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494 lines
12 KiB
494 lines
12 KiB
// Copyright 2020 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !appengine
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// +build gc
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// +build !noasm
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#include "textflag.h"
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// The asm code generally follows the pure Go code in decode_other.go, except
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// where marked with a "!!!".
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// func decode(dst, src []byte) int
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//
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// All local variables fit into registers. The non-zero stack size is only to
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// spill registers and push args when issuing a CALL. The register allocation:
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// - R2 scratch
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// - R3 scratch
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// - R4 length or x
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// - R5 offset
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// - R6 &src[s]
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// - R7 &dst[d]
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// + R8 dst_base
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// + R9 dst_len
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// + R10 dst_base + dst_len
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// + R11 src_base
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// + R12 src_len
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// + R13 src_base + src_len
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// - R14 used by doCopy
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// - R15 used by doCopy
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//
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// The registers R8-R13 (marked with a "+") are set at the start of the
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// function, and after a CALL returns, and are not otherwise modified.
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//
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// The d variable is implicitly R7 - R8, and len(dst)-d is R10 - R7.
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// The s variable is implicitly R6 - R11, and len(src)-s is R13 - R6.
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TEXT ·decode(SB), NOSPLIT, $56-56
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// Initialize R6, R7 and R8-R13.
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MOVD dst_base+0(FP), R8
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MOVD dst_len+8(FP), R9
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MOVD R8, R7
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MOVD R8, R10
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ADD R9, R10, R10
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MOVD src_base+24(FP), R11
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MOVD src_len+32(FP), R12
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MOVD R11, R6
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MOVD R11, R13
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ADD R12, R13, R13
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loop:
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// for s < len(src)
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CMP R13, R6
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BEQ end
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// R4 = uint32(src[s])
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//
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// switch src[s] & 0x03
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MOVBU (R6), R4
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MOVW R4, R3
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ANDW $3, R3
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MOVW $1, R1
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CMPW R1, R3
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BGE tagCopy
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// ----------------------------------------
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// The code below handles literal tags.
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// case tagLiteral:
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// x := uint32(src[s] >> 2)
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// switch
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MOVW $60, R1
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LSRW $2, R4, R4
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CMPW R4, R1
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BLS tagLit60Plus
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// case x < 60:
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// s++
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ADD $1, R6, R6
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doLit:
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// This is the end of the inner "switch", when we have a literal tag.
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//
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// We assume that R4 == x and x fits in a uint32, where x is the variable
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// used in the pure Go decode_other.go code.
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// length = int(x) + 1
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//
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// Unlike the pure Go code, we don't need to check if length <= 0 because
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// R4 can hold 64 bits, so the increment cannot overflow.
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ADD $1, R4, R4
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// Prepare to check if copying length bytes will run past the end of dst or
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// src.
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//
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// R2 = len(dst) - d
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// R3 = len(src) - s
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MOVD R10, R2
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SUB R7, R2, R2
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MOVD R13, R3
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SUB R6, R3, R3
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// !!! Try a faster technique for short (16 or fewer bytes) copies.
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//
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// if length > 16 || len(dst)-d < 16 || len(src)-s < 16 {
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// goto callMemmove // Fall back on calling runtime·memmove.
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// }
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//
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// The C++ snappy code calls this TryFastAppend. It also checks len(src)-s
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// against 21 instead of 16, because it cannot assume that all of its input
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// is contiguous in memory and so it needs to leave enough source bytes to
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// read the next tag without refilling buffers, but Go's Decode assumes
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// contiguousness (the src argument is a []byte).
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CMP $16, R4
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BGT callMemmove
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CMP $16, R2
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BLT callMemmove
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CMP $16, R3
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BLT callMemmove
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// !!! Implement the copy from src to dst as a 16-byte load and store.
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// (Decode's documentation says that dst and src must not overlap.)
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//
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// This always copies 16 bytes, instead of only length bytes, but that's
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// OK. If the input is a valid Snappy encoding then subsequent iterations
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// will fix up the overrun. Otherwise, Decode returns a nil []byte (and a
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// non-nil error), so the overrun will be ignored.
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//
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// Note that on arm64, it is legal and cheap to issue unaligned 8-byte or
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// 16-byte loads and stores. This technique probably wouldn't be as
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// effective on architectures that are fussier about alignment.
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LDP 0(R6), (R14, R15)
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STP (R14, R15), 0(R7)
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// d += length
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// s += length
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ADD R4, R7, R7
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ADD R4, R6, R6
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B loop
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callMemmove:
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// if length > len(dst)-d || length > len(src)-s { etc }
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CMP R2, R4
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BGT errCorrupt
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CMP R3, R4
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BGT errCorrupt
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// copy(dst[d:], src[s:s+length])
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//
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// This means calling runtime·memmove(&dst[d], &src[s], length), so we push
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// R7, R6 and R4 as arguments. Coincidentally, we also need to spill those
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// three registers to the stack, to save local variables across the CALL.
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MOVD R7, 8(RSP)
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MOVD R6, 16(RSP)
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MOVD R4, 24(RSP)
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MOVD R7, 32(RSP)
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MOVD R6, 40(RSP)
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MOVD R4, 48(RSP)
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CALL runtime·memmove(SB)
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// Restore local variables: unspill registers from the stack and
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// re-calculate R8-R13.
