From: Matthew Dempsky <mdempsky@google.com>
Date: Fri, 30 Oct 2015 02:16:20 +0000 (-0700)
Subject: math: fix bad shift in Expm1
X-Git-Tag: go1.6beta1~664
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=7832c82bf5ebd32e24e3f324f5284194e505f3e4;p=gostls13.git

math: fix bad shift in Expm1

Noticed by cmd/vet.

Expected values array produced by Python instead of Keisan because:

1) Keisan's website calculator is painfully difficult to copy/paste
values into and out of, and

2) after tediously computing e^(vf[i] * 10) - 1 via Keisan I
discovered that Keisan computing vf[i]*10 in a higher precision was
giving substantially different output values.

Also, testing uses "close" instead of "veryclose" because 386's
assembly implementation produces values for some of the test cases
that fail "veryclose".  Curiously, Expm1(vf[i]*10) is identical to
Exp(vf[i]*10)-1 on 386, whereas with the portable implementation
they're only "veryclose".

Investigating these questions is left to someone else.  I just wanted
to fix the cmd/vet warning.

Fixes #13101.

Change-Id: Ica8f6c267d01aa4cc31f53593e95812746942fbc
Reviewed-on: https://go-review.googlesource.com/16505
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Klaus Post <klauspost@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
---

diff --git a/src/math/all_test.go b/src/math/all_test.go
index 4838ffc5e1..968a7b1837 100644
--- a/src/math/all_test.go
+++ b/src/math/all_test.go
@@ -234,6 +234,18 @@ var expm1 = []float64{
 	1.842068661871398836913874273e-02,
 	-8.3193870863553801814961137573e-02,
 }
+var expm1Large = []float64{
+	4.2031418113550844e+21,
+	4.0690789717473863e+33,
+	-0.9372627915981363e+00,
+	-1.0,
+	7.077694784145933e+41,
+	5.117936223839153e+12,
+	5.124137759001189e+22,
+	7.03546003972584e+11,
+	8.456921800389698e+07,
+	-1.0,
+}
 var exp2 = []float64{
 	3.1537839463286288034313104e+01,
 	2.1361549283756232296144849e+02,
@@ -1987,6 +1999,12 @@ func TestExpm1(t *testing.T) {
 			t.Errorf("Expm1(%g) = %g, want %g", a, f, expm1[i])
 		}
 	}
+	for i := 0; i < len(vf); i++ {
+		a := vf[i] * 10
+		if f := Expm1(a); !close(expm1Large[i], f) {
+			t.Errorf("Expm1(%g) = %g, want %g", a, f, expm1Large[i])
+		}
+	}
 	for i := 0; i < len(vfexpm1SC); i++ {
 		if f := Expm1(vfexpm1SC[i]); !alike(expm1SC[i], f) {
 			t.Errorf("Expm1(%g) = %g, want %g", vfexpm1SC[i], f, expm1SC[i])
diff --git a/src/math/expm1.go b/src/math/expm1.go
index 064e131161..670b96de5e 100644
--- a/src/math/expm1.go
+++ b/src/math/expm1.go
@@ -227,7 +227,7 @@ func expm1(x float64) float64 {
 			y = Float64frombits(Float64bits(y) + uint64(k)<<52) // add k to y's exponent
 			return y
 		}
-		t := Float64frombits(uint64((0x3ff - k) << 52)) // 2**-k
+		t := Float64frombits(uint64(0x3ff-k) << 52) // 2**-k
 		y := x - (e + t)
 		y += 1
 		y = Float64frombits(Float64bits(y) + uint64(k)<<52) // add k to y's exponent