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6.41 Assembler Instructions with C Expression Operands

In an assembler instruction using asm, you can specify the operands of the instruction using C expressions. This means you need not guess which registers or memory locations contain the data you want to use.

You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.

For example, here is how to use the 68881’s fsinx instruction:

asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here angle is the C expression for the input operand while result is that of the output operand. Each has ‘"f"’ as its operand constraint, saying that a floating-point register is required. The ‘=’ in ‘=f’ indicates that the operand is an output; all output operands’ constraints must use ‘=’. The constraints use the same language used in the machine description (see section Constraints for asm Operands).

Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.

If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.

As of GCC version 3.1, it is also possible to specify input and output operands using symbolic names which can be referenced within the assembler code. These names are specified inside square brackets preceding the constraint string, and can be referenced inside the assembler code using %[name] instead of a percentage sign followed by the operand number. Using named operands the above example could look like:

asm ("fsinx %[angle],%[output]"
     : [output] "=f" (result)
     : [angle] "f" (angle));

Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but you must ensure that no two operands within the same assembler construct use the same symbolic name.

Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input. The extended asm feature is most often used for machine instructions the compiler itself does not know exist. If the output expression cannot be directly addressed (for example, it is a bit-field), your constraint must allow a register. In that case, GCC uses the register as the output of the asm, and then stores that register into the output.

The ordinary output operands must be write-only; GCC assumes that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character ‘+’ to indicate such an operand and list it with the output operands.

You may, as an alternative, logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints that say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) ‘combine’ instruction with bar as its read-only source operand and foo as its read-write destination:

asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint ‘"0"’ for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.

Only a number in the constraint can guarantee that one operand is in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they are in the same place in the generated assembler code. The following does not work reliably:

asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GCC knows no reason not to do so. For example, the compiler might find a copy of the value of foo in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to foo’s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GCC can’t tell that.

As of GCC version 3.1, one may write [name] instead of the operand number for a matching constraint. For example:

asm ("cmoveq %1,%2,%[result]"
     : [result] "=r"(result)
     : "r" (test), "r"(new), "[result]"(old));

Sometimes you need to make an asm operand be a specific register, but there’s no matching constraint letter for that register by itself. To force the operand into that register, use a local variable for the operand and specify the register in the variable declaration. See section Variables in Specified Registers. Then for the asm operand, use any register constraint letter that matches the register:

register int *p1 asm ("r0") = …;
register int *p2 asm ("r1") = …;
register int *result asm ("r0");
asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));

In the above example, beware that a register that is call-clobbered by the target ABI will be overwritten by any function call in the assignment, including library calls for arithmetic operators. Also a register may be clobbered when generating some operations, like variable shift, memory copy or memory move on x86. Assuming it is a call-clobbered register, this may happen to r0 above by the assignment to p2. If you have to use such a register, use temporary variables for expressions between the register assignment and use:

int t1 = …;
register int *p1 asm ("r0") = …;
register int *p2 asm ("r1") = t1;
register int *result asm ("r0");
asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));

Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:

asm volatile ("movc3 %0,%1,%2"
              : /* no outputs */
              : "g" (from), "g" (to), "g" (count)
              : "r0", "r1", "r2", "r3", "r4", "r5");

You may not write a clobber description in a way that overlaps with an input or output operand. For example, you may not have an operand describing a register class with one member if you mention that register in the clobber list. Variables declared to live in specific registers (see section Variables in Specified Registers), and used as asm input or output operands must have no part mentioned in the clobber description. There is no way for you to specify that an input operand is modified without also specifying it as an output operand. Note that if all the output operands you specify are for this purpose (and hence unused), you then also need to specify volatile for the asm construct, as described below, to prevent GCC from deleting the asm statement as unused.

If you refer to a particular hardware register from the assembler code, you probably have to list the register after the third colon to tell the compiler the register’s value is modified. In some assemblers, the register names begin with ‘%’; to produce one ‘%’ in the assembler code, you must write ‘%%’ in the input.

If your assembler instruction can alter the condition code register, add ‘cc’ to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; ‘cc’ serves to name this register. On other machines, the condition code is handled differently, and specifying ‘cc’ has no effect. But it is valid no matter what the machine.

If your assembler instructions access memory in an unpredictable fashion, add ‘memory’ to the list of clobbered registers. This causes GCC to not keep memory values cached in registers across the assembler instruction and not optimize stores or loads to that memory. You also should add the volatile keyword if the memory affected is not listed in the inputs or outputs of the asm, as the ‘memory’ clobber does not count as a side-effect of the asm. If you know how large the accessed memory is, you can add it as input or output but if this is not known, you should add ‘memory’. As an example, if you access ten bytes of a string, you can use a memory input like:

{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.

Note that in the following example the memory input is necessary, otherwise GCC might optimize the store to x away:

int foo ()
{
  int x = 42;
  int *y = &x;
  int result;
  asm ("magic stuff accessing an 'int' pointed to by '%1'"
       : "=&d" (r) : "a" (y), "m" (*y));
  return result;
}

You can put multiple assembler instructions together in a single asm template, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as ‘\n\t’). Sometimes semicolons can be used, if the assembler allows semicolons as a line-breaking character. Note that some assembler dialects use semicolons to start a comment. The input operands are guaranteed not to use any of the clobbered registers, and neither do the output operands’ addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes the subroutine _foo accepts arguments in registers 9 and 10:

asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
     : /* no outputs */
     : "g" (from), "g" (to)
     : "r9", "r10");

Unless an output operand has the ‘&’ constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use ‘&’ for each output operand that may not overlap an input. See section Constraint Modifier Characters.

