In my source code I saw a weird behavior of arm compiler where it did redundant iteration over a string, which unnecessary. I display here a minimal example that shows that,and ask my question below that
#include <string.h>
#define MIN(x, y) (((x) < (y)) ? (x) : (y))
int MAX_FILE_NAME = 2500;
int F(char *file){
int file_len = MIN(strlen(file), MAX_FILE_NAME - 1);
return file_len;
}
int main(void) {
F(__FILE__);
return 0 ;
}
compiled with:
arm-none-eabi-gcc -nostdlib -Xlinker -Map="m7_experiments.map" -Xlinker --cref -Xlinker --gc-sections -Xlinker -print-memory-usage -mcpu=cortex-m7 -mfpu=fpv5-sp-d16 -mfloat-abi=hard -mthumb -T "m7_experiments_Debug.ld" -o "m7_experiments.axf" ./src/cr_startup_cm7.o ./src/crp.o ./src/flashconfig.o ./src/m7_experiments.o
Leads to:
Dump of assembler code for function F:
0x00000104 <+0>: push {r4, lr}
0x00000106 <+2>: mov r4, r0
0x00000108 <+4>: bl 0x13c <strlen>
0x0000010c <+8>: mov r2, r0
0x0000010e <+10>: ldr r3, [pc, #20] ; (0x124 <F+32>)
0x00000110 <+12>: ldr r0, [r3, #0]
0x00000112 <+14>: subs r0, #1
0x00000114 <+16>: cmp r2, r0
0x00000116 <+18>: bcc.n 0x11a <F+22>
0x00000118 <+20>: pop {r4, pc}
0x0000011a <+22>: mov r0, r4
0x0000011c <+24>: bl 0x13c <strlen>
0x00000120 <+28>: b.n 0x118 <F+20>
0x00000122 <+30>: nop
0x00000124 <+32>: lsls r0, r3, #6
0x00000126 <+34>: movs r0, r0
Note how in the case that the file length is shorter than the defined one, instead of just getting it's length from $r2
it's being computed again, worsening the time run to be as long as 2* file length. which seems unnecessary. Is there some way to justify the compiler behavior in this case? I'm interested to know.
It is redundant. But that is because of your code, not the compiler. That macro is going to expand to this:
// x = strlen(file)
// y = MAX_FILE_NAME - 1
(((strlen(file)) < (MAX_FILE_NAME - 1)) ? (strlen(file)) : (MAX_FILE_NAME - 1))
Remember, the preprocessor is essentially just a glorified copy and paste machine. You're calling strlen
twice. Try this:
size_t file_len = strlen(file);
file_len = MIN(file_len, MAX_FILE_NAME - 1);