On 12 December 2017 at 18:32, James Morse <james.morse@xxxxxxx> wrote:
> As this is over my head, I've been pushing random encodings through gas/objdump
> and then tracing them through here.... can this encode 0xf80000000fffffff?
>
> gas thinks this is legal:
> | 0: 92458000 and x0, x0, #0xf80000000fffffff
>
> I make that N=1, S=0x20, R=0x05.
> (I'm still working out what 'S' means)
This comment from QEMU (describing the decode direction, ie
immn,imms,immr => immediate) might assist:
/* The bit patterns we create here are 64 bit patterns which
* are vectors of identical elements of size e = 2, 4, 8, 16, 32 or
* 64 bits each. Each element contains the same value: a run
* of between 1 and e-1 non-zero bits, rotated within the
* element by between 0 and e-1 bits.
*
* The element size and run length are encoded into immn (1 bit)
* and imms (6 bits) as follows:
* 64 bit elements: immn = 1, imms = <length of run - 1>
* 32 bit elements: immn = 0, imms = 0 : <length of run - 1>
* 16 bit elements: immn = 0, imms = 10 : <length of run - 1>
* 8 bit elements: immn = 0, imms = 110 : <length of run - 1>
* 4 bit elements: immn = 0, imms = 1110 : <length of run - 1>
* 2 bit elements: immn = 0, imms = 11110 : <length of run - 1>
* Notice that immn = 0, imms = 11111x is the only combination
* not covered by one of the above options; this is reserved.
* Further, <length of run - 1> all-ones is a reserved pattern.
*
* In all cases the rotation is by immr % e (and immr is 6 bits).
*/
so N=1 S=0x20 means run length 33, element size 64 (and
indeed your immediate has a run of 33 set bits).
(The Arm ARM pseudocode is confusing here because it merges
the handling of logical-immediates and bitfield instructions
together, which is nice if you're a hardware engineer. For
software you're much better off keeping the two separate.)
thanks
-- PMM
