RFC: [PATCH] X86: Add pseudo prefixes to control encoding

[Florian Weimer]

On 11/04/2016 07:24 PM, H.J. Lu wrote:
> Many x86 instructions have more than one encodings.  Assembler picks
> the default one, usually the shortest one.  Although the ".s", ".d8"
> and ".d32" suffixes can be used to swap register operands or specify
> displacement size, they aren't very flexible.  This patch adds pseudo
> prefixes, {xxx}, to control instruction encoding.  The available
> pseudo prefixes are {disp8}, {disp32}, {swap}, {vex2}, {vex3} and
> {evex}.  Pseudo prefixes are preferred over the ".s", ".d8" and ".d32"
> suffixes, which are deprecated.

For certain ways of writing jump tables, instruction length 
specifications would be quite useful.  The assembler would be free to 
choose any encoding, as long as it has the required instruction length.

Here's a bug which  was caused by optimizing

   mov 0(%esi,%ecx,4),%edx   to   mov (%esi,%ecx,4),%edx

in GAS:

<https://lists.debian.org/debian-ocaml-maint/2005/11/msg00223.html>

Florian