Inline assembly syntax

Ian Lance Taylor
Sat May 8 00:56:00 GMT 2004

I'm going to try to write this from the point of a view of a gcc user
rather than a gcc developer.  Contributions welcome.  In particular, I
haven't used CW asm syntax, so I am, to some extent, guessing.

Traditional GCC asm syntax
    * Permits reasonably precise description of types of operands
      which may appear in assembler code.
    * Explicitly describes which operands are modified and which are
      not, including read-write operands.
    * Explicitly describes anything else which may be affected by the
      assembler code.
    * Precise description of side-effects permit various
      optimizations, including scheduling the asm or simply
      eliminating it entirely.
    * Mapping from C variables to assembler operands is awkward.
    * Use of constraints is cryptic.  Documentation is insufficient.
    * Particular pairs of constraints are especially confusing, such
      as 'm' vs. 'o'.
    * No way to describe affects on particular memory locations.
    * No way disable scheduling without introducing inappropriate
      clobbers--e.g., you can't safely grab a semaphore without
      clobbering all of memory.
    * Requiring \n or \; between instructions is tedious.
    * In fact, string constants and backslash handling in general is

CW asm syntax
    * Mapping from C variables to assembler operands is
    * Avoiding string constants makes code easier to write.
    * Compiler already knows side-effects of ordinary instructions.
    * Compiler knows register requirements of ordinary instructions.
    * No way to describe side-effects for unusual operations, such as
      supervisor calls.
    * No way to disable scheduling across instructions.
    * No obvious way to encourage compiler to put operand in the right
      place, other than using an instruction to move it there, unless
      the compiler is smart enough to detect that case and eliminate
      the instruction if it is not required.
    * Requires extensive assembly language parsing for any fully
      supported target.

So, putting these together, the ideal assembler syntax would have
characteristics like these:

* Straightforward mapping from C variables to assembler operands.
* Avoid string constants and associated backslash syntax.
* Compiler would automatically determine side effects of known
* Support specifying additional side effects.
* Scheduling barriers could be inserted.
* Clobbering of precise memory locations could be discerned and
* Compiler would automatically determine required locations for
* Some mechanism would be available to specify required operand

These lead us in the direction of

    add r0, foo, bar    // register r0 = variable foo + variable bar
    add r0, "r1", r2    // register r0 = variable r1 + register r2
    {                   // instructions in block may not be rearranged
      cli               // disable interrupts
      add 0(r0), 1      // * (register r0) += 1
      sti               // enable interuppts
    svc 21 __clobber__(r0,r1,r2) // supervisor call
    __noschedule__      // scheduling barrier
    addi r0, r1, constant __constraint__(i) // specify constraint



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