Beyond Hello World: Advanced Arm Compiler 5 Features
Introduction
Beyond "Hello World": advanced compiler features
The Building "hello world" using Arm Compiler tutorial shows you how to build a simple C program with the Arm Compiler 5 toolchain.
This tutorial moves beyond the basics to explore some of the more advanced features of the Arm Compiler 5 toolchain.
This tutorial assumes you have installed and licensed Arm DS-5 Development Studio. For more information, see Getting Started with Arm DS-5 Development Studio.
1. Compiling mixed C and assembly source files
The Arm assembler, armasm
reads assembly language source code and outputs object code.
The Arm compiler, armcc
compiles C and C++ source to object code.
The Arm linker, armlink
combines the contents of one or more object files with any required libraries to produce an executable program.
The following example shows how to use armasm
, armcc
, and armlink
from DS-5 to build a project containing both C and assembly source files.
-
Create a new C project and add a new source file
my_strcopy.s
containing the following assembly code:PRESERVE8 AREA SCopy, CODE, READONLY EXPORT my_strcopy ; Export symbol my_strcopy ; R0 -> dest string ; R1 -> source string LDRB R2, [R1],#1 ; Load byte + update addr STRB R2, [R0],#1 ; Store byte + update addr CMP R2, #0 ; Check for null BNE my_strcopy ; Keep going if not BX lr ; Return END
The function
my_strcopy()
is exported so that it is available to be used from C.

-
Add a new source file to the project with the name
test.c
containing the following C code:#include <stdio.h> /* Declare the assembly function */ extern void my_strcopy(char *d, const char *s); int main() { const char *srcstr = "First string - source "; char dststr[] = "Second string - dest "; printf("Before copying:\n"); printf(" %s\n %s\n",srcstr,dststr); my_strcopy(dststr,srcstr); printf("After copying:\n"); printf(" %s\n %s\n",srcstr,dststr); return (0); }

-
Build the project.
The Arm Compiler toolchain does the following:
- Assembles
my_strcopy.s
witharmasm
to produce the object filemy_strcopy.o
. - Compiles
test.c
witharmcc
to produce the object filetest.o
. - Links the object files with
armlink
to produce an executable image.
When you run the executable image, it produces the following output:
Before copying: First string - source Second string - dest After copying: First string - source First string - source
- Assembles

2. Sharing header files between C and assembly code
The usual way to define constants in C code is to use #define
s, or in assembly code to use EQU
directives. If your project contains a mixture of C and assembly code, there might be some constant definitions that are common to both. If so, to avoid maintaining two separate lists, you can create one list of common definitions and include them in both your C and assembly code.
To do this, you can use C-style #include
and #define
directives directly in your assembly source code. You can pass this source code through the armcc
C preprocessor. This outputs a preprocessed version of your assembly code which armasm
can then assemble.
The following example shows how to do this.
-
Add a header file called
my_strcopy.h
to the project, containing the following line:#define ONE_CONSTANT 1
-
Add this line to the top of
my_strcopy.s
, created in the previous example:#include "my_strcopy.h"
-
In
my_strcopy.s
, replace the occurrences of#1
with#ONE_CONSTANT
, for example:LDRB R2, [R1], #ONE_CONSTANT

-
Pass
my_strcopy.s
through the C preprocessor. If you tried to build the project without first doing this,armasm
would report a syntax error for the#include
statement you added tomy_strcopy.s
.Open the Project Settings dialog. Then, under C/C++ build, select Settings.
In the Tool Settings tab, under Arm Assembler 5, select Preprocessor, then tick the box marked Preprocess input before assembling (--cpreproc).

armasm automatically passes some command-line options to the C preprocessor. If you need to pass other simple command-line options to the C preprocessor, for example –D or –I, specify them in the field marked Preprocessor options (--cpreproc_opts).
3. Improving optimization with linker feedback
Linker feedback lets the compiler and linker collaborate to improve the removal of unused code.
The linker can produce a text file containing a list of unused functions and functions that have been inlined. This information can be fed back to the compiler, which rebuilds the objects, placing these functions in their own sections. These sections can then be removed by the linker during usual unused section elimination.
The following example shows how linker feedback works.
-
Create a new C project and add a new source file
fb.c
containing the following code:
#include <stdio.h>
void legacy()
{
printf("This is an unused function.\n");
}
int cubed(int i)
{
return i*i*i;
}
int main(void)
{
int n = 3;
printf("%d cubed = %d\n",n,cubed(n));
}
- Open the Properties dialog box for your project, and select C/C++ Build > Settings.
-
On the Settings tab, select Arm C Compiler 5 > Optimizations and specify "Feedback file (--feedback)"
fb.txt
.This tells the compiler to look for a feedback file.
-
On the Settings tab, select Arm Linker 5 > Optimizations and specify "Feedback file (--feedback)"
fb.txt
.This tells the linker to create a feedback file.
-
On the Tool Settings tab, select Arm Linker 5 > Additional Information and specify "Redirect diagnostics output to file (--list)"
fbout.txt
.This tells the linker to save diagnostics to a file.
-
Build the project.
For this first compilation, the feedback file does not exist at compilation time. At link time, the linker identifies
legacy()
as the unused function, and creates a feedback filefb.txt
containing this information. - Clean the project (Project > Clean...), to remove the object and image files.
- Rename the diagnostics file
fbout.txt
tofbout_orig.txt
to let you compare it to the next build. -
Build the project again.
This time, the feedback file does exist at compilation time. Because the feedback file informs the compiler that
legacy()
is an unused function, the compiler can now omit this function from the generated object code.
You can compare the two diagnostics files, fbout.txt
and fbout_orig.txt
, to see the sizes of the image components (for example, Code, RO Data, RW Data, and ZI Data). The Code component is smaller, because armlink
has removed the legacy()
function from the final image.