The asm keyword allows you to embed assembler instructions within C code. GCC provides two forms of inline asm statements. A basic asm statement is one with no operands (see Basic Asm — Assembler Instructions Without Operands ), while an extended asm statement (see Extended Asm - Assembler Instructions with C Expression Operands ) includes one or more operands. The extended form is preferred for mixing C and assembly language within a function, but to include assembly language at top level you must use basic asm . You can also use the asm keyword to override the assembler name for a C symbol, or to place a C variable in a specific register.
asm asm-qualifiers ( AssemblerInstructions )
For the C language, the asm keyword is a GNU extension. When writing C code that can be compiled with -ansi and the -std options that select C dialects without GNU extensions, use __asm__ instead of asm (see Alternate Keywords ). For the C++ language, asm is a standard keyword, but __asm__ can be used for code compiled with -fno-asm .
volatile The optional volatile qualifier has no effect. All basic asm blocks are implicitly volatile. inline If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see Size of an asm ).
AssemblerInstructions This is a literal string that specifies the assembler code. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. You may place multiple assembler instructions together in a single asm string, 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 (written as \n\t ). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment.
Safely accessing C data and calling functions from basic asm is more complex than it may appear. To access C data, it is better to use extended asm .
Do not expect a sequence of asm statements to remain perfectly consecutive after compilation. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement. Note that GCC’s optimizers can move asm statements relative to other code, including across jumps.
asm statements may not perform jumps into other asm statements. GCC does not know about these jumps, and therefore cannot take account of them when deciding how to optimize. Jumps from asm to C labels are only supported in extended asm .
Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. This can lead to unexpected duplicate symbol errors during compilation if your assembly code defines symbols or labels.
The C standards do not specify semantics for asm , making it a potential source of incompatibilities between compilers. These incompatibilities may not produce compiler warnings/errors.
GCC does not parse basic asm ‘s AssemblerInstructions , which means there is no way to communicate to the compiler what is happening inside them. GCC has no visibility of symbols in the asm and may discard them as unreferenced. It also does not know about side effects of the assembler code, such as modifications to memory or registers. Unlike some compilers, GCC assumes that no changes to general purpose registers occur. This assumption may change in a future release.
To avoid complications from future changes to the semantics and the compatibility issues between compilers, consider replacing basic asm with extended asm . See How to convert from basic asm to extended asm for information about how to perform this conversion.
The compiler copies the assembler instructions in a basic asm verbatim to the assembly language output file, without processing dialects or any of the % operators that are available with extended asm . This results in minor differences between basic asm strings and extended asm templates. For example, to refer to registers you might use %eax in basic asm and %%eax in extended asm .
On targets such as x86 that support multiple assembler dialects, all basic asm blocks use the assembler dialect specified by the -masm command-line option (see x86 Options ). Basic asm provides no mechanism to provide different assembler strings for different dialects.
For basic asm with non-empty assembler string GCC assumes the assembler block does not change any general purpose registers, but it may read or write any globally accessible variable.
Here is an example of basic asm for i386:
/* Note that this code will not compile with -masm=intel */ #define DebugBreak() asm("int $3")
With extended asm you can read and write C variables from assembler and perform jumps from assembler code to C labels. Extended asm syntax uses colons ( : ) to delimit the operand parameters after the assembler template:
asm asm-qualifiers ( AssemblerTemplate : OutputOperands [ : InputOperands [ : Clobbers ] ]) asm asm-qualifiers ( AssemblerTemplate : OutputOperands : InputOperands : Clobbers : GotoLabels)
where in the last form, asm-qualifiers contains goto (and in the first form, not).
The asm keyword is a GNU extension. When writing code that can be compiled with -ansi and the various -std options, use __asm__ instead of asm (see Alternate Keywords ).
The typical use of extended asm statements is to manipulate input values to produce output values. However, your asm statements may also produce side effects. If so, you may need to use the volatile qualifier to disable certain optimizations. See Volatile .
If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see Size of an asm ).
This qualifier informs the compiler that the asm statement may perform a jump to one of the labels listed in the GotoLabels . See Goto Labels .
This is a literal string that is the template for the assembler code. It is a combination of fixed text and tokens that refer to the input, output, and goto parameters. See Assembler Template .