If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the asm construct, as follows:

asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
     : "g" (result)
     : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.

Speaking of labels, jumps from one asm to another are not supported. The compiler’s optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize. See Extended asm with goto.

Usually the most convenient way to use these asm instructions is to encapsulate them in macros that look like functions. For example,

#define sin(x)       \
({ double __value, __arg = (x);   \
   asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
   __value; })

Here the variable __arg is used to make sure that the instruction operates on a proper double value, and to accept only those arguments x that can convert automatically to a double.

Another way to make sure the instruction operates on the correct data type is to use a cast in the asm. This is different from using a variable __arg in that it converts more different types. For example, if the desired type is int, casting the argument to int accepts a pointer with no complaint, while assigning the argument to an int variable named __arg warns about using a pointer unless the caller explicitly casts it.

If an asm has output operands, GCC assumes for optimization purposes the instruction has no side effects except to change the output operands. This does not mean instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren’t used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register.

You can prevent an asm instruction from being deleted by writing the keyword volatile after the asm. For example:

#define get_and_set_priority(new)              \
({ int __old;                                  \
   asm volatile ("get_and_set_priority %0, %1" \
                 : "=g" (__old) : "g" (new));  \
   __old; })

The volatile keyword indicates that the instruction has important side-effects. GCC does not delete a volatile asm if it is reachable. (The instruction can still be deleted if GCC can prove that control flow never reaches the location of the instruction.) Note that even a volatile asm instruction can be moved relative to other code, including across jump instructions. For example, on many targets there is a system register that can be set to control the rounding mode of floating-point operations. You might try setting it with a volatile asm, like this PowerPC example:

       asm volatile("mtfsf 255,%0" : : "f" (fpenv));
       sum = x + y;

This does not work reliably, as the compiler may move the addition back before the volatile asm. To make it work you need to add an artificial dependency to the asm referencing a variable in the code you don’t want moved, for example:

    asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
    sum = x + y;

Similarly, you can’t expect a sequence of volatile asm instructions to remain perfectly consecutive. If you want consecutive output, use a single asm. Also, GCC performs some optimizations across a volatile asm instruction; GCC does not “forget everything” when it encounters a volatile asm instruction the way some other compilers do.

An asm instruction without any output operands is treated identically to a volatile asm instruction.

It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which result in additional following “store” instructions. On most machines, these instructions alter the condition code before there is time to test it. This problem doesn’t arise for ordinary “test” and “compare” instructions because they don’t have any output operands.

For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.

As of GCC version 4.5, asm goto may be used to have the assembly jump to one or more C labels. In this form, a fifth section after the clobber list contains a list of all C labels to which the assembly may jump. Each label operand is implicitly self-named. The asm is also assumed to fall through to the next statement.

This form of asm is restricted to not have outputs. This is due to a internal restriction in the compiler that control transfer instructions cannot have outputs. This restriction on asm goto may be lifted in some future version of the compiler. In the meantime, asm goto may include a memory clobber, and so leave outputs in memory.

int frob(int x)
{
  int y;
  asm goto ("frob %%r5, %1; jc %l[error]; mov (%2), %%r5"
            : : "r"(x), "r"(&y) : "r5", "memory" : error);
  return y;
 error:
  return -1;
}

In this (inefficient) example, the frob instruction sets the carry bit to indicate an error. The jc instruction detects this and branches to the error label. Finally, the output of the frob instruction (%r5) is stored into the memory for variable y, which is later read by the return statement.

void doit(void)
{
  int i = 0;
  asm goto ("mfsr %%r1, 123; jmp %%r1;"
            ".pushsection doit_table;"
            ".long %l0, %l1, %l2, %l3;"
            ".popsection"
            : : : "r1" : label1, label2, label3, label4);
  __builtin_unreachable ();

 label1:
  f1();
  return;
 label2:
  f2();
  return;
 label3:
  i = 1;
 label4:
  f3(i);
}

In this (also inefficient) example, the mfsr instruction reads an address from some out-of-band machine register, and the following jmp instruction branches to that address. The address read by the mfsr instruction is assumed to have been previously set via some application-specific mechanism to be one of the four values stored in the doit_table section. Finally, the asm is followed by a call to __builtin_unreachable to indicate that the asm does not in fact fall through.

#define TRACE1(NUM)                         \
  do {                                      \
    asm goto ("0: nop;"                     \
              ".pushsection trace_table;"   \
              ".long 0b, %l0;"              \
              ".popsection"                 \
              : : : : trace#NUM);           \
    if (0) { trace#NUM: trace(); }          \
  } while (0)
#define TRACE  TRACE1(__COUNTER__)

In this example (which in fact inspired the asm goto feature) we want on rare occasions to call the trace function; on other occasions we’d like to keep the overhead to the absolute minimum. The normal code path consists of a single nop instruction. However, we record the address of this nop together with the address of a label that calls the trace function. This allows the nop instruction to be patched at run time to be an unconditional branch to the stored label. It is assumed that an optimizing compiler moves the labeled block out of line, to optimize the fall through path from the asm.

If you are writing a header file that should be includable in ISO C programs, write __asm__ instead of asm. See section Alternate Keywords.


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