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Replace UTF-8 decoder with one based on branchless version by Christopher Wellons. (not branchless anymore tho)

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Decoding performance increase ~30%
This commit is contained in:
Rokas Kupstys
2020-11-03 14:32:44 +01:00
committed by ocornut
parent 3777fbbd81
commit 9cca1b2e97
2 changed files with 50 additions and 55 deletions
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102
imgui.cpp
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@@ -1554,66 +1554,58 @@ void* ImFileLoadToMemory(const char* filename, const char* mode, size_t* out_f
//-----------------------------------------------------------------------------
// Convert UTF-8 to 32-bit character, process single character input.
// Based on stb_from_utf8() from github.com/nothings/stb/
// A nearly-branchless UTF-8 decoder, based on work of Christopher Wellons (https://github.com/skeeto/branchless-utf8).
// We handle UTF-8 decoding error by skipping forward.
int ImTextCharFromUtf8(unsigned int* out_char, const char* in_text, const char* in_text_end)
{
unsigned int c = (unsigned int)-1;
const unsigned char* str = (const unsigned char*)in_text;
if (!(*str & 0x80))
static const char lengths[32] = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 2, 3, 3, 4, 0 };
static const int masks[] = { 0x00, 0x7f, 0x1f, 0x0f, 0x07 };
static const uint32_t mins[] = { 0x400000, 0, 0x80, 0x800, 0x10000 };
static const int shiftc[] = { 0, 18, 12, 6, 0 };
static const int shifte[] = { 0, 6, 4, 2, 0 };
int len = lengths[*(const unsigned char*)in_text >> 3];
int wanted = len + !len;
if (in_text_end == NULL)
in_text_end = in_text + wanted; // Max length, nulls will be taken into account.
// Copy at most 'len' bytes, stop copying at 0 or past in_text_end. Branch predictor does a good job here,
// so it is fast even with excessive branching.
unsigned char s[4];
s[0] = in_text + 0 < in_text_end ? in_text[0] : 0;
s[1] = in_text + 1 < in_text_end ? in_text[1] : 0;
s[2] = in_text + 2 < in_text_end ? in_text[2] : 0;
s[3] = in_text + 3 < in_text_end ? in_text[3] : 0;
// Assume a four-byte character and load four bytes. Unused bits are shifted out.
*out_char = (uint32_t)(s[0] & masks[len]) << 18;
*out_char |= (uint32_t)(s[1] & 0x3f) << 12;
*out_char |= (uint32_t)(s[2] & 0x3f) << 6;
*out_char |= (uint32_t)(s[3] & 0x3f) << 0;
*out_char >>= shiftc[len];
// Accumulate the various error conditions.
int e = 0;
e = (*out_char < mins[len]) << 6; // non-canonical encoding
e |= ((*out_char >> 11) == 0x1b) << 7; // surrogate half?
e |= (*out_char > IM_UNICODE_CODEPOINT_MAX) << 8; // out of range?
e |= (s[1] & 0xc0) >> 2;
e |= (s[2] & 0xc0) >> 4;
e |= (s[3] ) >> 6;
e ^= 0x2a; // top two bits of each tail byte correct?
e >>= shifte[len];
if (e)
{
c = (unsigned int)(*str++);
*out_char = c;
return 1;
// No bytes are consumed when *in_text == 0 || in_text == in_text_end.
// One byte is consumed in case of invalid first byte of in_text.
// All available bytes (at most `len` bytes) are consumed on incomplete/invalid second to last bytes.
// Invalid or incomplete input may consume less bytes than wanted, therefore every byte has to be inspected in s.
wanted = ImMin(wanted, !!s[0] + !!s[1] + !!s[2] + !!s[3]);
*out_char = IM_UNICODE_CODEPOINT_INVALID;
}
if ((*str & 0xe0) == 0xc0)
{
*out_char = IM_UNICODE_CODEPOINT_INVALID; // will be invalid but not end of string
if (in_text_end && in_text_end - (const char*)str < 2) return 1;
if (*str < 0xc2) return 2;
c = (unsigned int)((*str++ & 0x1f) << 6);
if ((*str & 0xc0) != 0x80) return 2;
c += (*str++ & 0x3f);
*out_char = c;
return 2;
}
if ((*str & 0xf0) == 0xe0)
{
*out_char = IM_UNICODE_CODEPOINT_INVALID; // will be invalid but not end of string
if (in_text_end && in_text_end - (const char*)str < 3) return 1;
if (*str == 0xe0 && (str[1] < 0xa0 || str[1] > 0xbf)) return 3;
if (*str == 0xed && str[1] > 0x9f) return 3; // str[1] < 0x80 is checked below
c = (unsigned int)((*str++ & 0x0f) << 12);
if ((*str & 0xc0) != 0x80) return 3;
c += (unsigned int)((*str++ & 0x3f) << 6);
if ((*str & 0xc0) != 0x80) return 3;
c += (*str++ & 0x3f);
*out_char = c;
return 3;
}
if ((*str & 0xf8) == 0xf0)
{
*out_char = IM_UNICODE_CODEPOINT_INVALID; // will be invalid but not end of string
if (in_text_end && in_text_end - (const char*)str < 4) return 1;
if (*str > 0xf4) return 4;
if (*str == 0xf0 && (str[1] < 0x90 || str[1] > 0xbf)) return 4;
if (*str == 0xf4 && str[1] > 0x8f) return 4; // str[1] < 0x80 is checked below
c = (unsigned int)((*str++ & 0x07) << 18);
if ((*str & 0xc0) != 0x80) return 4;
c += (unsigned int)((*str++ & 0x3f) << 12);
if ((*str & 0xc0) != 0x80) return 4;
c += (unsigned int)((*str++ & 0x3f) << 6);
if ((*str & 0xc0) != 0x80) return 4;
c += (*str++ & 0x3f);
// utf-8 encodings of values used in surrogate pairs are invalid
if ((c & 0xFFFFF800) == 0xD800) return 4;
// If codepoint does not fit in ImWchar, use replacement character U+FFFD instead
if (c > IM_UNICODE_CODEPOINT_MAX) c = IM_UNICODE_CODEPOINT_INVALID;
*out_char = c;
return 4;
}
*out_char = 0;
return 0;
return wanted;
}
int ImTextStrFromUtf8(ImWchar* buf, int buf_size, const char* in_text, const char* in_text_end, const char** in_text_remaining)