A comma-separated list of the C variables modified by the instructions in the AssemblerTemplate . An empty list is permitted. See Output Operands .
A comma-separated list of C expressions read by the instructions in the AssemblerTemplate . An empty list is permitted. See Input Operands .
A comma-separated list of registers or other values changed by the AssemblerTemplate , beyond those listed as outputs. An empty list is permitted. See Clobbers and Scratch Registers .
When you are using the goto form of asm , this section contains the list of all C labels to which the code in the AssemblerTemplate may jump. See Goto Labels .
asm statements may not perform jumps into other asm statements, only to the listed GotoLabels . GCC’s optimizers do not know about other jumps; therefore they cannot take account of them when deciding how to optimize.
The total number of input + output + goto operands is limited to 30.
The asm statement allows you to include assembly instructions directly within C code. This may help you to maximize performance in time-sensitive code or to access assembly instructions that are not readily available to C programs.
Note that extended asm statements must be inside a function. Only basic asm may be outside functions (see Basic Asm — Assembler Instructions Without Operands ). Functions declared with the naked attribute also require basic asm (see Declaring Attributes of Functions ).
While the uses of asm are many and varied, it may help to think of an asm statement as a series of low-level instructions that convert input parameters to output parameters. So a simple (if not particularly useful) example for i386 using asm might look like this:
int src = 1; int dst; asm ("mov %1, %0\n\t" "add $1, %0" : "=r" (dst) : "r" (src)); printf("%d\n", dst);
This code copies src to dst and add 1 to dst .
GCC’s optimizers sometimes discard asm statements if they determine there is no need for the output variables. Also, the optimizers may move code out of loops if they believe that the code will always return the same result (i.e. none of its input values change between calls). Using the volatile qualifier disables these optimizations. asm statements that have no output operands and asm goto statements, are implicitly volatile.
This i386 code demonstrates a case that does not use (or require) the volatile qualifier. If it is performing assertion checking, this code uses asm to perform the validation. Otherwise, dwRes is unreferenced by any code. As a result, the optimizers can discard the asm statement, which in turn removes the need for the entire DoCheck routine. By omitting the volatile qualifier when it isn’t needed you allow the optimizers to produce the most efficient code possible.
void DoCheck(uint32_t dwSomeValue) uint32_t dwRes; // Assumes dwSomeValue is not zero. asm ("bsfl %1,%0" : "=r" (dwRes) : "r" (dwSomeValue) : "cc"); assert(dwRes > 3); >
The next example shows a case where the optimizers can recognize that the input ( dwSomeValue ) never changes during the execution of the function and can therefore move the asm outside the loop to produce more efficient code. Again, using the volatile qualifier disables this type of optimization.
void do_print(uint32_t dwSomeValue) uint32_t dwRes; for (uint32_t x=0; x 5; x++) // Assumes dwSomeValue is not zero. asm ("bsfl %1,%0" : "=r" (dwRes) : "r" (dwSomeValue) : "cc"); printf("%u: %u %u\n", x, dwSomeValue, dwRes); > >
The following example demonstrates a case where you need to use the volatile qualifier. It uses the x86 rdtsc instruction, which reads the computer’s time-stamp counter. Without the volatile qualifier, the optimizers might assume that the asm block will always return the same value and therefore optimize away the second call.
uint64_t msr; asm volatile ( "rdtsc\n\t" // Returns the time in EDX:EAX. "shl $32, %%rdx\n\t" // Shift the upper bits left. "or %%rdx, %0" // 'Or' in the lower bits. : "=a" (msr) : : "rdx"); printf("msr: %llx\n", msr); // Do other work. // Reprint the timestamp asm volatile ( "rdtsc\n\t" // Returns the time in EDX:EAX. "shl $32, %%rdx\n\t" // Shift the upper bits left. "or %%rdx, %0" // 'Or' in the lower bits. : "=a" (msr) : : "rdx"); printf("msr: %llx\n", msr);
GCC’s optimizers do not treat this code like the non-volatile code in the earlier examples. They do not move it out of loops or omit it on the assumption that the result from a previous call is still valid.
