Re: [RFC] gcc feature request: Moving blocks into sections

From: H. Peter Anvin
Date: Tue Aug 06 2013 - 00:28:38 EST

On 08/05/2013 09:14 PM, Mathieu Desnoyers wrote:
>> For unconditional jmp that should be pretty safe barring any fundamental
>> changes to the instruction set, in which case we can enable it as
>> needed, but for extra robustness it probably should skip prefix bytes.
> On x86-32, some prefixes are actually meaningful. AFAIK, the 0x66 prefix
> is used for:
> E9 cw jmp rel16 relative jump, only in 32-bit
> Other prefixes can probably be safely skipped.

Yes. Some of them are used as hints or for MPX.

> Another question is whether anything prevents the assembler from
> generating a jump near (absolute indirect), or far jump. The code above
> seems to assume that we have either a short or near relative jump.

Absolutely something prevents! It would be a very serious error for the
assembler to generate such instructions.


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