Re: [Qemu-devel] [PATCH] target-m68k: add rol/ror/roxl/roxr instructions

[Richard Henderson <rth@twiddle.net>]

On 11/09/2016 06:30 PM, Laurent Vivier wrote:
> +        /* create [src:X:..] */
> +
> +        tcg_gen_deposit_i32(t0, QREG_CC_X, src, 1, size);
> +        tcg_gen_shli_i32(t0, t0, 31 - size);
> +
> +        /* rotate */
> +
> +        tcg_gen_rotl_i32(t0, t0, shift);
> +
> +        /* result is [src:..:src:X] */
> +
> +        tcg_gen_andi_i32(X, t0, 1);
> +        tcg_gen_shri_i32(t0, t0, 1);

I don't see how this is supposed to work.  If you form [src:x:...], and rotate 
by 0, then X gets garbage.  Of course, you're actually forming [0:src:x].  But 
for a rol of 2, the lsb of src gets 0's instead of the msb of src.

If you want to use a 32-bit rotate here, you have to (1) reduce the rotate by 
modulo size + 1 and (2) form [src:...:src:x].  E.g.

     tcg_gen_deposit_i32(t0, Q_REG_CC_X, src, 32 - size, size);
     tcg_gen_deposit_i32(t0, t0, src, 1, size);


r~