Re: [PATCH] x86: suppress emission of zero displacements in memoryoperands

[ctm at ardi dot com (Clifford T. Matthews)]

Howdy,

I'm a little over a year late to the discussion, but it was only
yesterday that I figured out that I was being bitten by a change that
was made in May of 2005.

Syn68k is software (written by Mat Hostetter) that dynamically
translates 68k assembly to x86 assembly.  It doesn't want to invoke a
full-blown assembler as it does its work, so instead, at build time,
it uses gas to compile a whole bunch of templates and figure out the
significance of the bits that get created.

Starting with binutils-2.16.90.0.3, instructions like

    lea 0x0(%eax),%eax

began compiling into the exact same byte sequence as

    lea (%eax),%eax

That change broke Syn68k, because it observed that the byte sequence
for a zero displacement was significantly different from the rest (two
bytes versus three bytes).  In theory, Syn68k should have been able to
construct translation code that understood the difference.  In
reality, it created code that didn't work.  Oops.

I "fixed" Syn68k by putting in special-case code that tells it to
ignore the templates that have 0 displacements.  That happens to work
for our current template set.  A better fix would be to fix the
pattern matching code to be able to deal with this new change, but
there's not sufficient use of Syn68k to justify the effort.

An alternative fix would be to use whatever syntax (or command line
switch) gas would take to allow Syn68k to tell gas that it really
does want a zero displacement rather than no displacement.  However,
AFAICT, there no longer is such a syntax, nor is there a command line
option to suppress the elision.

I do exceedingly small amounts of assembly work these days and when
things don't go my way, I can still hack tools that I have the source
for (well... actually I can hack tools that I don't have the source
for, but that takes even longer).  As such, the lack of syntax or a
command line switch to request a zero displacement doesn't really
matter to me, but on May 6th 2005, DaveK wrote:

   "Nothing the user _explicitly_ specifies should ever be discarded
    IMO.  What if someone wants to write self-modifying code that
    stores varying offsets into that field?"

I largely agree with that sentiment, but with 20/20 hindsight and
standing on his shoulders, perhaps I'd say

    Removing a way for a user to explicitly request something is bad.
    What if someone wants what he requested for some reason that we
    haven't anticipated?

Either way, my experience might be an interesting data point for some
of you.

-- 
Cliff Matthews <ctm@ardi.com>