c++ - Referencing memory operands in .intel_syntax GNU C inline assembly

Question

I'm catching a link error when compiling and linking a source file with inline assembly.

Here are the test files:

via:$ cat test.cxx
extern int libtest();
int main(int argc, char* argv[])
{
    return libtest();
}

$ cat lib.cxx
#include <stdint.h>
int libtest()
{
    uint32_t rnds_00_15;    
    __asm__ __volatile__
    (
        ".intel_syntax noprefix         ;
"
        "mov DWORD PTR [rnds_00_15], 1  ;
"
        "cmp DWORD PTR [rnds_00_15], 1  ;
"
        "je  done                       ;
"
        "done:                          ;
"
        ".att_syntax noprefix           ;
"
        :
        : [rnds_00_15] "m" (rnds_00_15)
        : "memory", "cc"
    );

    return 0;
}

Compiling and linking the program results in:

via:$ g++ -fPIC test.cxx lib.cxx -c
via:$ g++ -fPIC lib.o test.o -o test.exe
lib.o: In function `libtest()':
lib.cxx:(.text+0x1d): undefined reference to `rnds_00_15'
lib.cxx:(.text+0x27): undefined reference to `rnds_00_15'
collect2: error: ld returned 1 exit status

The real program is more complex. The routine is out of registers so the flag rnds_00_15 must be a memory operand. Use of rnds_00_15 is local to the asm block. It is declared in the C code to ensure the memory is allocated on the stack and nothing more. We don't read from it or write to it as far as the C code is concerned. We list it as a memory input so GCC knows we use it and wire up the "C variable name" in the extended ASM.

Why am I receiving a link error, and how do I fix it?

Answer 1

Compile with gcc -masm=intel and don't try to switch modes inside the asm template string. AFAIK there's no equivalent for clang (note that MacOS installs clang as gcc / g++ by default.)

Also, of course you need to use valid GNU C inline asm, using operands to tell the compiler which C objects you want to read and write.

---

I don't believe Intel syntax uses the percent sign. Perhaps I am missing something?

You're getting mixed up between %operand substitutions into the Extended-Asm template (which use a single %), vs. the final asm that the assembler sees.

You need %% to use a literal % in the final asm. You wouldn't use "mov %%eax, 1" in Intel-syntax inline asm, but you do still use "mov %0, 1" or %[named_operand].

See https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html. In Basic asm (no operands), there is no substitution and % isn't special in the template, so you'd write mov $1, %eax in Basic asm vs. mov $1, %%eax in Extended, if for some reason you weren't using an operand like mov $1, %[tmp] or mov $1, %0.

---

uint32_t rnds_00_15; is a local with automatic storage. Of course it there's no asm symbol with that name.

Use %[rnds_00_15] and compile with -masm=intel (And remove the .att_syntax at the end; that would break the compiler-generate asm that comes after.)

You also need to remove the DWORD PTR, because the operand-expansion already includes that, e.g. DWORD PTR [rsp - 4], and clang errors on DWORD PTR DWORD PTR [rsp - 4]. (GAS accepts it just fine, but the 2nd one takes precendence so it's pointless and potentially misleading.)

And you'll want a "=m" output operand if you want the compiler to reserve you some scratch space on the stack. You must not modify input-only operands, even if it's unused in the C. Maybe the compiler decides it can overlap something else because it's not written and not initialized (i.e. UB). (I'm not sure if your "memory" clobber makes it safe, but there's no reason not to use an early-clobber output operand here.)

And you'll want to avoid label name conflicts by using %= to get a unique number.

Working example (GCC and ICC, but not clang unfortunately), on the Godbolt compiler explorer (which uses -masm=intel depending on options in the dropdown). You can use "binary mode" (the 11010 button) to prove that it actually assembles after compiling to asm without warnings.

int libtest_intel()
{
    uint32_t rnds_00_15;
    // Intel syntax operand-size can only be overridden with operand modifiers
    // because the expansion includes an explicit DWORD PTR
    __asm__ __volatile__
    (  // ".intel_syntax noprefix 
"
        "mov %[rnds_00_15], 1  
"
        "cmp %[rnds_00_15], 1  
"
        "je  .Ldone%=                 
"
        ".Ldone%=:                    
"
        : [rnds_00_15] "=&m" (rnds_00_15)
        :
        : // no clobbers
    );
    return 0;
}

Compiles (with gcc -O3 -masm=intel) to this asm. Also works with gcc -m32 -masm=intel of course:

libtest_intel:
    mov DWORD PTR [rsp-4], 1  
    cmp DWORD PTR [rsp-4], 1  
    je  .Ldone8                 
.Ldone8:                    

    xor     eax, eax
    ret

I couldn't get this to work with clang: It choked on .intel_syntax noprefix when I left that in explicitly.

---

Operand-size overrides:

You have to use %b[tmp] to get the compiler to substitute in BYTE PTR [rsp-4] to only access the low byte of a dword input operand. I'd recommend AT&T syntax if you want to do much of this.

---

---

Using %[rnds_00_15] results in Error: junk '(%ebp)' after expression.

That's because you switched to Intel syntax without telling the compiler. If you want it to use Intel addressing modes, compile with -masm=intel so the compiler can substitute into the template with the correct syntax.

This is why I avoid that crappy GCC inline assembly at nearly all costs. Man I despise this crappy tool.

You're just using it wrong. It's a bit cumbersome, but makes sense and mostly works well if you understand how it's designed.

Repeat after me: The compiler doesn't parse the asm string at all, except to do text substitutions of %operand. This is why it doesn't notice your .intel_syntax noprefex and keeps substituting AT&T syntax.

It does work better and more easily with AT&T syntax though, e.g. for overriding the operand-size of a memory operand, or adding an offset. (e.g. 4 + %[mem] works in AT&T syntax).

---

Dialect alternatives:

If you want to write inline asm that doesn't depend on -masm=intel or not, use Dialect alternatives (which makes your code super-ugly; not recommended for anything other than wrapping one or two instructions):

Also demonstrates operand-size overrides

#include <stdint.h>
int libtest_override_operand_size()
{
    uint32_t rnds_00_15;
    // Intel syntax operand-size can only be overriden with operand modifiers
    // because the expansion includes an explicit DWORD PTR
    __asm__ __volatile__
    (
        "{movl $1, %[rnds_00_15] | mov %[rnds_00_15], 1}  
"
        "{cmpl $1, %[rnds_00_15] | cmp %k[rnds_00_15], 1}  
"
        "{cmpw $1, %[rnds_00_15] | cmp %w[rnds_00_15], 1}  
"
        "{cmpb $1, %[rnds_00_15] | cmp %b[rnds_00_15], 1}  
"
        "je  .Ldone%=                     
"
        ".Ldone%=:                        
"
        : [rnds_00_15] "=&m" (rnds_00_15)
    );
    return 0;
}

With Intel syntax, gcc compiles it to:

     mov DWORD PTR [rsp-4], 1  
     cmp DWORD PTR [rsp-4], 1  
     cmp WORD PTR [rsp-4], 1  
     cmp BYTE PTR [rsp-4], 1  
    je  .Ldone38                     
.Ldone38:                        

    xor     eax, eax
    ret

With AT&T syntax, compiles to:

    movl $1, -4(%rsp)   
    cmpl $1, -4(%rsp)   
    cmpw $1, -4(%rsp)   
    cmpb $1, -4(%rsp)   
    je  .Ldone38                     
.Ldone38:                        

    xorl    %eax, %eax
    ret