bench-forgejo/vendor/github.com/RoaringBitmap/roaring/setutil_arm64.s

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// +build arm64,!gccgo,!appengine
#include "textflag.h"
// This implements union2by2 using golang's version of arm64 assembly
// The algorithm is very similar to the generic one,
// but makes better use of arm64 features so is notably faster.
// The basic algorithm structure is as follows:
// 1. If either set is empty, copy the other set into the buffer and return the length
// 2. Otherwise, load the first element of each set into a variable (s1 and s2).
// 3. a. Compare the values of s1 and s2.
// b. add the smaller one to the buffer.
// c. perform a bounds check before incrementing.
// If one set is finished, copy the rest of the other set over.
// d. update s1 and or s2 to the next value, continue loop.
//
// Past the fact of the algorithm, this code makes use of several arm64 features
// Condition Codes:
// arm64's CMP operation sets 4 bits that can be used for branching,
// rather than just true or false.
// As a consequence, a single comparison gives enough information to distinguish the three cases
//
// Post-increment pointers after load/store:
// Instructions like `MOVHU.P 2(R0), R6`
// increment the register by a specified amount, in this example 2.
// Because uint16's are exactly 2 bytes and the length of the slices
// is part of the slice header,
// there is no need to separately track the index into the slice.
// Instead, the code can calculate the final read value and compare against that,
// using the post-increment reads to move the pointers along.
//
// TODO: CALL out to memmove once the list is exhausted.
// Right now it moves the necessary shorts so that the remaining count
// is a multiple of 4 and then copies 64 bits at a time.
TEXT ·union2by2(SB), NOSPLIT, $0-80
// R0, R1, and R2 for the pointers to the three slices
MOVD set1+0(FP), R0
MOVD set2+24(FP), R1
MOVD buffer+48(FP), R2
//R3 and R4 will be the values at which we will have finished reading set1 and set2.
// R3 should be R0 + 2 * set1_len+8(FP)
MOVD set1_len+8(FP), R3
MOVD set2_len+32(FP), R4
ADD R3<<1, R0, R3
ADD R4<<1, R1, R4
//Rather than counting the number of elements added separately
//Save the starting register of buffer.
MOVD buffer+48(FP), R5
// set1 is empty, just flush set2
CMP R0, R3
BEQ flush_right
// set2 is empty, just flush set1
CMP R1, R4
BEQ flush_left
// R6, R7 are the working space for s1 and s2
MOVD ZR, R6
MOVD ZR, R7
MOVHU.P 2(R0), R6
MOVHU.P 2(R1), R7
loop:
CMP R6, R7
BEQ pop_both // R6 == R7
BLS pop_right // R6 > R7
//pop_left: // R6 < R7
MOVHU.P R6, 2(R2)
CMP R0, R3
BEQ pop_then_flush_right
MOVHU.P 2(R0), R6
JMP loop
pop_both:
MOVHU.P R6, 2(R2) //could also use R7, since they are equal
CMP R0, R3
BEQ flush_right
CMP R1, R4
BEQ flush_left
MOVHU.P 2(R0), R6
MOVHU.P 2(R1), R7
JMP loop
pop_right:
MOVHU.P R7, 2(R2)
CMP R1, R4
BEQ pop_then_flush_left
MOVHU.P 2(R1), R7
JMP loop
pop_then_flush_right:
MOVHU.P R7, 2(R2)
flush_right:
MOVD R1, R0
MOVD R4, R3
JMP flush_left
pop_then_flush_left:
MOVHU.P R6, 2(R2)
flush_left:
CMP R0, R3
BEQ return
//figure out how many bytes to slough off. Must be a multiple of two
SUB R0, R3, R4
ANDS $6, R4
BEQ long_flush //handles the 0 mod 8 case
SUBS $4, R4, R4 // since possible values are 2, 4, 6, this splits evenly
BLT pop_single // exactly the 2 case
MOVW.P 4(R0), R6
MOVW.P R6, 4(R2)
BEQ long_flush // we're now aligned by 64 bits, as R4==4, otherwise 2 more
pop_single:
MOVHU.P 2(R0), R6
MOVHU.P R6, 2(R2)
long_flush:
// at this point we know R3 - R0 is a multiple of 8.
CMP R0, R3
BEQ return
MOVD.P 8(R0), R6
MOVD.P R6, 8(R2)
JMP long_flush
return:
// number of shorts written is (R5 - R2) >> 1
SUB R5, R2
LSR $1, R2, R2
MOVD R2, size+72(FP)
RET