Re: [RFC v4 02/11] build: Implement libnative library and the build machinery for libnative

[Richard Henderson <richard.henderson@linaro.org>]

On 8/8/23 07:17, Yeqi Fu wrote:
> +#if defined(i386) || defined(x86_64)
> +/*
> + * An unused instruction is utilized to mark a native call.
> + */
> +#define __CALL_EXPR ".byte 0x0f, 0xff;"
> +#endif

This is 2 of the 3 (or more) bytes of the UD0 instruction.
At minimum you should include a third byte for the modrm.

For example,

	0F FF C0	ud0	%eax, %eax

If you want to encode more data, or simply magic numbers, you can use

	0F FF 80
	78 56 34 12	ud0	0x12345678(%eax), %eax

or with modrm + sib bytes,

	0F FF 84 00
	78 56 34 12	ud0	0x12345678(%eax, %eax, 0), %eax

So you have up to 32 (displacement) + 3 * 3 (registers) + 2 (shift) = 43 bits that you can 
vary while staying within the encoding of UD0.

You can even have the assembler help encode a displacement to associated data:

	.text
0:	ud0	label-0b(%eax), %eax
	.rodata
label:	.byte	some stuff


r~