[Bug inline-asm/97708] Inline asm does not use the local register asm specified with register ... asm() as input

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97708

--- Comment #30 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Asm operand binding should work by looking at bound lvalue: "c"(a) binds an
lvalue so if 'a' is a register var the compiler must remember its associated
register; "c"(a+0) binds an rvalue, so what kind of variable 'a' is is
irrelevant.

The way it should work internally is at gimplification time the compiler should
produce an additional asm statement argument that stores the associated
register names for each operand. For example. for

  register int a asm("%eax");
  asm("" : "r"(a+0), "r"(a), "r"(0));

gcc could internally produce a string ",%eax," signifying that operand 1 is
bound to %eax and operands 0 and 2 are not bound to any particular register.

Then all passes up to IRA don't need to give any particular care for asm
operands, and IRA can use the string to place operands in appropriate registers
(and diagnose a mismatch).

This is also the only proper way to fix PR 87984 as far as I can tell.