🚀 go-pugleaf

You need two steps: an address computation and a jmp.

asm("shll %1, %0"
    "addl _first_case, %0"
    "jmpl %0"
    : /* no output */
    : "r" (offset), "I" (align_size)
    : /* no clobbers */);

Alternatively, to let the compiler potentially reschedule the shift:
asm("addl _first_case, %0"
    "jmpl %0"
    : /* no output */
    : "r" (offset * align_size)
    : /* no clobbers */);

As long as you keep the code smaller than the alignment size, you should be
safe. Be -extremely- careful not to have a case larger than the alignment
size.

-Matt

"Vis Mike" <visionary25@_nospam_hotmail.com> wrote in message
news:b2d1sj$8j4$1@bob.news.rcn.net...
> I found a nice feature in gcc - -malign-jumps=n
>
> > wcc -O3 -malign-jumps=3 vmtest2.cpp
> > a.exe
> Testing stack4...
> &&addi = 0x401180 +4 +8 +16
>
> 0x401180 is the address of the first label, followed by the offset of the
> rest of the labels.  So if I can squeeze the code in, all I need is a "jmp
> (opcode*4)".  This is sooo unportable :)  I'll just use conditional
> compilation.
>
> I don't quite understand gcc's inline assembly syntax, any help for the
jmp?
> Or am I just going overboard?
>
> Cheers!
> Mike
>
> "James Van Buskirk" <not_valid@attbi.com> wrote in message
> news:dCk2a.80786$HN5.307889@rwcrnsc51.ops.asp.att.net...
> >
> > "Matt Taylor" <para@tampabay.rr.com> wrote in message
> > news:h2j2a.61691$AQ3.981480@twister.tampabay.rr.com...
> >
> > > Any good compiler will convert switch/case to a jump table for you. VC
> > does
> > > this; I'm quite sure that GCC does too. Unless you're writing in
> assembly,
> > > don't try to pad things out. The compiler should do this for you. I
> don't
> > > know when specifically VC/GCC do and when they don't.
> >
> > I think I have a different interpretation of Mike's question than
> > you: my take on it is that he has code where all branches are just
> > little snippets no longer than some nice easy number of bytes to
> > multiply by, e.g. if all his branches were less than 32 bytes
> > long and he had 16 possible cases numbered 0:15 he could, assuming
> > the branch number were in EAX do something like:
> >
> > lea EAX, [4*EAX]
> > jmp branch_implementation_start[8*EAX]
> >
> > In this way he could avoid waiting for the address of a previous
> > write to memory to be computed before prefetching the code
> > following the branch.  Maybe ALIGN directives or ORG
> > directives could tell the assembler where to insert padding
> > without having to maintain it by hand every time the code
> > is modified?