[Bug inline-asm/82701] New: RFE: x86: double word operands in inline assembly

["hpa at zytor dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82701

            Bug ID: 82701
           Summary: RFE: x86: double word operands in inline assembly
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hpa at zytor dot com
  Target Milestone: ---

x86 inline assembly currently has no sensible way to use doubleword operands
(long long on x86-32, __int128 on x86-64) without restricting them to the d:a
register pair via the "A" constraint, and hard-coding the register names.  It
would be highly desirable to have a clean way to emit into the assembly code
the upper half of a doublewidth operand (register or memory) without
artificially constraining code generation.

In some ways, this is similar to the "H" modifier, except it would be +4 for
memory operands on 32 bits.  I'm calling it U below.

For an artificial example:

uint64_t a, b;

/* a += b; */
asm("add %1,%0; adc %U1,%U0" : "+rm,r" (a) : "ri,m" (b) : "cc");

For a register: print the high register of the pair.
For a memory operand: print the address + the word size
For an immediate: print the value >> (word size*8)