unoptimized behaviour of arm compiler

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.

>Solution :

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:

int file_len = strlen(file);
file_len = MIN(file_len, MAX_FILE_NAME - 1);

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