Note that the compiler can move even volatile asm instructions relative to other code, including across jump instructions. For example, on many targets there is a system register that controls the rounding mode of floating-point operations. Setting it with a volatile asm statement, as in the following PowerPC example, does not work reliably.
asm volatile("mtfsf 255, %0" : : "f" (fpenv)); sum = x + y;
The compiler may move the addition back before the volatile asm statement. To make it work as expected, add an artificial dependency to the asm by referencing a variable in the subsequent code, for example:
asm volatile ("mtfsf 255,%1" : "=X" (sum) : "f" (fpenv)); sum = x + y;
Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. This can lead to unexpected duplicate symbol errors during compilation if your asm code defines symbols or labels. Using %= (see Assembler Template ) may help resolve this problem.
An assembler template is a literal string containing assembler instructions. The compiler replaces tokens in the template that refer to inputs, outputs, and goto labels, and then outputs the resulting string to the assembler. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. However, it does count the statements (see Size of an asm ).
You may place multiple assembler instructions together in a single asm string, 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 ). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment.
Do not expect a sequence of asm statements to remain perfectly consecutive after compilation, even when you are using the volatile qualifier. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement.
Accessing data from C programs without using input/output operands (such as by using global symbols directly from the assembler template) may not work as expected. Similarly, calling functions directly from an assembler template requires a detailed understanding of the target assembler and ABI.
Since GCC does not parse the assembler template, it has no visibility of any symbols it references. This may result in GCC discarding those symbols as unreferenced unless they are also listed as input, output, or goto operands.
In addition to the tokens described by the input, output, and goto operands, these tokens have special meanings in the assembler template:
Outputs a single % into the assembler code.
Outputs a number that is unique to each instance of the asm statement in the entire compilation. This option is useful when creating local labels and referring to them multiple times in a single template that generates multiple assembler instructions.
Outputs < , | , and >characters (respectively) into the assembler code. When unescaped, these characters have special meaning to indicate multiple assembler dialects, as described below.
On targets such as x86, GCC supports multiple assembler dialects. The -masm option controls which dialect GCC uses as its default for inline assembler. The target-specific documentation for the -masm option contains the list of supported dialects, as well as the default dialect if the option is not specified. This information may be important to understand, since assembler code that works correctly when compiled using one dialect will likely fail if compiled using another. See x86 Options .
If your code needs to support multiple assembler dialects (for example, if you are writing public headers that need to support a variety of compilation options), use constructs of this form:
dialect0 | dialect1 | dialect2. >
This construct outputs dialect0 when using dialect #0 to compile the code, dialect1 for dialect #1, etc. If there are fewer alternatives within the braces than the number of dialects the compiler supports, the construct outputs nothing.
For example, if an x86 compiler supports two dialects ( att , intel ), an assembler template such as this:
"bt; jc %l2"
is equivalent to one of
"btl %[Offset],%[Base] ; jc %l2" /* att dialect */ "bt %[Base],%[Offset]; jc %l2" /* intel dialect */
Using that same compiler, this code:
"xchg \tebx, %1"
corresponds to either
"xchgl\t%%ebx, %1" /* att dialect */ "xchg\tebx, %1" /* intel dialect */
There is no support for nesting dialect alternatives.
An asm statement has zero or more output operands indicating the names of C variables modified by the assembler code.
In this i386 example, old (referred to in the template string as %0 ) and *Base (as %1 ) are outputs and Offset ( %2 ) is an input:
bool old; __asm__ ("btsl %2,%1\n\t" // Turn on zero-based bit #Offset in Base. "sbb %0,%0" // Use the CF to calculate old. : "=r" (old), "+rm" (*Base) : "Ir" (Offset) : "cc"); return old;
Operands are separated by commas. Each operand has this format:
[ [asmSymbolicName] ] constraint (cvariablename)asmSymbolicName
Specifies a symbolic name for the operand. Reference the name in the assembler template by enclosing it in square brackets (i.e. %[Value] ). The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code. No two operands within the same asm statement can use the same symbolic name.
When not using an asmSymbolicName , use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are three output operands, use %0 in the template to refer to the first, %1 for the second, and %2 for the third.
A string constant specifying constraints on the placement of the operand; See Constraints for asm Operands , for details.
Output constraints must begin with either = (a variable overwriting an existing value) or + (when reading and writing). When using = , do not assume the location contains the existing value on entry to the asm , except when the operand is tied to an input; see Input Operands .
