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zstd: Branchless getBits for amd64 w/o BMI2 #640

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merged 1 commit into from Jul 12, 2022
Merged

zstd: Branchless getBits for amd64 w/o BMI2 #640

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59 changes: 15 additions & 44 deletions zstd/_generate/gen.go
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Expand Up @@ -298,7 +298,7 @@ func (o options) generateBody(name string, executeSingleTriple func(ctx *execute
MOVBQZX(total.As8(), total) // total = llState.As8() + mlState.As8() + ofState.As8()

// Read `total` bits
bits := o.getBitsValue(name+"_getBits", total, brValue, brBitsRead)
bits := o.getBits(total, brValue, brBitsRead)

// Update states
Comment("Update Offset State")
Expand Down Expand Up @@ -632,14 +632,13 @@ func (o options) updateState(name string, state, brValue, brBitsRead reg.GPVirtu
}

{
lowBits := o.getBits(name+"_getBits", AX, brValue, brBitsRead, LabelRef(name+"_skip_zero"))
lowBits := o.getBits(AX, brValue, brBitsRead)
// Check if below tablelog
assert(func(ok LabelRef) {
CMPQ(lowBits, U32(512))
JB(ok)
})
ADDQ(lowBits, DX)
Label(name + "_skip_zero")
}

// Load table pointer
Expand Down Expand Up @@ -695,53 +694,25 @@ func (o options) nextState(name string, state, lowBits reg.GPVirtual, table stri
}

// getBits will return nbits bits from brValue.
// If nbits == 0 it *may* jump to jmpZero, otherwise 0 is returned.
func (o options) getBits(name string, nBits, brValue, brBitsRead reg.GPVirtual, jmpZero LabelRef) reg.GPVirtual {
func (o options) getBits(nBits, brValue, brBitsRead reg.GPVirtual) reg.GPVirtual {
BX := GP64()
CX := reg.CL
if o.bmi2 {
LEAQ(Mem{Base: brBitsRead, Index: nBits, Scale: 1}, CX.As64())
MOVQ(brValue, BX)
MOVQ(CX.As64(), brBitsRead)
ROLQ(CX, BX)
BZHIQ(nBits, BX, BX)
} else {
CMPQ(nBits, U8(0))
JZ(jmpZero)
MOVQ(brBitsRead, CX.As64())
ADDQ(nBits, brBitsRead)
MOVQ(brValue, BX)
SHLQ(CX, BX)
MOVQ(nBits, CX.As64())
NEGQ(CX.As64())
SHRQ(CX, BX)
}
return BX
}

// getBits will return nbits bits from brValue.
// If nbits == 0 then 0 is returned.
func (o options) getBitsValue(name string, nBits, brValue, brBitsRead reg.GPVirtual) reg.GPVirtual {
BX := GP64()
CX := reg.CL
LEAQ(Mem{Base: brBitsRead, Index: nBits, Scale: 1}, CX.As64())
MOVQ(brValue, BX)
MOVQ(CX.As64(), brBitsRead)
ROLQ(CX, BX)

// BX &= (1<<nBits) - 1
if o.bmi2 {
LEAQ(Mem{Base: brBitsRead, Index: nBits, Scale: 1}, CX.As64())
MOVQ(brValue, BX)
MOVQ(CX.As64(), brBitsRead)
ROLQ(CX, BX)
BZHIQ(nBits, BX, BX)
} else {
XORQ(BX, BX)
CMPQ(nBits, U8(0))
JZ(LabelRef(name + "_get_bits_value_zero"))
MOVQ(brBitsRead, CX.As64())
ADDQ(nBits, brBitsRead)
MOVQ(brValue, BX)
SHLQ(CX, BX)
MOVQ(nBits, CX.As64())
NEGQ(CX.As64())
SHRQ(CX, BX)
Label(name + "_get_bits_value_zero")
mask := GP32()
MOVL(U32(1), mask)
MOVB(nBits.As8(), CX)
SHLL(CX, mask)
DECL(mask)
ANDQ(mask.As64(), BX)
}
return BX
}
Expand Down