math: improve math.Log to handle subnormal floating number on amd64 #56601

Original file line number	Diff line number	Diff line change
Expand Up		@@ -1511,6 +1511,27 @@ var logSC = []float64{
		NaN(),
		}

	var vflogBC = []float64{
	SmallestNonzeroFloat64,
	LargestSubnormalFloat64,
	SmallestNormalFloat64,
	MaxFloat64,
	-SmallestNonzeroFloat64,
	-LargestSubnormalFloat64,
	-SmallestNormalFloat64,
	-MaxFloat64,
	}
	var logBC = []float64{
	-744.4400719213812,
	-708.3964185322641,
	-708.3964185322641,
	709.782712893384,
	NaN(),
	NaN(),
	NaN(),
	NaN(),
	}

		var vflogbSC = []float64{
		Inf(-1),
		0,
Expand Down Expand Up		@@ -2717,6 +2738,12 @@ func TestLog(t *testing.T) {
		t.Errorf("Log(%g) = %g, want %g", vflogSC[i], f, logSC[i])
		}
		}

	for i := 0; i < len(vflogBC); i++ {
	if f := Log(vflogBC[i]); !alike(logBC[i], f) {
	t.Errorf("Log(%g) = %g, want %g", vflogBC[i], f, logBC[i])
	}
	}
		}

		func TestLogb(t *testing.T) {
Expand Down

11 changes: 11 additions & 0 deletions src/math/log_amd64.s

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -14,6 +14,7 @@
		#define L5 1.818357216161805012e-01 // 0x3FC7466496CB03DE
		#define L6 1.531383769920937332e-01 // 0x3FC39A09D078C69F
		#define L7 1.479819860511658591e-01 // 0x3FC2F112DF3E5244
	#define Two52 4.503599627370496e15 // 0x4330000000000000 (2**52)
		#define NaN 0x7FF8000000000001
		#define NegInf 0xFFF0000000000000
		#define PosInf 0x7FF0000000000000
Expand All		@@ -32,7 +33,16 @@ TEXT ·archLog(SB),NOSPLIT,0ドル
		CMPQ AX, BX
		JLE isInfOrNaN
		// f1, ki := math.Frexp(x); k := float64(ki)
	MOVQ 0,ドル CX
		MOVQ BX, X0
	ANDQ BX, AX
	JNE isNormal
	// f, exp = normalize(f)
	MOVSD $Two52, X1
	MULSD X1, X0
	MOVQ X0, BX
	MOVQ $-52, CX
	isNormal:
		MOVQ 0ドルx000FFFFFFFFFFFFF, AX
		MOVQ AX, X2
		ANDPD X0, X2
Expand All		@@ -41,6 +51,7 @@ TEXT ·archLog(SB),NOSPLIT,0ドル
		SHRQ 52,ドル BX
		ANDL 0ドルx7FF, BX
		SUBL 0ドルx3FE, BX
	ADDQ CX, BX
		XORPS X1, X1 // break dependency for CVTSL2SD
		CVTSL2SD BX, X1 // x1= k, x2= f1
		// if f1 < math.Sqrt2/2 { k -= 1; f1 *= 2 }
Expand Down

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