ARM GCC Inline Assembler Paper
About this Document
Written by batched ~ batched@gmail.com
The GNU C compiler for ARM RISC processors offers, to embed assembly
language code into C programs. This cool feature may be used for
manually optimizing time critical parts of the software or to use
specific processor instruction, which are not available in the C
language.
It's assumed, that you are familiar with writing ARM assembler programs,
because this is not an ARM assembler programming tutorial. It's not a C
language tutorial either.
This document describes version 3.4 of the compiler. GCC asm Statement
Let's start with a simple example of rotating bits. It takes the value
of one integer variable, right rotates the bits by one and stores the
result in a second integer variable.
asm("mov %0, %1, ror #1" : "=r" (result) : "r" (value));
Each asm statement is devided by colons into up to four parts:
1. The assembler instructions, defined as a single string constant:
"mov %0, %1, ror #1"
2. A list of output operands, separated by commas. Our example uses just one:
"=r" (result)
3. A comma separated list of input operands. Again our example uses one operand only:
"r" (value)
4. Clobbered registers, left empty in our example.
You can write assembler instructions in much the same way as you would
write assembler programs. However, registers and constants are used in a
different way if they refer to expressions of your C program. The
connection between registers and C operands is specified in the second
and third part of the asm instruction, the list of output and input
operands, respectively. The general form is
asm(code : output operand list : input operand list : clobber list);
In the code section, operands are referenced by a percent sign followed
by a single digit. %0 refers to the first %1 to the second operand and
so forth. From the above example:
%0 refers to "=r" (result) and
%1 refers to "r" (value)
The last part of the asm instruction, the clobber list, is mainly used
to tell the compiler about modifications done by the assembler code.
This may still look a little odd now, but the syntax of an operand list
will be explained soon. Let us first examine the part of a compiler
listing which may have been generated from our example:
00309DE5 ldr r3, [sp, #0] @ value, value
E330A0E1 mov r3, r3, ror #1 @ tmp69, value
04308DE5 str r3, [sp, #4] @ tmp71, result
The compiler selected register r3 for bit rotation. It could have
selected any other register, though. It may not explicitly load or store
the value and it may even decide not to include your assembler code at
all. All these decisions are part of the compiler's optimization
strategy. For example, if you never use the variable value in the
remaining part of the C program, the compiler will most likely remove
your code unless you switched off optimization.
You can add the volatile attribute to the asm statement to instruct the
compiler not to optimize your assembler code.
asm volatile("mov %0, %1, ror #1" : "=r" (result) : "r" (value));
As with the clobber list in our example, trailing parts of the asm
statement may be omitted, if unused. The following statement does
nothing but consuming CPU time and provides the code part only. It is
also known as a NOP (no operation) statement and is typically used for
tiny delays.
asm volatile ("mov r0, r0");
If an unused part is followed by one which is used, it must be left
empty. The following example uses an input, but no output value.
asm volatile ("msr cpsr, %0" : : "r" (status));
Even the code part may be left empty, though an empty string is reuired.
The next statement specifies a special clobber to tell the compiler,
that memory contents may have changed.
asm volatile ("" : : : "memory");
With inline assembly you can use the same assembler instruction
mnemonics as you'd use for writing pure ARM assemly code. And you can
write more than one assembler instruction in a single inline asm
statement. To make it more readable, you should put each instruction on
a seperate line.
asm volatile(
"mov r0, r0\n\t"
"mov r0, r0\n\t"
"mov r0, r0\n\t"
"mov r0, r0"
);
The linefeed and tab characters will make the assembler listing
generated by the compiler more readable. It may look a bit odd for the
first time, but that's the way the compiler creates it's own assembler
code. Also note, that eight characters are reserved for the assembler
instruction mnemonic.
Input and Output Operands
Each input and output operand is described by a constraint string
followed by a C expression in parantheses. For ARM processors, GCC 3.4
provides the following constraint characters.
Constraint Used for Range
f Floating point registers
I Immediate operands 8 bits, possibly shifted.
J Indexing constants -4095 .. 4095
K Negated value in rhs -4095 .. 4095
L Negative value in rhs -4095 .. 4095
M For shifts. 0..32 or power of 2
r General registers
Constraint characters may be prepended by a single constraint modifier.
Contraints without a modifier specify read-only operands. Modifiers are:
Modifier Specifies
= Write-only operand, usually used for all output operands.
+ Read-write operand (not supported by inline assembler)
& Register should be used for output only
Output operands must be write-only and the C expression result must be
an lvalue, which means that the operands must be valid on the left side
of assignments. Note, that the compiler will not check if the operands
are of reasonable type for the kind of operation used in the assembler
instructions.
Input operands are, you guessed it, read-only. Never ever write to an
input operand. But what if you need the same operand for input and
output? As stated above, read-write operands are not supported in inline
assembler code. But there is another solution.
For input operators it is possible to use a single digit in the
constraint string. Using digit n tells the compiler to use the same
register as for the n-th operand, starting with zero. Here is an
example:
asm volatile("mov %0, %0, ror #1" : "=r" (value) : "0" (value));
This is similar to our initial example. It rotates the contents of the
variable value to the right by one bit. In opposite to our first
example, the result is not stored in another variable. Instead the
original contents of input variable will be modified. Constraint "0"
tells the compiler, to use the same input register as for the first
output operand.