After the prefix, there must be one or more additional constraints (see Constraints for asm Operands ) that describe where the value resides. Common constraints include r for register and m for memory. When you list more than one possible location (for example, "=rm" ), the compiler chooses the most efficient one based on the current context. If you list as many alternates as the asm statement allows, you permit the optimizers to produce the best possible code. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Specifying Registers for Local Variables ).
Specifies a C lvalue expression to hold the output, typically a variable name. The enclosing parentheses are a required part of the syntax.
When the compiler selects the registers to use to represent the output operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers ).
Output operand expressions must be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. For output expressions that are not directly addressable (for example a bit-field), the 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.
Operands using the + constraint modifier count as two operands (that is, both as input and output) towards the total maximum of 30 operands per asm statement.
Use the & constraint modifier (see Constraint Modifier Characters ) on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction.
The same problem can occur if one output parameter ( a ) allows a register constraint and another output parameter ( b ) allows a memory constraint. The code generated by GCC to access the memory address in b can contain registers which might be shared by a , and GCC considers those registers to be inputs to the asm. As above, GCC assumes that such input registers are consumed before any outputs are written. This assumption may result in incorrect behavior if the asm statement writes to a before using b . Combining the & modifier with the register constraint on a ensures that modifying a does not affect the address referenced by b . Otherwise, the location of b is undefined if a is modified before using b .
asm supports operand modifiers on operands (for example %k2 instead of simply %2 ). Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found at x86 Operand Modifiers .
If the C code that follows the asm makes no use of any of the output operands, use volatile for the asm statement to prevent the optimizers from discarding the asm statement as unneeded (see Volatile ).
This code makes no use of the optional asmSymbolicName . Therefore it references the first output operand as %0 (were there a second, it would be %1 , etc). The number of the first input operand is one greater than that of the last output operand. In this i386 example, that makes Mask referenced as %1 :
uint32_t Mask = 1234; uint32_t Index; asm ("bsfl %1, %0" : "=r" (Index) : "r" (Mask) : "cc");
That code overwrites the variable Index ( = ), placing the value in a register ( r ). Using the generic r constraint instead of a constraint for a specific register allows the compiler to pick the register to use, which can result in more efficient code. This may not be possible if an assembler instruction requires a specific register.
The following i386 example uses the asmSymbolicName syntax. It produces the same result as the code above, but some may consider it more readable or more maintainable since reordering index numbers is not necessary when adding or removing operands. The names aIndex and aMask are only used in this example to emphasize which names get used where. It is acceptable to reuse the names Index and Mask .
uint32_t Mask = 1234; uint32_t Index; asm ("bsfl %[aMask], %[aIndex]" : [aIndex] "=r" (Index) : [aMask] "r" (Mask) : "cc");
Here are some more examples of output operands.
uint32_t c = 1; uint32_t d; uint32_t *e = &c; asm ("mov %[e], %[d]" : [d] "=rm" (d) : [e] "rm" (*e));
Here, d may either be in a register or in memory. Since the compiler might already have the current value of the uint32_t location pointed to by e in a register, you can enable it to choose the best location for d by specifying both constraints.
Some targets have a special register that holds the ‘flags’ for the result of an operation or comparison. Normally, the contents of that register are either unmodifed by the asm, or the asm statement is considered to clobber the contents.
On some targets, a special form of output operand exists by which conditions in the flags register may be outputs of the asm. The set of conditions supported are target specific, but the general rule is that the output variable must be a scalar integer, and the value is boolean. When supported, the target defines the preprocessor symbol __GCC_ASM_FLAG_OUTPUTS__ .
Because of the special nature of the flag output operands, the constraint may not include alternatives.
Most often, the target has only one flags register, and thus is an implied operand of many instructions. In this case, the operand should not be referenced within the assembler template via %0 etc, as there’s no corresponding text in the assembly language.
The flag output constraints for the ARM family are of the form =@cc cond where cond is one of the standard conditions defined in the ARM ARM for ConditionHolds .
Z flag set, or equal
Z flag clear or not equal
C flag set or unsigned greater than equal
C flag clear or unsigned less than
N flag set or ‘minus’
N flag clear or ‘plus’
V flag set or signed overflow
unsigned greater than
unsigned less than equal
signed greater than equal
signed less than
signed greater than
signed less than equal
The flag output constraints are not supported in thumb1 mode.
