Tomas Lindquist Olsen wrote:
On Wed, Jan 7, 2009 at 12:41 PM, Don <nos...@nospam.com
<mailto:nos...@nospam.com>> wrote:
Tomas Lindquist Olsen wrote:
On Fri, Jan 2, 2009 at 9:21 AM, Don <nos...@nospam.com
<mailto:nos...@nospam.com> <mailto:nos...@nospam.com
<mailto:nos...@nospam.com>>> wrote:
Duane Bailey wrote:
I am currently porting LDC to PowerPC and, hopefully,
eventually the POWER and CELL platforms as well. The
first bit
requires me to port the inline assembler, allowing me to
review the problems that the D language presents LLVM.
Cool!!!!
LLVM is not a toy virtual machine. It is, perhaps, the most
flexible and powerful compiler toolset ever, spanning massive
numbers of computing platforms. It even supports (in a
limited
manner) the PIC16 platform, require insane constraints: there
are no registers, memory can only be accessed in one byte
amounts, and some processors only have 35 instructions.
That's pretty impressive. I'm currently using a PIC, but it's so
memory-limited it's hard to believe D ever being workable on it.
LLVM, however, is not able to do everything. For some reason,
its current API does not allow the restriction of
prologue and
epilogue generation; to allow so would not make sense: the
language itself depends on the maintenance of the stack. The
only way to establish a 'naked' function in *c* is to
'omit' the
frame pointer—technically not allowed in most OS's
ABIs—and then
explicitly avoid using all variables (and hence the
stack), OR
to use top level assembly to write the assembly yourself.
Now, neither of those options are really what D should
use, but
I have some other recommendations based on this. 'naked'
functions should not be allowed to have any D, except to
reference arguments passed to it. In other words, it
should not
touch the stack. in fact, there's really no reason at all to
have the 'naked' statement in the inline assembly. It's
not a
property of the assembly, it's a property of the
*function*. And
because D code should not be used (except perhaps for
macros?),
'naked' functions should intrinsically be assembly functions.
I agree with this. Mixing run-time D and naked asm doesn't
make any
sense. But, something which I've done which is _very_ useful
is to
mixin CTFE functions. You get something like:
void foo() {
asm {
naked;
}
mixin(someasm("EBX")); // becomes asm {mov EAX, EBX; }
asm { ret; }
}
char [] someasm(char [] c) {
return "asm { mov EAX," ~ c ~"; }";
}
I see this as crucial functionality since it gives you an
unbelievably powerful macro language in the assembler.
it should be no problem to merge asm blocks in a function, the
only problem is mixing normal dcode in there as well.
I've decided to make this an error in LDC since there is no
sensible way to implement D-style function parameters *and* make
sure the function really is naked. function parameters in llvm
are ssa values, so you manually have to alloca a stack slot and
copy the argument into that to make sure the parameter is an l-value
I don't understand those last two sentences. Does it mean that you'd
only allow 'naked' on extern(C) functions? Or only that mixing D and
naked asm would be illegal?
Sorry, I can see I was a bit unclear. All I meant was that I can't
sensibly implement naked and allow arbitrary D code in the mix as well,
it's either or... naked should eventually work fine for any calling
conventions, it'll just bail out with an error if you try and mix D in
between the asm blocks.
By the way, in DMD, mixing naked and D code only works if you put a
"push EBP; mov EBP, ESP;" before the first bit of D code, which
isn't documented anywhere as far as I know; and I don't know how to
make inner functions work. So it's pretty much unspecified behaviour
right now.
nested delegates inside naked functions pose some problems, since they
will modify the stack frame as well (like allowing full access to
function parameters, like taking the address of one, which was what I
was trying to talk about in my reply)
Consider taking the address of a parameter that is passed by value in
EAX, this will allocate a stack slot...
Hope I was a bit more clear this time :)
Yes. And that's awesome! It means my existing Bigint naked asm code in
Tango should work on LDC without modification.
