The new instructions
SSE 4.2 introduces four instructions (PcmpEstrI, PcmpEstrM, PcmpIstrI, and PcmpIstrM) that can be used to speed up text processing code (including strcmp, memcmp, strstr, and strspn functions).
Intel had published the description for new instruction formats, but no sample code nor high-level guidelines. This article tries to provide them.
MovDqU xmm0, dqword[str1] PcmpIstrI xmm0, dqword[str2], imm
PcmpIstrI is one of the new string-handling instructions comparing their operands. The first operand is always an SSE register (typically loaded with MovDqU). The second operand can be a memory location. Note the immediate operand, which consists of several bit fields controlling operation modes.
The heart of a string-processing instruction is the aggregation operation (immediate bits [3:2]).
Equal each (imm[3:2] = 10). This operation compares two strings (think of strcmp or memcmp). The result of comparison is a bit mask (1 if the corresponding bytes are equal, 0 if not equal). For example:
operand1 = "UseFlatAssembler" operand2 = "UsingAnAssembler" IntRes1 = 1100000111111111
Equal any (imm[3:2] = 00). The first operand is a character set, the second is a string (think of strspn or strcspn). The bit mask includes 1 if the character belongs to a set, 0 if not:
operand2 = "You Drive Me Mad", operand1 = "aeiouy" IntRes1 = 0110001010010010
Ranges (imm[3:2] = 01). The first operand consists of ranges, for example, "azAZ" means "all characters from a to z and all characters from A to Z":
operand2 = "I'm here because", operand1 = "azAZ" IntRes1 = 1010111101111111
Equal ordered (imm[3:2] = 11). Substring search (strstr). The first operand contains a string to search for, the second is a string to search in. The bit mask includes 1 if the substring is found at the corresponding position:
operand2 = "WhenWeWillBeWed!", operand1 = "We" IntRes1 = 000010000000100
After computing the aggregation function, IntRes1 can be complemented, expanded into byte mask or shrinked into index. The result is written into xmm0 or ECX registers. Intel manual explains these details well, so there is no need to repeate them here.
Other features of SSE 4.2
- The strings do not need to be aligned.
- The processor properly handles end-of-the-string case for zero-terminated strings and Pascal-style strings.
- You can use the instructions with Unicode characters, signed or unsigned bytes.
- Four aggregation operations can be used to implement a wide range of string-processing functions.
The following strcmp and strlen functions were written and published when the processors with SSE 4.2 support were not available yet. Later they were tested on real hardware and found to be correct.
; compile with FASM ; Immediate byte constants EQUAL_ANY = 0000b RANGES = 0100b EQUAL_EACH = 1000b EQUAL_ORDERED = 1100b NEGATIVE_POLARITY = 010000b BYTE_MASK = 1000000b ; ==== strcmp ==== strcmp_sse42: ; Using __fastcall convention, ecx = string1, edx = string2 mov eax, ecx sub eax, edx ; eax = ecx - edx sub edx, 16 STRCMP_LOOP: add edx, 16 MovDqU xmm0, dqword[edx] ; find the first *different* bytes, hence negative polarity PcmpIstrI xmm0, dqword[edx + eax], EQUAL_EACH + NEGATIVE_POLARITY ja STRCMP_LOOP jc STRCMP_DIFF ; the strings are equal xor eax, eax ret STRCMP_DIFF: ; subtract the first different bytes add eax, edx movzx eax, byte[eax + ecx] movzx edx, byte[edx + ecx] sub eax, edx ret ; ==== strlen ==== strlen_sse42: ; ecx = string mov eax, -16 mov edx, ecx pxor xmm0, xmm0 STRLEN_LOOP: add eax, 16 PcmpIstrI xmm0, dqword[edx + eax], EQUAL_EACH jnz STRLEN_LOOP add eax, ecx ret ; ==== strstr ==== strstr_sse42: ; ecx = haystack, edx = needle push esi push edi MovDqU xmm2, dqword[edx] ; load the first 16 bytes of neddle Pxor xmm3, xmm3 lea eax, [ecx - 16] ; find the first possible match of 16-byte fragment in haystack STRSTR_MAIN_LOOP: add eax, 16 PcmpIstrI xmm2, dqword[eax], EQUAL_ORDERED ja STRSTR_MAIN_LOOP jnc STRSTR_NOT_FOUND add eax, ecx ; save the possible match start mov edi, edx mov esi, eax sub edi, esi sub esi, 16 ; compare the strings @@: add esi, 16 MovDqU xmm1, dqword[esi + edi] ; mask out invalid bytes in the haystack PcmpIstrM xmm3, xmm1, EQUAL_EACH + NEGATIVE_POLARITY + BYTE_MASK MovDqU xmm4, dqword[esi] PAnd xmm4, xmm0 PcmpIstrI xmm1, xmm4, EQUAL_EACH + NEGATIVE_POLARITY ja @B jnc STRSTR_FOUND ; continue searching from the next byte sub eax, 15 jmp STRSTR_MAIN_LOOP STRSTR_NOT_FOUND: xor eax, eax STRSTR_FOUND: pop edi pop esi ret
- Intel Software Developer Manuals.
- comp.arch discussion about the new string-processing instructions.
- Optimizing strlen on processors without SSE4 support.
- A review of Intel Nehalem processor, which includes support for the text processing instructions.
Typo in Intel manual: on figure 5-1, "imm8[6:5]" near Optional boolean negation should be "imm8[5:4]".