The flag output constraints for the x86 family are of the form =@cc cond where cond is one of the standard conditions defined in the ISA manual for jcc or setcc .
‘above’ or unsigned greater than
‘above or equal’ or unsigned greater than or equal
‘below’ or unsigned less than
‘below or equal’ or unsigned less than or equal
‘equal’ or zero flag set
signed greater than
signed greater than or equal
signed less than
signed less than or equal
overflow flag set
parity flag set
na nae nb nbe nc ne ng nge nl nle no np ns nz
‘not’ flag , or inverted versions of those above
Input operands make values from C variables and expressions available to the assembly code.
Operands are separated by commas. Each operand has this format:
[ [asmSymbolicName] ] constraint (cexpression)asmSymbolicName
Specifies a symbolic name for the operand. Reference the name in the assembler template by enclosing it in square brackets (i.e. %[Value] ). The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code. No two operands within the same asm statement can use the same symbolic name.
When not using an asmSymbolicName , use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are two output operands and three inputs, use %2 in the template to refer to the first input operand, %3 for the second, and %4 for the third.
A string constant specifying constraints on the placement of the operand; See Constraints for asm Operands , for details.
Input constraint strings may not begin with either = or + . When you list more than one possible location (for example, "irm" ), the compiler chooses the most efficient one based on the current context. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Specifying Registers for Local Variables ).
Input constraints can also be digits (for example, "0" ). This indicates that the specified input must be in the same place as the output constraint at the (zero-based) index in the output constraint list. When using asmSymbolicName syntax for the output operands, you may use these names (enclosed in brackets [] ) instead of digits.
This is the C variable or expression being passed to the asm statement as input. The enclosing parentheses are a required part of the syntax.
When the compiler selects the registers to use to represent the input operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers ).
If there are no output operands but there are input operands, place two consecutive colons where the output operands would go:
__asm__ ("some instructions" : /* No outputs. */ : "r" (Offset / 8));
Do not modify the contents of input-only operands (except for inputs tied to outputs). The compiler assumes that on exit from the asm statement these operands contain the same values as they had before executing the statement.
It is not possible to use clobbers to inform the compiler that the values in these inputs are changing. One common work-around is to tie the changing input variable to an output variable that never gets used. Note, however, that if the code that follows the asm statement makes no use of any of the output operands, the GCC optimizers may discard the asm statement as unneeded (see Volatile ).
asm supports operand modifiers on operands (for example %k2 instead of simply %2 ). Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found at x86 Operand Modifiers .
In this example using the fictitious combine instruction, the constraint "0" for input operand 1 says that it must occupy the same location as output operand 0. Only input operands may use numbers in constraints, and they must each refer to an output operand. Only a number (or the symbolic assembler name) 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.
asm ("combine %2, %0" : "=r" (foo) : "0" (foo), "g" (bar));
Here is an example using symbolic names.
asm ("cmoveq %1, %2, %[result]" : [result] "=r"(result) : "r" (test), "r" (new), "[result]" (old));
While the compiler is aware of changes to entries listed in the output operands, the inline asm code may modify more than just the outputs. For example, calculations may require additional registers, or the processor may overwrite a register as a side effect of a particular assembler instruction. In order to inform the compiler of these changes, list them in the clobber list. Clobber list items are either register names or the special clobbers (listed below). Each clobber list item is a string constant enclosed in double quotes and separated by commas.
Clobber descriptions may not in any way overlap with an input or output operand. For example, you may not have an operand describing a register class with one member when listing that register in the clobber list. Variables declared to live in specific registers (see Variables in Specified Registers ) and used as asm input or output operands must have no part mentioned in the clobber description. In particular, there is no way to specify that input operands get modified without also specifying them as output operands.
When the compiler selects which registers to use to represent input and output operands, it does not use any of the clobbered registers. As a result, clobbered registers are available for any use in the assembler code.
Another restriction is that the clobber list should not contain the stack pointer register. This is because the compiler requires the value of the stack pointer to be the same after an asm statement as it was on entry to the statement. However, previous versions of GCC did not enforce this rule and allowed the stack pointer to appear in the list, with unclear semantics. This behavior is deprecated and listing the stack pointer may become an error in future versions of GCC.