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MOVD 32(RSP), R7
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MOVD 40(RSP), R6
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MOVD 48(RSP), R4
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MOVD dst_base+0(FP), R8
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MOVD dst_len+8(FP), R9
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MOVD R8, R10
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ADD R9, R10, R10
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MOVD src_base+24(FP), R11
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MOVD src_len+32(FP), R12
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MOVD R11, R13
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ADD R12, R13, R13
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// d += length
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// s += length
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ADD R4, R7, R7
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ADD R4, R6, R6
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B loop
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tagLit60Plus:
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// !!! This fragment does the
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//
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// s += x - 58; if uint(s) > uint(len(src)) { etc }
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//
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// checks. In the asm version, we code it once instead of once per switch case.
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ADD R4, R6, R6
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SUB $58, R6, R6
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MOVD R6, R3
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SUB R11, R3, R3
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CMP R12, R3
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BGT errCorrupt
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// case x == 60:
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MOVW $61, R1
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CMPW R1, R4
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BEQ tagLit61
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BGT tagLit62Plus
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// x = uint32(src[s-1])
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MOVBU -1(R6), R4
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B doLit
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tagLit61:
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// case x == 61:
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// x = uint32(src[s-2]) | uint32(src[s-1])<<8
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MOVHU -2(R6), R4
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B doLit
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tagLit62Plus:
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CMPW $62, R4
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BHI tagLit63
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// case x == 62:
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// x = uint32(src[s-3]) | uint32(src[s-2])<<8 | uint32(src[s-1])<<16
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MOVHU -3(R6), R4
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MOVBU -1(R6), R3
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ORR R3<<16, R4
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B doLit
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tagLit63:
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// case x == 63:
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// x = uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24
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MOVWU -4(R6), R4
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B doLit
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// The code above handles literal tags.
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// ----------------------------------------
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// The code below handles copy tags.
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tagCopy4:
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// case tagCopy4:
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// s += 5
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ADD $5, R6, R6
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// if uint(s) > uint(len(src)) { etc }
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MOVD R6, R3
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SUB R11, R3, R3
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CMP R12, R3
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BGT errCorrupt
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// length = 1 + int(src[s-5])>>2
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MOVD $1, R1
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ADD R4>>2, R1, R4
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// offset = int(uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24)
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MOVWU -4(R6), R5
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B doCopy
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tagCopy2:
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// case tagCopy2:
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// s += 3
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ADD $3, R6, R6
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// if uint(s) > uint(len(src)) { etc }
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MOVD R6, R3
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SUB R11, R3, R3
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CMP R12, R3
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BGT errCorrupt
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// length = 1 + int(src[s-3])>>2
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MOVD $1, R1
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ADD R4>>2, R1, R4
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// offset = int(uint32(src[s-2]) | uint32(src[s-1])<<8)
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MOVHU -2(R6), R5
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B doCopy
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tagCopy:
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// We have a copy tag. We assume that:
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// - R3 == src[s] & 0x03
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// - R4 == src[s]
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CMP $2, R3
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BEQ tagCopy2
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BGT tagCopy4
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// case tagCopy1:
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// s += 2
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ADD $2, R6, R6
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// if uint(s) > uint(len(src)) { etc }
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MOVD R6, R3
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SUB R11, R3, R3
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CMP R12, R3
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BGT errCorrupt
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// offset = int(uint32(src[s-2])&0xe0<<3 | uint32(src[s-1]))
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MOVD R4, R5
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AND $0xe0, R5
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MOVBU -1(R6), R3
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ORR R5<<3, R3, R5
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// length = 4 + int(src[s-2])>>2&0x7
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MOVD $7, R1
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AND R4>>2, R1, R4
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ADD $4, R4, R4
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doCopy:
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// This is the end of the outer "switch", when we have a copy tag.
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//
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// We assume that:
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// - R4 == length && R4 > 0
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// - R5 == offset
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// if offset <= 0 { etc }
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MOVD $0, R1
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CMP R1, R5
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BLE errCorrupt
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// if d < offset { etc }
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MOVD R7, R3
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SUB R8, R3, R3
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CMP R5, R3
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BLT errCorrupt
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// if length > len(dst)-d { etc }
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MOVD R10, R3
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SUB R7, R3, R3
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CMP R3, R4
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BGT errCorrupt
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// forwardCopy(dst[d:d+length], dst[d-offset:]); d += length
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//
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// Set:
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// - R14 = len(dst)-d
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// - R15 = &dst[d-offset]
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MOVD R10, R14
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SUB R7, R14, R14
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MOVD R7, R15
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SUB R5, R15, R15
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// !!! Try a faster technique for short (16 or fewer bytes) forward copies.
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//
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// First, try using two 8-byte load/stores, similar to the doLit technique
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// above. Even if dst[d:d+length] and dst[d-offset:] can overlap, this is
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// still OK if offset >= 8. Note that this has to be two 8-byte load/stores
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// and not one 16-byte load/store, and the first store has to be before the
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// second load, due to the overlap if offset is in the range [8, 16).
