ARM GCC Inline Assembler Cookbook
About this Document
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:
The assembler instructions, defined as a single string constant:
"mov %0, %1, ror #1"
A list of output operands, separated by commas. Our example uses just one:
"=r" (result)
A comma separated list of input operands. Again our example uses one operand
only:
"r" (value)
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/
출처:
http://www.ethernut.de/en/documents/arm-inline-asm.html
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