Asm in c program

We can name our function however we want. The assembly code is machine-dependent. We want to verify if n is even. For even numbers, it will be 0. For odd numbers, it will be 1.

First, we decremented n by one. If it was 0 , it will be 1 after decrementing n. If it was 1 , it will be turned to 0. 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. 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. 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.

It might be a symbolic memory reference to the object pointed to by x. 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. 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. This is why a1 has an early-clobber. It is also not desirable in this case. Note that tying an input to an output is the way to set up an initialized temporary register modified by an asm statement.

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 GotoLabels section in an asm goto statement contains a comma-separated list of all C labels to which the assembler code may jump. 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.

Be careful when you set output operands inside asm goto only on some possible control flow paths. With this modifier you will have the correct values on all possible paths from the asm goto.

Alternately, you can reference labels using the actual C label name enclosed in brackets. The label must still be listed in the GotoLabels section when using this approach.

The following artificial example shows an asm goto that sets up an output only on one path inside the asm goto. 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.

On x86 targets, there are several rules on the usage of stack-like registers in the operands of an asm. These rules apply only to the operands that are stack-like registers:.

An input register that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand. All implicitly popped input registers must be closer to the top of the reg-stack than any input that is not implicitly popped.

It is possible that if an input dies in an asm , the compiler might use the input register for an output reload. Consider this example:.

This code says that input b is not popped by the asm , and that the asm pushes a result onto the reg-stack, i. But, it is possible that reload may think that it can use the same register for both the input and the output.

Output operands must specifically indicate which register an output appears in after an asm. This asm takes two inputs, which are popped by the fyl2xp1 opcode, and replaces them with one output. The st 1 clobber is necessary for the compiler to know that fyl2xp1 pops both inputs. Print the address in Double Integer DImode mode 8 bytes when the target is bit. Otherwise mode is unspecified VOIDmode. Add 8 bytes to an offsettable memory reference.

Useful when accessing the high 8 bytes of SSE values. If used for a constant, drop all syntax-specific prefixes and issue the bare constant. See p above. Here " add " is the output operand referred to by register eax. And arg1 and arg2 are input operands referred to by registers eax and ebx respectively. Let us see a complete example using extended inline assembly statements. It performs simple arithmetic operations on integer operands and displays the result available as arithmetic.

If our assembly statement must execute where we put it, i. Refer to the following example, which computes the Greatest Common Divisor using well known Euclid's Algorithm honoured as first algorithm.

Did you include it? Is the asm library added to the project so that it gets linked? Maybe this thread will help you: stackoverflow.

What operating system and architecture are you programming for? Can you show us the relevant code? And what compiler do you use? And what assembler? The exact way you interface between C and assembler is dependent on such things Add a comment. Active Oldest Votes. Improve this answer. I will think further about it.

You said inline assembly is an advanced topic and compiler dependent. Do you mean prototyping asm in C can avoid the problem of compiler difference? If you want to learn asm, learn asm, THEN learn specific compiler stuff, same goes with C learn C then learn compiler specific stuff Yes, my target assembly library is a Intel one.

I successfully built it and the function I need can be found in xxx. Like you said, what I need is just a C wrapper function to connect java and assembly.