The C language implementation method of recording and calculating leading zeros.
Leading zeros: that is, a hexadecimal number starts from the highest digit until it encounters the first 1, with a few zeros in front. For example, 0x00FFFFFF leading zero is 8; 0xF0000000 leading zero is 0; 0x08010002 leading zero is 4.
Calculating leading zeros Some hardware can be supported by assembly instructions, such as STM32F4 series, which can quickly get leading zeros using assembly instructions.
Now it is implemented in C language using table lookup, the code is as follows: taken from UCOSIII3.03 version cpu_core.c
static const CPU_INT08U CPU_CntLeadZerosTbl[256] = { /* Data vals : */
/* 0 1 2 3 4 5 6 7 8 9 A B C D E F */
8u, 7u, 6u, 6u, 5u, 5u, 5u, 5u, 4u, 4u, 4u, 4u, 4u, 4u, 4u, 4u, /* 0x00 to 0x0F */
3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, /* 0x10 to 0x1F */
2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, /* 0x20 to 0x2F */
2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, /* 0x30 to 0x3F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x40 to 0x4F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x50 to 0x5F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x60 to 0x6F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x70 to 0x7F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0x80 to 0x8F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0x90 to 0x9F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xA0 to 0xAF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xB0 to 0xBF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xC0 to 0xCF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xD0 to 0xDF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xE0 to 0xEF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u /* 0xF0 to 0xFF */
};
/*8bit类型数据计算前导零*/
CPU_DATA CPU_CntLeadZeros08 (CPU_INT08U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
/* ----------- C-OPTIMIZED ------------ */
/* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
return (nbr_lead_zeros);
}
/*16bit类型数据计算前导零*/
CPU_DATA CPU_CntLeadZeros16 (CPU_INT16U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
if (val > 0x00FFu) { /* Chk bits [15:08] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 8u); /* .. lookup tbl ix = 'val' >> 8 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
} else { /* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 8u); /* .. plus nbr msb lead zeros = 8 bits.*/
}
return (nbr_lead_zeros);
}
/*32bit类型数据计算前导零*/
CPU_DATA CPU_CntLeadZeros32 (CPU_INT32U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
/* ---------- ASM-OPTIMIZED ----------- */
if (val > 0x0000FFFFu) {
if (val > 0x00FFFFFFu) { /* Chk bits [31:24] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 24u); /* .. lookup tbl ix = 'val' >> 24 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
} else { /* Chk bits [23:16] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 16u); /* .. lookup tbl ix = 'val' >> 16 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 8u); /* .. plus nbr msb lead zeros = 8 bits.*/
}
} else {
if (val > 0x000000FFu) { /* Chk bits [15:08] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 8u); /* .. lookup tbl ix = 'val' >> 8 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 16u); /* .. plus nbr msb lead zeros = 16 bits.*/
} else { /* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 24u); /* .. plus nbr msb lead zeros = 24 bits.*/
}
}
return (nbr_lead_zeros);
}