Implementation of cx_bn_cnt_bits function is broken?

[aido]

Hi,

I think the implementation of cx_bn_cnt_bits in Speculos may be broken. It is exhibiting some strange behaviour.

I have written the following three tests:

cx_bn_t bn_temp;
CX_CHECK(cx_bn_alloc(&bn_temp, 16));

uint32_t nbits, temp_u32 = 0;

for (uint8_t i = 0; i <= 8; i++) {
    CX_CHECK(cx_bn_set_u32(bn_temp, (uint32_t) i));
    CX_CHECK(cx_bn_cnt_bits(bn_temp, &nbits));
    PRINTF("Test 1: Number of bits set to 1 in number %d is %d\n", i, nbits);
}

for (uint8_t i = 8; i >= 0; i--) {
    CX_CHECK(cx_bn_set_u32(bn_temp, (uint32_t) i));
    CX_CHECK(cx_bn_cnt_bits(bn_temp, &nbits));
    PRINTF("Test 2: Number of bits set to 1 in number %d is %d\n", i, nbits);
}

CX_CHECK(cx_bn_set_u32(bn_temp, (uint32_t) 8));
CX_CHECK(cx_bn_cnt_bits(bn_temp, &nbits));

while (nbits > 0) {
    CX_CHECK(cx_bn_get_u32(bn_temp, &temp_u32));
    PRINTF("Test 3: Number of bits set to 1 in number %d is %d\n", temp_u32, nbits);
    CX_CHECK(cx_bn_shr(bn_temp, 1));
}

Each test results in an infinite loop. It would seem that rather than count the set bits in a Big Num cx_bn_cnt_bits instead calculates the position of the MSB.

09:29:27.929:seproxyhal: printf: Test 1: Number of bits set to 1 in number 0 is -127
09:29:27.931:seproxyhal: printf: Test 1: Number of bits set to 1 in number 1 is 1
09:29:27.932:seproxyhal: printf: Test 1: Number of bits set to 1 in number 2 is 2
09:29:27.934:seproxyhal: printf: Test 1: Number of bits set to 1 in number 3 is 2
09:29:27.935:seproxyhal: printf: Test 1: Number of bits set to 1 in number 4 is 3
09:29:27.936:seproxyhal: printf: Test 1: Number of bits set to 1 in number 5 is 3
09:29:27.938:seproxyhal: printf: Test 1: Number of bits set to 1 in number 6 is 3
09:29:27.939:seproxyhal: printf: Test 1: Number of bits set to 1 in number 7 is 3
09:29:27.941:seproxyhal: printf: Test 1: Number of bits set to 1 in number 8 is 4
09:29:27.942:seproxyhal: printf: Test 2: Number of bits set to 1 in number 8 is 4
09:29:27.943:seproxyhal: printf: Test 2: Number of bits set to 1 in number 7 is 3
09:29:27.945:seproxyhal: printf: Test 2: Number of bits set to 1 in number 6 is 3
09:29:27.946:seproxyhal: printf: Test 2: Number of bits set to 1 in number 5 is 3
09:29:27.947:seproxyhal: printf: Test 2: Number of bits set to 1 in number 4 is 3
09:29:27.949:seproxyhal: printf: Test 2: Number of bits set to 1 in number 3 is 2
09:29:27.950:seproxyhal: printf: Test 2: Number of bits set to 1 in number 2 is 2
09:29:27.952:seproxyhal: printf: Test 2: Number of bits set to 1 in number 1 is 1
09:29:27.953:seproxyhal: printf: Test 2: Number of bits set to 1 in number 0 is 0
09:29:27.954:seproxyhal: printf: Test 2: Number of bits set to 1 in number 255 is 8
09:29:27.956:seproxyhal: printf: Test 2: Number of bits set to 1 in number 254 is 8
09:29:27.957:seproxyhal: printf: Test 2: Number of bits set to 1 in number 253 is 8
09:29:27.959:seproxyhal: printf: Test 2: Number of bits set to 1 in number 252 is 8
09:29:27.960:seproxyhal: printf: Test 2: Number of bits set to 1 in number 251 is 8
09:29:27.962:seproxyhal: printf: Test 2: Number of bits set to 1 in number 250 is 8
09:29:27.963:seproxyhal: printf: Test 2: Number of bits set to 1 in number 249 is 8
09:29:27.965:seproxyhal: printf: Test 2: Number of bits set to 1 in number 248 is 8
09:29:27.966:seproxyhal: printf: Test 2: Number of bits set to 1 in number 247 is 8
09:29:27.967:seproxyhal: printf: Test 2: Number of bits set to 1 in number 246 is 8
09:29:27.969:seproxyhal: printf: Test 2: Number of bits set to 1 in number 245 is 8
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09:36:07.109:seproxyhal: printf: Test 2: Number of bits set to 1 in number 8 is 4
09:36:07.110:seproxyhal: printf: Test 2: Number of bits set to 1 in number 7 is 3
09:36:07.112:seproxyhal: printf: Test 2: Number of bits set to 1 in number 6 is 3
09:36:07.113:seproxyhal: printf: Test 2: Number of bits set to 1 in number 5 is 3
09:36:07.114:seproxyhal: printf: Test 2: Number of bits set to 1 in number 4 is 3
09:36:07.116:seproxyhal: printf: Test 2: Number of bits set to 1 in number 3 is 2
09:36:07.117:seproxyhal: printf: Test 2: Number of bits set to 1 in number 2 is 2
09:36:07.119:seproxyhal: printf: Test 2: Number of bits set to 1 in number 1 is 1
09:36:07.120:seproxyhal: printf: Test 2: Number of bits set to 1 in number 0 is 0
09:36:07.109:seproxyhal: printf: Test 2: Number of bits set to 1 in number 255 is 8
09:36:07.123:seproxyhal: printf: Test 2: Number of bits set to 1 in number 254 is 8
09:36:07.124:seproxyhal: printf: Test 2: Number of bits set to 1 in number 253 is 8
09:36:07.126:seproxyhal: printf: Test 2: Number of bits set to 1 in number 252 is 8
09:36:07.127:seproxyhal: printf: Test 2: Number of bits set to 1 in number 251 is 8
09:36:07.129:seproxyhal: printf: Test 2: Number of bits set to 1 in number 250 is 8
09:36:07.130:seproxyhal: printf: Test 2: Number of bits set to 1 in number 249 is 8
09:36:07.131:seproxyhal: printf: Test 2: Number of bits set to 1 in number 248 is 8
09:36:07.133:seproxyhal: printf: Test 2: Number of bits set to 1 in number 247 is 8
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09:41:28.614:seproxyhal: printf: Test 3: Number of bits set to 1 in number 8 is 4
09:41:28.615:seproxyhal: printf: Test 3: Number of bits set to 1 in number 4 is 4
09:41:28.617:seproxyhal: printf: Test 3: Number of bits set to 1 in number 2 is 4
09:41:28.618:seproxyhal: printf: Test 3: Number of bits set to 1 in number 1 is 4
09:41:28.619:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.609:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.622:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.623:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.625:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.626:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.628:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.629:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.630:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
09:41:28.632:seproxyhal: printf: Test 3: Number of bits set to 1 in number 0 is 4
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