Here is a realistic example for the VAX showing the use of clobbered registers:
asm volatile ("movc3 %0, %1, %2" : /* No outputs. */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5", "memory");
Also, there are two special clobber arguments:
The "cc" clobber indicates that the assembler code modifies the flags register. On some machines, GCC represents the condition codes as a specific hardware register; "cc" serves to name this register. On other machines, condition code handling is different, and specifying "cc" has no effect. But it is valid no matter what the target.
The "memory" clobber tells the compiler that the assembly code performs memory reads or writes to items other than those listed in the input and output operands (for example, accessing the memory pointed to by one of the input parameters). To ensure memory contains correct values, GCC may need to flush specific register values to memory before executing the asm . Further, the compiler does not assume that any values read from memory before an asm remain unchanged after that asm ; it reloads them as needed. Using the "memory" clobber effectively forms a read/write memory barrier for the compiler.
Note that this clobber does not prevent the processor from doing speculative reads past the asm statement. To prevent that, you need processor-specific fence instructions.
Flushing registers to memory has performance implications and may be an issue for time-sensitive code. You can provide better information to GCC to avoid this, as shown in the following examples. At a minimum, aliasing rules allow GCC to know what memory doesn’t need to be flushed.
Here is a fictitious sum of squares instruction, that takes two pointers to floating point values in memory and produces a floating point register output. Notice that x , and y both appear twice in the asm parameters, once to specify memory accessed, and once to specify a base register used by the asm . You won’t normally be wasting a register by doing this as GCC can use the same register for both purposes. However, it would be foolish to use both %1 and %3 for x in this asm and expect them to be the same. In fact, %3 may well not be a register. It might be a symbolic memory reference to the object pointed to by x .
asm ("sumsq %0, %1, %2" : "+f" (result) : "r" (x), "r" (y), "m" (*x), "m" (*y));
Here is a fictitious *z++ = *x++ * *y++ instruction. Notice that the x , y and z pointer registers must be specified as input/output because the asm modifies them.
asm ("vecmul %0, %1, %2" : "+r" (z), "+r" (x), "+r" (y), "=m" (*z) : "m" (*x), "m" (*y));
An x86 example where the string memory argument is of unknown length.
asm("repne scasb" : "=c" (count), "+D" (p) : "m" (*(const char (*)[]) p), "0" (-1), "a" (0));
If you know the above will only be reading a ten byte array then you could instead use a memory input like: "m" (*(const char (*)[10]) p) .
Here is an example of a PowerPC vector scale implemented in assembly, complete with vector and condition code clobbers, and some initialized offset registers that are unchanged by the asm .
void dscal (size_t n, double *x, double alpha) asm ("/* lots of asm here */" : "+m" (*(double (*)[n]) x), "+&r" (n), "+b" (x) : "d" (alpha), "b" (32), "b" (48), "b" (64), "b" (80), "b" (96), "b" (112) : "cr0", "vs32","vs33","vs34","vs35","vs36","vs37","vs38","vs39", "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47"); >
Rather than allocating fixed registers via clobbers to provide scratch registers for an asm statement, an alternative is to define a variable and make it an early-clobber output as with a2 and a3 in the example below. This gives the compiler register allocator more freedom. You can also define a variable and make it an output tied to an input as with a0 and a1 , tied respectively to ap and lda . Of course, with tied outputs your asm can’t use the input value after modifying the output register since they are one and the same register. What’s more, if you omit the early-clobber on the output, it is possible that GCC might allocate the same register to another of the inputs if GCC could prove they had the same value on entry to the asm . This is why a1 has an early-clobber. Its tied input, lda might conceivably be known to have the value 16 and without an early-clobber share the same register as %11 . On the other hand, ap can’t be the same as any of the other inputs, so an early-clobber on a0 is not needed. It is also not desirable in this case. An early-clobber on a0 would cause GCC to allocate a separate register for the "m" (*(const double (*)[]) ap) input. Note that tying an input to an output is the way to set up an initialized temporary register modified by an asm statement. An input not tied to an output is assumed by GCC to be unchanged, for example "b" (16) below sets up %11 to 16, and GCC might use that register in following code if the value 16 happened to be needed. You can even use a normal asm output for a scratch if all inputs that might share the same register are consumed before the scratch is used. The VSX registers clobbered by the asm statement could have used this technique except for GCC’s limit on the number of asm parameters.
