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.
So, I recommend the following:
+ Remove the naked keyword as an assembly 'instruction'.
+ Instate it as a function property, similarly to 'extern (C)'. So you might
see the following declaration:
extern naked void flushIDT() {
mov EAX, [ESP+4]
lidt [EAX]
ret
}
It doesn't need to part of the function signature, though, does it?
Though, if the assembly is implicit, it might be better to rename the keyword
'asm' or something like that to make it clearer. Anyway, these changes will, in
my humble opinion, make the language cleaner and my life easier because I can
simply declare this function by myself.
Because of what I wrote above, I think this removes too much
functionality. I think it would be quite reasonable, though, to make
non-asm code generation illegal inside a naked function. BTW this
probably includes contracts (at present, contracts don't work for naked
functions even in DMD).
Would that restriction be enough?
Cheers!
-Duane
