Re: [Nasm-devel] [nosplit eax]

[anonymous c. <nas...@us...>]

> Apparently [nosplit eax] is treated as if it was [nosplit eax*1], which
> was a deliberate patch introduced by Debbie in 0.98.34.  This is rather
> unexpected behavior, and I would really like to change it so that
> nosplit means "the term with the multiplication is the index".  This is
> a change, but does anyone anticipate a problem with doing this?

tl;dr = keep NOSPLIT unchanged, but prevent *2 opt with MPX

Historically NASM's goal has always been to emit the shortest
possible encoding. As a result, (nobase+)reg(*1) becomes just
reg (which does emit a sib-less encoding), and (nobase+)reg*2
becomes reg+reg*1 (which does emit a disp-less sib encoding
rather than a sib encoding with a signed dword disp). To give a
user the ability to emit the longer encodings when desired, the
NOSPLIT qualifier was added. Initially it only handled the case
of *2 (and that is documented) but eventually support for *1 got
added as well (but apparently not documented properly). Under

  http://sourceforge.net/p/nasm/feature-requests/5/

you can see the original feature request. From me, actually.  :)

That said, NOSPLIT is working as intended (though the *1 case
ought to be documented). Please refrain from breaking it, since
that will break existing code. Likewise, please don't change the
existing reg*1 or reg*2 behavior, i.e. don't suddenly interpret an
explicit *1 or *2 as a request for a longer encoding -- since that
too will break existing code.

In terms of BNDMK, BNDLDX, and BNDSTX, the [reg] case as
well as the [NOSPLIT reg] case are fine. The [NOSPLIT reg*2]
case is fine as well (though BNDLDX and BNDSTX now need a
warning for the resulting scale=2); however, the [reg*2] case is
of course problematic since NASM turns it into [reg+reg] which
has a base that the user did not specify. Since MPX is new, it
looks like disabling that case of *2 splitting is the right choice.