ARM GCC Inline Assembler Cookbook

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ARM GCC Inline Assembler Cookbook

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GCC asm statement

Let's start with a simple example. The following statement may be included in your code like any other C statement.

/* NOP example */

It moves the contents of register r0 to register r0. In other words, it doesn't do much more than nothing. It is also known as a

NOP (no operation) statement and is typically used for very short delays.

Stop! Before adding this example right away to your C code, keep on reading and learn, why this may not work as expected.

With inline assembly you can use the same assembler instruction mnemonics as you'd use for writing pure ARM assembly

code. And you can write more than one assembler instruction in a single inline asm statement. To make it more readable, you

can put each instruction on a separate line.

The special sequence of linefeed and tab characters will keep the assembler listing looking nice. It may seem a bit odd for the

first time, but that's the way the compiler creates its own assembler code while compiling C statements.

So far, the assembler instructions are much the same as they'd appear in pure assembly language programs. However,

registers and constants are specified in a different way, if they refer to C expressions. The general form of an inline assembler

statement is

The connection between assembly language and C operands is provided by an optional second and third part of the asm

statement, the list of output and input operands. We will explain the third optional part, the list of clobbers, later.

The next example of rotating bits passes C variables to assembly language. It takes the value of one integer variable, right

rotates the bits by one and stores the result in a second integer variable.

3. A comma separated list of input operands, which uses the same syntax as the list of output operands. Again, this is

4. Optional list of clobbered registers, omitted in our example.

As shown in the initial NOP example, trailing parts of the asm statement may be omitted, if unused. Inline asm statements

containing assembler instruction only are also known as basic inline assembly, while statements containing optional parts are

called extended inline assembly. If an unused part is followed by one which is used, it must be left empty. The following

You can insert spaces, newlines and even C comments to increase readability.

In the code section, operands are referenced by a percent sign followed by the related symbolic name enclosed in square

brackets. It refers to the entry in one of the operand lists that contains the same symbolic name. From the rotating bits example:

%[result] refers to output operand, the C variable y, and

must be written as

Operands are referenced by a percent sign followed by a single digit, where %0 refers to the first %1 to the second operand

and so forth. This format is still supported by the latest GCC releases, but quite error-prone and difficult to maintain. Imagine,

that you have written a large number of assembler instructions, where operands have to be renumbered manually after

inserting a new output operand.

If all this stuff still looks a little odd, don't worry. Beside the mysterious clobber list, you have the strong feeling that something

else is missing, right? Indeed, we didn't talk about the constraint strings in the operand lists. I'd like to ask for your patience.

There's something more important to highlight in the next chapter.

C code optimization

been generated from our rotating bits example:

The compiler selected register r3 for bit rotation. It could have selected any other register or two registers, one for each C

variable. It may not explicitly load the value or store the result. Here is another listing, generated by a different compiler version

with different compile options:

The compiler selected a unique register for each operand, using the value already cached in r4 and passing the result to the

following code in r2. Did you get the picture?

code optimization. Remember, that you have been warned to use the initial example. Here is the revised version:

/* NOP example, revised */

But there is more trouble waiting for us. A sophisticated optimizer will re-arrange the code. The following C snippet had been

left over after several last minute changes:

The optimizer will recognize, that the two increments do not have any impact on the conditional statement. Furthermore it