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zstd: Less branching in asm version of adjustOffset #897

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zstd: Less branching in asm version of adjustOffset #897

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74 changes: 17 additions & 57 deletions zstd/_generate/gen.go
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Original file line number Diff line number Diff line change
Expand Up @@ -747,18 +747,10 @@ func (o options) adjustOffset(name string, moP, llP Mem, offsetB reg.GPVirtual,
// offset++
// }
{
if true {
CMPQ(llP, U32(0))
JNE(LabelRef(name + "_offset_maybezero"))
INCQ(offset)
JMP(LabelRef(name + "_offset_nonzero"))
} else {
// No idea why this doesn't work:
tmp := GP64()
LEAQ(Mem{Base: offset, Disp: 1}, tmp)
CMPQ(llP, U32(0))
CMOVQEQ(tmp, offset)
}
tmp := GP64()
LEAQ(Mem{Base: offset, Disp: 1}, tmp)
CMPQ(llP, U32(0))
CMOVQEQ(tmp, offset)

// if offset == 0 {
// return s.prevOffset[0]
Expand All @@ -771,56 +763,25 @@ func (o options) adjustOffset(name string, moP, llP Mem, offsetB reg.GPVirtual,
JMP(end)
}
}

Label(name + "_offset_nonzero")
{
// if offset == 3 {
// temp = s.prevOffset[0] - 1
// } else {
// temp = s.prevOffset[offset]
// }
temp := GP64()
CMPQ(offset, U8(1))
JB(LabelRef(name + "_zero"))
JEQ(LabelRef(name + "_one"))
CMPQ(offset, U8(2))
JA(LabelRef(name + "_three"))
JMP(LabelRef(name + "_two"))

Label(name + "_zero")
MOVQ(offsets[0], temp)
JMP(LabelRef(name + "_test_temp_valid"))

Label(name + "_one")
MOVQ(offsets[1], temp)
JMP(LabelRef(name + "_test_temp_valid"))

Label(name + "_two")
MOVQ(offsets[2], temp)
JMP(LabelRef(name + "_test_temp_valid"))

Label(name + "_three")
// temp = s.prevOffset[0] - 1, for when offset == 3.
LEAQ(Mem{Base: offsets[0], Disp: -1}, temp)

Label(name + "_test_temp_valid")
// if temp == 0 {
// temp = 1
// }
CMPQ(offset, U8(2))
CMOVQEQ(offsets[2], temp) // If offset == 2.
CMOVQCS(offsets[1], temp) // If offset == 1.
CMOVQCC(offsets[1], offsets[2]) // If offset >= 2.

// if temp == 0 { temp = 1 } to handle invalid inputs.
// This is a perfectly predictable branch, no need for CMOV.
TESTQ(temp, temp)
JNZ(LabelRef(name + "_temp_valid"))
MOVQ(U32(1), temp)

INCL(temp.As32())
Label(name + "_temp_valid")
// if offset != 1 {
// s.prevOffset[2] = s.prevOffset[1]
// }
CMPQ(offset, U8(1))
if false {
JZ(LabelRef(name + "_skip"))
MOVQ(offsets[1], offsets[2]) // s.prevOffset[2] = s.prevOffset[1]
Label(name + "_skip")
} else {
CMOVQNE(offsets[1], offsets[2])
}

// s.prevOffset[1] = s.prevOffset[0]
// s.prevOffset[0] = temp
MOVQ(offsets[0], offsets[1])
Expand Down Expand Up @@ -914,13 +875,12 @@ func (o options) adjustOffsetInMemory(name string, moP, llP Mem, offsetB reg.GPV
// }
// temp := s.prevOffset[ofs] + shift
// TODO: This should be easier...
CX, DX, R15 := GP64(), GP64(), GP64()
CX, DX := GP64(), GP64()
MOVQ(offset, CX)
XORQ(DX, DX)
MOVQ(I32(-1), R15)
CMPQ(offset, U8(3))
CMOVQEQ(DX, CX)
CMOVQEQ(R15, DX)
ADCQ(I32(-1), DX) // DX += (offset < 0) - 1
assert(func(ok LabelRef) {
CMPQ(CX, U32(0))
JAE(ok)
Expand Down
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