https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102124
--- Comment #1 from Tomas Chang <changyp6 at gmail dot com> ---
Test Program is as below:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t byte;
int buf_eq_const(const void *_a, const void *_b, size_t len)
{
const byte *a = _a;
const byte *b = _b;
int ab, ba;
size_t i;
/* Constant-time compare. */
for (i = 0, ab = 0, ba = 0; i < len; i++)
{
/* If a[i] != b[i], either ab or ba will be negative. */
ab |= a[i] - b[i];
ba |= b[i] - a[i];
}
/* 'ab | ba' is negative when buffers are not equal. */
printf("(ab | ba) = %d(%08x), ab = %d(%08x), ba = %d(%08x)\n", (ab | ba),
(ab | ba), ab, ab, ba, ba);
return (ab | ba) >= 0;
}
void print_array(const char *name, uint8_t *a, size_t len)
{
printf("%s[%lu] = {", name, len);
for (size_t i = 0; i < len; ++ i) {
printf("\'%c\', ", a[i]);
}
printf("}\n");
}
int main(int argc, char *argv[])
{
uint8_t a[32] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', 'a', 'a', 'a'};
uint8_t b[32] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', 'a', 'a', 'a'};
uint8_t c[32] = {'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b',
'b', 'b', 'b', 'b', 'b'};
printf("Comparing array a and array b:\n");
print_array("a", a, 16);
print_array("b", b, 16);
if (buf_eq_const(a, b, 16)) {
printf("a == b\n");
} else {
printf("a != b\n");
}
printf("Comparing array a and array c:\n");
print_array("a", a, 16);
print_array("c", c, 16);
if (buf_eq_const(a, c, 16)) {
printf("a == c\n");
} else {
printf("a != c\n");
}
return 0;
}
Compile this file with the following command:
gcc -Wall -O2 -march=armv8.2-a+crypto+fp16+rcpc+dotprod -mtune=cortex-a76 -o $@
$<
Running Results:
./test-gcc-O2
Comparing array a and array b:
a[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
b[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
(ab | ba) = 0(00000000), ab = 0(00000000), ba = 0(00000000)
a == b
Comparing array a and array c:
a[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
c[16] = {'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b',
'b', 'b', }
(ab | ba) = -1(ffffffff), ab = -1(ffffffff), ba = 1(00000001)
a != c
Compile this file with the following command:
test-gcc-O2-tree-loop-vectorize: test-gcc.c
gcc -Wall -O2 -march=armv8.2-a+crypto+fp16+rcpc+dotprod
-mtune=cortex-a76 -ftree-loop-vectorize -o $@ $<
Running Results:
./test-gcc-O2-tree-loop-vectorize
Comparing array a and array b:
a[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
b[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
(ab | ba) = 0(00000000), ab = 0(00000000), ba = 0(00000000)
a == b
Comparing array a and array c:
a[16] = {'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', }
c[16] = {'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b',
'b', 'b', }
(ab | ba) = 65535(0000ffff), ab = 65535(0000ffff), ba = 1(00000001)
a == c
