Bug in Clang

test-bug.c

/*

This program triggers a bug in Clang when compiling with optimizations.

When compiling with no-optimizations, the correct output is produced.

    $ clang -O0 test-bug.c && ./a.out
    value = 1


When compiling with optimizations (-O2 or -O3), the output is wrong.

    $ clang -O3 test-bug.c && ./a.out
    value = 0


*/


#include <stdio.h>
#include <stdlib.h>
#include <string.h>


//
// Helpers
//

typedef ssize_t Int;

typedef struct {
    char *begin;
    char *end;
} String;

String make_string(char *ptr, Int n) {
    String s = { ptr, ptr+n };
    return s;
}

Int string_size(String s) {
    return s.end - s.begin;
}

String c_string(char *s) {
    return make_string(s, (Int)strlen(s));
}

String alloc_string(String s) {
    Int n = string_size(s);
    char *ptr = malloc(n);
    memcpy(ptr, s.begin, n);
    return make_string(ptr, n);
}

void free_string(String s) {
    free(s.begin);
}


//
// count_py_files
//

int are_strings_equal(String a, String b) {
    if (string_size(a) != string_size(b)) { return 0; }
    char *a_ptr = a.begin;
    char *b_ptr = b.begin;
    while (1) {
        if (a_ptr == a.end) { break; }
        if (*a_ptr != *b_ptr) { return 0; }
        a_ptr += 1;
        b_ptr += 1;
    }
    return 1;
}

int ends_with(String a, String b) {
    Int na = string_size(a);
    Int nb = string_size(b);
    if (nb > na) { return 0; }
    String tmp = make_string(a.begin + (na-nb), nb);
    return are_strings_equal(tmp, b);
}

__attribute__((noinline))
Int count_py_files(String *files_begin, String *files_end) {
    String suffix = c_string(".py");
    Int n = 0;
    String *files_ptr = files_begin;
    while (files_ptr != files_end) {
        if (ends_with(*files_ptr, suffix)) {
            n += 1;
        }
        files_ptr += 1;
    }
    return n;
}


//
// main
//

int main() {
    String file = alloc_string(c_string("foo.py"));
    String *files_begin = &file;
    String *files_end = files_begin + 1;
    Int n = count_py_files(files_begin, files_end);
    free_string(file);
    printf("value = %d\n", (int)n);
    return 0;
}