math: handle denormals in AMD64 Exp

Fixes #23164

Change-Id: I6e8c6443f3ef91df71e117cce1cfa1faba647dd7
Reviewed-on: https://go-review.googlesource.com/87337
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/math/all_test.go b/src/math/all_test.go
index 39b5b33071558b1f4416981c89f4f38a3d7c4c0c..a145c5e60b6aabdb9607f1ee5ca0650f9cce6450 100644 (file)
--- a/src/math/all_test.go
+++ b/src/math/all_test.go
@@ -1014,6 +1014,8 @@ var vfexpSC = []float64{
        1,
        // near zero
        3.725290298461915e-09,
+       // denormal
+       -740,
 }
 var expSC = []float64{
        0,
@@ -1026,6 +1028,7 @@ var expSC = []float64{
        Inf(1),
        2.718281828459045,
        1.0000000037252903,
+       4.2e-322,
 }
 
 var vfexp2SC = []float64{

diff --git a/src/math/exp_amd64.s b/src/math/exp_amd64.s
index 2f7fd76fe4bbbf59224ae0b4db513784a8e46164..1c2ac8f676c1dfdb4fc0bda4c708896a8ba86aaf 100644 (file)
--- a/src/math/exp_amd64.s
+++ b/src/math/exp_amd64.s
@@ -96,14 +96,13 @@ TEXT ·Exp(SB),NOSPLIT,$0
        MULSD   X1, X0
        ADDSD exprodata<>+8(SB), X0
        // return fr * 2**exponent
-lastStep:
-       MOVL    $0x3FF, AX // bias
-       ADDL    AX, BX
-       JLE     underflow
+ldexp:
+       ADDL    $0x3FF, BX // add bias
+       JLE     denormal
        CMPL    BX, $0x7FF
        JGE     overflow
-       MOVL    $52, CX
-       SHLQ    CX, BX
+lastStep:
+       SHLQ    $52, BX
        MOVQ    BX, X1
        MULSD   X1, X0
        MOVSD   X0, ret+8(FP)
@@ -115,14 +114,22 @@ notFinite:
        JNE     notNegInf
        // -Inf, return 0
 underflow: // return 0
-       MOVQ    $0, AX
-       MOVQ    AX, ret+8(FP)
+       MOVQ    $0, ret+8(FP)
        RET
 overflow: // return +Inf
        MOVQ    $PosInf, BX
 notNegInf: // NaN or +Inf, return x
        MOVQ    BX, ret+8(FP)
        RET
+denormal:
+       CMPL    BX, $-52
+       JL      underflow
+       ADDL    $0x3FE, BX // add bias - 1
+       SHLQ    $52, BX
+       MOVQ    BX, X1
+       MULSD   X1, X0
+       MOVQ    $1, BX
+       JMP     lastStep
 
 avxfma:
        MOVSD   $LN2U, X2
@@ -149,4 +156,4 @@ avxfma:
        MULSD   X1, X0
        VADDSD exprodata<>+16(SB), X0, X1
        VFMADD213SD   exprodata<>+8(SB), X1, X0
-       JMP lastStep
+       JMP ldexp