GCC inline asm: constraint for direct memory reference

Question

I need GCC to produce a consistent set of instructions for inline asm, but one of the instructions I'm using is sometimes compiled two different ways:

__asm__ ("mov %1,%%rax;" \
         : \
         : "m"(ref) \
         : "%rax");

Compile #1:

mov 0x200894(%rip),%rax

Compile #2:

mov 0x200894(%rip),%rdx
mov (%rdx),%rax

I'm not sure what the reason is for the second version, but I don't want it. Is there a constraint to specify that a memory reference should only be direct, i.e., not via register?

Update:

This variation always produces the exact same instruction:

    __asm__ ("mov ref@GOTPCREL(%rip),%rax");

Compiles to:

mov 0x200910(%rip),%rax

The two sequences of instructions are not equivalent. In the first 0x200894(%rip) contains the result directly, in the second 0x200894(%rip) contains a pointer to the result. What is ref? Does it have the same type in both instances? Is it pointer to a function or object that's exported from a DLL (which sometimes might have to be an indirect reference through a 'GOT')? — Michael Burr
– Michael Burr, Commented Jun 12, 2014 at 18:19
It's extern const char *ref, and the second version does only occur in a shared library--so it must have a GOT reference even though the variable is defined inside the library. Can I avoid that somehow? — Byron Hawkins
– Byron Hawkins, Commented Jun 12, 2014 at 18:27
I'm not sure. There's a good description of the GOT mechanism here: eli.thegreenplace.net/2011/11/03/… If there is a way to do what you want, that article might give you a clue (or a starting point) for how. — Michael Burr
– Michael Burr, Commented Jun 12, 2014 at 19:10
Byron, you should post your solution as an answer to your own question. That was an interesting one. — embedded_guy
– embedded_guy, Commented Jun 12, 2014 at 22:17

Byron Hawkins · Accepted Answer · 2014-06-13 08:23:40Z

3

Answering my own question:

This variation always produces the exact same instruction:

__asm__ ("mov ref@GOTPCREL(%rip),%rax");

Compiles to:

mov 0x200910(%rip),%rax

For x86 where RIP-relative is not available, it takes two instructions:

__asm__ ("mov $_GLOBAL_OFFSET_TABLE_,%%eax; \
          add ref@GOT,%%eax;");

Compiles to:

mov $0x2ff7,%eax
add 0xfffffff0,%eax

Two more instructions are required to make %eax RIP-relative, but I'm working in a binary translator where that's easier to do internally.

edited Jun 13, 2014 at 8:23

answered Jun 12, 2014 at 22:20

Byron Hawkins

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1 Comment

Peter Cordes Jan 14 at 0:49

Just for the record, these "Basic Asm" statements with no constraints aren't safe to use inside functions. You need asm("..." ::: "eax"). gcc.gnu.org/wiki/ConvertBasicAsmToExtended

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