Embedded Systems Programming Writing Optimised C code for ARM
Why write optimised C code? • For embedded system size and/or speed are of key importance • The compiler optimisation phase can only do so much • In order to write optimal C code you need to know details of the underlying hardware and the compiler
What compilers can’t do • void memclr( char * data, int N) • { • for (; N > 0; N--) • { • *data=0; • data++; • } • } • Is N == on first loop? • 0 – 1 is dangerous! • Is data array 4 byte aligned? • Can store using int • Is N a multiple of 4? • Could do 4 word blocks at a time • Compilers have to be conservative!
An example Program The program might seem fine – even resource friendly Using a char saves space for loops make good assembler Lets look at the assembler code • /* program showing inefficient • * variable and loop • * usage craig Nov 04 • */ • int checksum_1(int *data) • { • char i; int sum = 0; • for (i =0; i < 64; i++) • sum += data[i]; • return sum; • }
.text .align 2 .global checksum_1 .type checksum_1,functionchecksum_1: @ args = 0, pretend = 0, frame = 0 @ frame_needed = 1, current_function_anonymous_args = 0 mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4 mov r1, r0 mov r0, #0 @ sum = 0 mov r2, r0 @ i = 0.L6: ldr r3, [r1, r2, asl #2] @ data[i] add r0, r0, r3 @ sum = data[i] add r3, r2, #1 @ i ++ and r2, r3, #255 cmp r2, #63 @ i < 64 bls .L6 ldmea fp, {fp, sp, pc}.Lfe1: .size checksum_1,.Lfe1-checksum_1
What is wrong? • The use of char means that the compiler has to cast to look at 8 bits – using • and r2, r3, #255 • The loop variable requires a register and initialisation • If the loop is called often then the tests and branch is quite an overhead
Variable sizes • In general the compiler will use 32bit registers for local variables but will have to cast them when used as 8 or 16 bit values • If you can, use unsigned ints, if you can’t explicitly cast • Using signed shorts can be quite a problem for compilers
Watch your shorts! short add( short a, short b) { return a + (b >> 1); } • The above C code turns into the rather nasty assembler • The gnu C compiler is very cautious when confronted with short variables Becomes …. mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4 mov r1, r1, asl #16 mov r0, r0, asl #16 mov r0, r0, asr #16 add r0, r0, r1, asr #17 mov r0, r0, asl #16 mov r0, r0, asr #16 ldmea fp, {fp, sp, pc
Loops #1 • As well as using a char for a loop counter the loop counter could be redundant • Terminate loops by counting down to 0 the reduces register usage and means no initialisation • Use do..while instead of for loops
Efficient loop C */ * Program to show efficient use of * variables and loops */ int checksum_2(int *data) { int sum = 0, i = 64; do { sum += *(data++); } while ( --i != 0 ); return sum; }
Efficient loop assembler checksum_2: @ args = 0, pretend = 0, frame = 0 @ frame_needed = 1, current_function_anonymous_args = 0 mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4 mov r1, r0 mov r0, #0 @ sum = 0 mov r2, #64 @ i = 64.L6: ldr r3, [r1], #4 @ *(data++) add r0, r0, r3 @ sum = *(data++) subs r2, r2, #1 @ --i bne .L6 ldmea fp, {fp, sp, pc}
Loop unrolling • If a loop is going to be repeated often then the test and branch can be quite an overhead • If the loop is a multiple of 4 and is done quite a lot then the loop can be unrolled • This increases code a size but is more speed efficient • Sizes that are not multiples of 4 can be done but are less efficient.
An unrolled loop * Program to show efficient use of * variables and loops & loop unrolling */ int checksum_2(int *data) { int sum = 0, i = 64; do { sum += *(data++); sum += *(data++); sum += *(data++); sum += *(data++); i -= 4; } while ( i != 0 ); return sum; }
checksum_2: @ args = 0, pretend = 0, frame = 0 @ frame_needed = 1, current_function_anonymous_args = 0 mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4 mov r2, r0 mov r0, #0 mov r1, #64.L6: ldr r3, [r2], #4 add r0, r0, r3 ldr r3, [r2], #4 add r0, r0, r3 ldr r3, [r2], #4 add r0, r0, r3 ldr r3, [r2], #4 add r0, r0, r3 subs r1, r1, #4 bne .L6 ldmea fp, {fp, sp, pc}
Loop unrolling ! = 4 /* Program to show use of * loop unrolling */ int checksum_2(int *data, unsigned int N) { int sum = 0; unsigned int i; for ( i = N/4; i != 0; i--) { sum += *(data++); sum += *(data++); sum += *(data++); sum += *(data++); } for ( i = N&3; i != 0; i--) sum += *(data++); return sum; }
