Accessing structure members from inline assembly?

Question

I'm trying to use inline assembly to load a bunch of structure members (Particle is a pointer to such structure) into some registers. Here's my initial solution:

asm("mov    %1(%0), %%edx\n"
    "fld    %2(%0)\n"
    "fld    %3(%0)\n"
    "fld    %4(%0)\n"
    "fld    %5(%0)\n"
    "movups %6(%0), %%xmm1\n"
    "movups %7(%0), %%xmm2\n"
    "movups %8(%0), %%xmm3\n"
    "movups %9(%0), %%xmm4\n"
    :
    : "r" (Particle),
        "n" (offsetof(ptcParticle, Active)),
        "n" (offsetof(ptcParticle, Size)),
        "n" (offsetof(ptcParticle, Rotation)),
        "n" (offsetof(ptcParticle, Time)),
        "n" (offsetof(ptcParticle, TimeScale)),
        "n" (offsetof(ptcParticle, Colour)),
        "n" (offsetof(ptcParticle, Location)),
        "n" (offsetof(ptcParticle, Velocity)),
        "n" (offsetof(ptcParticle, Accel))
    : "%edx", "%st", "%st(1)", "%st(2)", "%st(3)", "%xmm1", "%xmm2",
        "%xmm3", "%xmm4"
);

It doesn't work, though, as GCC outputs these offsets as immediate number literals, like this:

mov     $0(%eax), %edx
fld     $44(%eax)
fld     $40(%eax)
fld     $8(%eax)
fld     $4(%eax)
movups  $12(%eax), %xmm1
movups  $28(%eax), %xmm2
movups  $48(%eax), %xmm3
movups  $60(%eax), %xmm4

As a result, gas treats (%eax) as junk after expression:

Error: junk `(%eax)' after expression

This would work if I could only get rid of the dollar sign in the output. Any idea how to access structure members?

The same problem with `structs` and the `$` is discussed at [GCC extended asm, struct element offset encoding](http://stackoverflow.com/q/13254512). This may be a duplicate of the question, but I'm not sure because the other question is in the context of Extended ASM. Also see [What does %c mean in GCC inline assembly code?](http://stackoverflow.com/q/1672900/608639) — jww, Aug 19 '15 at 17:22

score 6 · Accepted Answer · answered Dec 02 '12 at 10:03

Okay, I've figured it out - the %c operator is needed. I've written this helper macro:

#define DECLARE_STRUCT_OFFSET(Type, Member)     \
    [Member] "i" (offsetof(Type, Member))

And use it like this:

    asm("mov    %c[Active](%0), %%edx\n"
        "fld    %c[Size](%0)\n"
        "fld    %c[Rotation](%0)\n"
        "fld    %c[Time](%0)\n"
        "fld    %c[TimeScale](%0)\n"
        "movups %c[Colour](%0), %%xmm1\n"
        "movups %c[Location](%0), %%xmm2\n"
        "movups %c[Velocity](%0), %%xmm3\n"
        "movups %c[Accel](%0), %%xmm4\n"
        :
        : "r" (Particle),
            DECLARE_STRUCT_OFFSET(ptcParticle, Active),
            DECLARE_STRUCT_OFFSET(ptcParticle, Size),
            DECLARE_STRUCT_OFFSET(ptcParticle, Rotation),
            DECLARE_STRUCT_OFFSET(ptcParticle, Time),
            DECLARE_STRUCT_OFFSET(ptcParticle, TimeScale),
            DECLARE_STRUCT_OFFSET(ptcParticle, Colour),
            DECLARE_STRUCT_OFFSET(ptcParticle, Location),
            DECLARE_STRUCT_OFFSET(ptcParticle, Velocity),
            DECLARE_STRUCT_OFFSET(ptcParticle, Accel)
        : "%edx", "%st", "%st(1)", "%st(2)", "%st(3)", "%xmm1", "%xmm2",
            "%xmm3", "%xmm4"
    );

The generated assembly is now correct and everything seems to work.

Accessing structure members from inline assembly?

1 Answers1