On Wed, Oct 02, 2024 at 01:59:03PM +0200, Richard Biener wrote:
> As you are using input constraints to mark symbol uses maybe we can
> use output constraints with a magic identifier (and a constraint letter
> specifying 'identifier'):
>
> asm (".globl %0; %0: ret" : "_D" (extern int foo()) : ...);
>
> In the BOF it was noted that LTO wants to be able to rename / localize
> symbols so both use and definition should be used in a way to support
> this (though changing visibility is difficult - the assembler might
> tie to GOT uses, and .globl is hard to replace).
Seems we have quite a few free letters on the constraint side, I think
'-.:;[]@(){}|_`
are all currently rejected in both input and output constraints
(we only allow ISALPHA constraint chars for the target specific ones).
So, using one of those for "the inline asm defines this global symbol",
another for "the inline asm defines this local symbol", and another
"the inline asm uses this function" might be possible;
Of course it can be also just one of those special characters with
some qualifier after it, we'd just need to tweak the generated
insn_constraint_len for that. I think the asm uses some variable can
be expressed with "m" (var) just fine.
Anyway, for function and var definitions, should the compiler be told
more information (e.g. whether it is weak or non-weak), or should one
assume that from the actually used FUNCTION_DECL/VAR_DECL? Should the
global vs. local be also implied from it? Though for variables, how does
one declare a static variable declaration but not definition?
And another thing is what the argument should be, what you wrote about
would be hard to parse (parsing dependent on the constraints, we usually
don't parse the constraint until the expression is parsed).
extern int foo ();
asm ("... : : "_" (foo));
probably would (but maybe just extern "C" for C++).
And we'd need to decide whether for LTO toplevel inline asm with
extended asm (as a new extension) is actually required not to rely on
the exact function/variable name at least in the non-global case and
the compiler is allowed to rename them (i.e. use %c0 etc. rather than
the actual name).
Jakub
