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Re: [Qemu-devel] [PATCH v4 04/64] tcg/aarch64: Implement field extractio


From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v4 04/64] tcg/aarch64: Implement field extraction opcodes
Date: Tue, 6 Dec 2016 08:36:49 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 12/06/2016 04:24 AM, Alex Bennée wrote:
>> > +    case INDEX_op_extract_i64:
>> > +    case INDEX_op_extract_i32:
>> > +        tcg_out_ubfm(s, ext, a0, a1, a2, a2 + args[3] - 1);
>> > +        break;
>> > +
>> > +    case INDEX_op_sextract_i64:
>> > +    case INDEX_op_sextract_i32:
>> > +        tcg_out_sbfm(s, ext, a0, a1, a2, a2 + args[3] - 1);
>> > +        break;
>> > +
> This isn't right is it? As I'm reading it extract takes from a
> offset+len from the source register to low bits of the destination
> register. The Bitfield Move instructions are the other way around,
> moving from the low order bits in the source register to an offset+len
> in the destination.
> 

It is right.  Extract is written as ofs/len in assembly, but encoded as lsb/msb
in the opcode -- just like bitfield move.

Boot an armv7 guest and there should be enough uses to convince you.


r~



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