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//
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// if length > 16 || offset < 8 || len(dst)-d < 16 {
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// goto slowForwardCopy
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// }
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// copy 16 bytes
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// d += length
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CMP $16, R4
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BGT slowForwardCopy
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CMP $8, R5
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BLT slowForwardCopy
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CMP $16, R14
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BLT slowForwardCopy
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MOVD 0(R15), R2
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MOVD R2, 0(R7)
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MOVD 8(R15), R3
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MOVD R3, 8(R7)
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ADD R4, R7, R7
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B loop
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slowForwardCopy:
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// !!! If the forward copy is longer than 16 bytes, or if offset < 8, we
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// can still try 8-byte load stores, provided we can overrun up to 10 extra
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// bytes. As above, the overrun will be fixed up by subsequent iterations
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// of the outermost loop.
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//
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// The C++ snappy code calls this technique IncrementalCopyFastPath. Its
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// commentary says:
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//
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// ----
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//
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// The main part of this loop is a simple copy of eight bytes at a time
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// until we've copied (at least) the requested amount of bytes. However,
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// if d and d-offset are less than eight bytes apart (indicating a
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// repeating pattern of length < 8), we first need to expand the pattern in
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// order to get the correct results. For instance, if the buffer looks like
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// this, with the eight-byte <d-offset> and <d> patterns marked as
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// intervals:
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//
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// abxxxxxxxxxxxx
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// [------] d-offset
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// [------] d
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//
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// a single eight-byte copy from <d-offset> to <d> will repeat the pattern
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// once, after which we can move <d> two bytes without moving <d-offset>:
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//
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// ababxxxxxxxxxx
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// [------] d-offset
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// [------] d
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//
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// and repeat the exercise until the two no longer overlap.
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//
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// This allows us to do very well in the special case of one single byte
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// repeated many times, without taking a big hit for more general cases.
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//
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// The worst case of extra writing past the end of the match occurs when
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// offset == 1 and length == 1; the last copy will read from byte positions
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// [0..7] and write to [4..11], whereas it was only supposed to write to
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// position 1. Thus, ten excess bytes.
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//
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// ----
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//
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// That "10 byte overrun" worst case is confirmed by Go's
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// TestSlowForwardCopyOverrun, which also tests the fixUpSlowForwardCopy
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// and finishSlowForwardCopy algorithm.
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//
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// if length > len(dst)-d-10 {
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// goto verySlowForwardCopy
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// }
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SUB $10, R14, R14
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CMP R14, R4
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BGT verySlowForwardCopy
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makeOffsetAtLeast8:
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// !!! As above, expand the pattern so that offset >= 8 and we can use
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// 8-byte load/stores.
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//
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// for offset < 8 {
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// copy 8 bytes from dst[d-offset:] to dst[d:]
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// length -= offset
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// d += offset
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// offset += offset
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// // The two previous lines together means that d-offset, and therefore
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// // R15, is unchanged.
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// }
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CMP $8, R5
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BGE fixUpSlowForwardCopy
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MOVD (R15), R3
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MOVD R3, (R7)
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SUB R5, R4, R4
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ADD R5, R7, R7
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ADD R5, R5, R5
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B makeOffsetAtLeast8
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fixUpSlowForwardCopy:
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// !!! Add length (which might be negative now) to d (implied by R7 being
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// &dst[d]) so that d ends up at the right place when we jump back to the
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// top of the loop. Before we do that, though, we save R7 to R2 so that, if
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// length is positive, copying the remaining length bytes will write to the
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// right place.
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MOVD R7, R2
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ADD R4, R7, R7
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finishSlowForwardCopy:
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// !!! Repeat 8-byte load/stores until length <= 0. Ending with a negative
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// length means that we overrun, but as above, that will be fixed up by
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// subsequent iterations of the outermost loop.
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MOVD $0, R1
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CMP R1, R4
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BLE loop
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MOVD (R15), R3
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MOVD R3, (R2)
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ADD $8, R15, R15
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ADD $8, R2, R2
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SUB $8, R4, R4
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B finishSlowForwardCopy
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verySlowForwardCopy:
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// verySlowForwardCopy is a simple implementation of forward copy. In C
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// parlance, this is a do/while loop instead of a while loop, since we know
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// that length > 0. In Go syntax:
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//
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// for {
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// dst[d] = dst[d - offset]
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// d++
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// length--
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// if length == 0 {
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// break
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// }
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// }
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MOVB (R15), R3
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MOVB R3, (R7)
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ADD $1, R15, R15
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ADD $1, R7, R7
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SUB $1, R4, R4
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CBNZ R4, verySlowForwardCopy
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B loop
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// The code above handles copy tags.
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// ----------------------------------------
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end:
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// This is the end of the "for s < len(src)".
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//
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// if d != len(dst) { etc }
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CMP R10, R7
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BNE errCorrupt
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// return 0
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MOVD $0, ret+48(FP)
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RET
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errCorrupt:
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// return decodeErrCodeCorrupt
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MOVD $1, R2
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MOVD R2, ret+48(FP)
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RET
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