducing a new mechanism for flags writeback that avoids a
>    tricky failure
>
> - converting the more orthogonal parts of the one-byte opcode
>    map, as well as the CMOVcc and SETcc instructions.
>
> Tested by booting several 32-bit and 64-bit guests.
>
> The new decoder produces roughly 2% more ops, but after optimization there
> are just 0.5% more and almost all of them come from cmp instructions.
> For some reason that I have not investigated, these end up with an extra
> mov even after optimization:
>
>                                  sub_i64 tmp0,rax,$0x33
>   mov_i64 cc_src,$0x33           mov_i64 cc_dst,tmp0
>   sub_i64 cc_dst,rax,$0x33       mov_i64 cc_src,$0x33
>   discard cc_src2                discard cc_src2
>   discard cc_op                  discard cc_op
>
> It could be easily fixed by not reusing gen_SUB for cmp instructions,
> or by debugging what goes on in the optimizer.  However, it does not
> result in larger assembly.

This is expected behaviour out of the tcg optimizer.  We don't forward-propagate outputs 
at that point.  But during register allocation of the "mov cc_dst,tmp0" opcode, we will 
see that tmp0 is dead and re-assign the register from tmp0 to cc_dst without emitting an 
host instruction.


r~