Note however, that this doesn't automatically imply the reverse case.
The compiler may choose the same registers for input and output, even if
not told to do so. In our initial example it did indeed choose the same
register r3.
This is not a problem in most cases, but may be fatal if the output
operator is modified by the assembler code before the input operator is
used. In situations where your code depends on different registers used
for input and output operands, you must add the & constraint modifier to
your output operand. The following example demonstrates this problem.
asm volatile("ldr %0, [%1]" "\n\t"
"str %2, [%1, #4]" "\n\t"
: "=&r" (rdv)
: "r" (&table), "r" (wdv)
: "memory"
);
In this example a value is read from a table and then another value is
written to another location in this table. If the compiler would have
choosen the same register for input and output, then the output value
would have been destroyed on the first assembler instruction.
Fortunately, this example uses the & constraint modifier to instruct the
compiler not to select any register for the output value, which is used
for any of the input operands. Back to swapping. Here is the code to
swap high and low byte of a 16-bit value: Clobbers
If you are using registers, which had not been passed as operands, you
need to inform the compiler about this. The following code will adjust a
value to a multiple of four. It uses r3 as a scratch register and lets
the compiler know about this by specifying r3 in the clobber list.
Furthermore the CPU status flags are modified by the ands instruction.
Adding the pseudo register cc to the clobber list will keep the compiler
informed about this modification as well.
asm volatile("ands r3, %1, #3" "\n\t"
"eor %0, %0, r3" "\n\t"
"addne %0, #4"
: "=r" (len)
: "0" (len)
: "cc", "r3"
);
Our previous example, which stored a value in a table
asm volatile("ldr %0, [%1]" "\n\t"
"str %2, [%1, #4]" "\n\t"
: "=&r" (rdv)
: "r" (&table), "r" (wdv)
: "memory"
);
uses another so called pseudo register named "memory"in the clobber
list. This special clobber informs the compiler that the assembler code
may modify any memory location. It forces the compiler to update all
variables for which the contents are currently held in a register before
executing the assembler code. And of course, everything has to be
reloaded again after this code. Assembler Macros
In order to reuse your assembler language parts, it is useful to define
them as macros and put them into include files. Nut/OS comes with some
of them, which could be found in the subdirectory include. Using such
include files may produce compiler warnings, if they are used in
modules, which are compiled in strict ANSI mode. To avoid that, you can
write __asm__ instead of asm and __volatile__ instead of volatile. These
are equivalent aliases. C Stub Functions
Macro definitions will include the same assembler code whenever they are
referenced. This may not be acceptable for larger routines. In this case
you may define a C stub function, containing nothing other than your
assembler code.
unsigned long htonl(unsigned long val)
{
asm volatile ("eor r3, %1, %1, ror #16\n\t"
"bic r3, r3, #0x00FF0000\n\t"
"mov %0, %1, ror #8\n\t"
"eor %0, %0, r3, lsr #8"
: "=r" (val)
: "0"(val)
: "r3"
);
return val;
}
The purpose of this function is to swap all bytes of an unsigend 32 bit
value. In other words, it changes a big endian to a little endian value
or vice versa. C Names Used in Assembler Code
By default GCC uses the same symbolic names of functions or variables in
C and assembler code. You can specify a different name for the assembler
code by using a special form of the asm statement:
unsigned long value asm("clock") = 3686400;
This statement instructs the compiler to use the symbol name clock
rather than value. This makes sense only for external or static
variables, because local variables do not have symbolic names in the
assembler code. However, local variables may be held in registers.
With GCC you can further demand the use of a specific register:
void Count(void) {
register unsigned char counter asm("r3");
... some code...
asm volatile("eor r3, r3, r3");
... more code...
}
The assembler instruction, "eor r3, r3, r3", will clear the variable
counter. Be warned, that this sample is bad in most situations, because
it interfers with the compiler's optimizer. Furthermore, GCC will not
completely reserve the specified register. If the optimizer recognizes
that the variable will not be referenced any longer, the register may be
re-used. But the compiler is not able to check wether this register
usage conflicts with any predefined register. If you reserve too many
registers in this way, the compiler may even run out of registers during
code generation.
In order to change the name of a function, you need a prototype
declaration, because the compiler will not accept the asm keyword in the
function definition:
extern long Calc(void) asm ("CALCULATE");
Calling the function Calc() will create assembler instructions to call the function CALCULATE.
Register Usage
Typically the following registers are used by the compiler for specific purposes.
Register Alt. Name Usage
r0 a1 First function argument
Integer function result
Scratch register
r1 a2 Second function argument
Scratch register
r2 a3 Third function argument
Scratch register
r3 a4 Fourth function argument
Scratch register
r4 v1 Register variable
r5 v2 Register variable
r6 v3 Register variable
r7 v4 Register variable
r8 v5 Register variable
r9 v6
rfp Register variable
Real frame pointer
r10 sl Stack limit
r11 fp Argument pointer
r12 ip Temporary workspace
r13 sp Stack pointer
r14 lr Link register
Workspace
r15 pc Program counter
Links
For a more thorough discussion of inline assembly usage, see the gcc
user manual. The latest version of the gcc manual is always available
here:
http://gcc.gnu.org/onlinedocs/
# milw0rm.com [2006-12-30]