static void dgemv_kernel_4x4 (long n, const double *ap, long lda, const double *x, double *y, double alpha) double *a0; double *a1; double *a2; double *a3; __asm__ ( /* lots of asm here */ "#n=%1 ap=%8=%12 lda=%13 x=%7=%10 y=%0=%2 alpha=%9 o16=%11\n" "#a0=%3 a1=%4 a2=%5 a3=%6" : "+m" (*(double (*)[n]) y), "+&r" (n), // 1 "+b" (y), // 2 "=b" (a0), // 3 "=&b" (a1), // 4 "=&b" (a2), // 5 "=&b" (a3) // 6 : "m" (*(const double (*)[n]) x), "m" (*(const double (*)[]) ap), "d" (alpha), // 9 "r" (x), // 10 "b" (16), // 11 "3" (ap), // 12 "4" (lda) // 13 : "cr0", "vs32","vs33","vs34","vs35","vs36","vs37", "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47" ); >
asm goto allows assembly code to jump to one or more C labels. The GotoLabels section in an asm goto statement contains a comma-separated list of all C labels to which the assembler code may jump. GCC assumes that asm execution falls through to the next statement (if this is not the case, consider using the __builtin_unreachable intrinsic after the asm statement). Optimization of asm goto may be improved by using the hot and cold label attributes (see Label Attributes ).
If the assembler code does modify anything, use the "memory" clobber to force the optimizers to flush all register values to memory and reload them if necessary after the asm statement.
Also note that an asm goto statement is always implicitly considered volatile.
Be careful when you set output operands inside asm goto only on some possible control flow paths. If you don’t set up the output on given path and never use it on this path, it is okay. Otherwise, you should use + constraint modifier meaning that the operand is input and output one. With this modifier you will have the correct values on all possible paths from the asm goto .
To reference a label in the assembler template, prefix it with %l (lowercase L ) followed by its (zero-based) position in GotoLabels plus the number of input and output operands. Output operand with constraint modifier + is counted as two operands because it is considered as one output and one input operand. For example, if the asm has three inputs, one output operand with constraint modifier + and one output operand with constraint modifier = and references two labels, refer to the first label as %l6 and the second as %l7 ).
Alternately, you can reference labels using the actual C label name enclosed in brackets. For example, to reference a label named carry , you can use %l[carry] . The label must still be listed in the GotoLabels section when using this approach. It is better to use the named references for labels as in this case you can avoid counting input and output operands and special treatment of output operands with constraint modifier + .
Here is an example of asm goto for i386:
asm goto ( "btl %1, %0\n\t" "jc %l2" : /* No outputs. */ : "r" (p1), "r" (p2) : "cc" : carry); return 0; carry: return 1;
The following example shows an asm goto that uses a memory clobber.
int frob(int x) int y; asm goto ("frob %%r5, %1; jc %l[error]; mov (%2), %%r5" : /* No outputs. */ : "r"(x), "r"(&y) : "r5", "memory" : error); return y; error: return -1; >
The following example shows an asm goto that uses an output.
int foo(int count) asm goto ("dec %0; jb %l[stop]" : "+r" (count) : : : stop); return count; stop: return 0; >
The following artificial example shows an asm goto that sets up an output only on one path inside the asm goto . Usage of constraint modifier = instead of + would be wrong as factor is used on all paths from the asm goto .
int foo(int inp) int factor = 0; asm goto ("cmp %1, 10; jb %l[lab]; mov 2, %0" : "+r" (factor) : "r" (inp) : : lab); lab: return inp * factor; /* return 2 * inp or 0 if inp < 10 */>
References to input, output, and goto operands in the assembler template of extended asm statements can use modifiers to affect the way the operands are formatted in the code output to the assembler. For example, the following code uses the h and b modifiers for x86:
uint16_t num; asm volatile ("xchg %h0, %b0" : "+a" (num) );
These modifiers generate this assembler code:
xchg %ah, %al
The rest of this discussion uses the following code for illustrative purposes.
int main() int iInt = 1; top: asm volatile goto ("some assembler instructions here" : /* No outputs. */ : "q" (iInt), "X" (sizeof(unsigned char) + 1), "i" (42) : /* No clobbers. */ : top); >
With no modifiers, this is what the output from the operands would be for the att and intel dialects of assembler: