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math: ensure Erfc is not called with out-of-expected-range arguments on s390x

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The existing implementation produces correct results with a wide range of inputs,
but invalid results asymptotically. With this change we ensure correct asymptotic results
on s390x

Fixes #26477

Change-Id: I760c1f8177f7cab2d7622ab9a926dfb1f8113b49
Reviewed-on: https://go-review.googlesource.com/127119
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
bill_ofarrell 2018-07-31 19:48:52 -04:00 committed by Ian Lance Taylor
parent db810b6e39
commit f04a002e5a
1 changed files with 13 additions and 12 deletions
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25
src/math/erfc_s390x.s
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@ -4,7 +4,8 @@
#include "textflag.h"
#define NegInf 0xFFF0000000000000
#define Neg2p11 0xC000E147AE147AE1
#define Pos15 0x402E
// Minimax polynomial coefficients and other constants
DATA ·erfcrodataL38<> + 0(SB)/8, $.234875460637085087E-01
@ -136,21 +137,24 @@ GLOBL ·erfctab2069<> + 0(SB), RODATA, $128
// Erfc(NaN) = NaN
// The algorithm used is minimax polynomial approximation
// with coefficients determined with a Remez exchange algorithm.
// This assembly implementation handles inputs in the range [-2.11, +15].
// For all other inputs we call the generic Go implementation.
TEXT ·erfcAsm(SB), NOSPLIT, $0-16
//special case Erfc(+Inf) = 0
TEXT ·erfcAsm(SB), NOSPLIT|NOFRAME, $0-16
MOVD x+0(FP), R1
MOVD $NegInf, R2
CMPUBEQ R1, R2, erfcIsPosInf
MOVD $Neg2p11, R2
CMPUBGT R1, R2, usego
FMOVD x+0(FP), F0
MOVD $·erfcrodataL38<>+0(SB), R9
WORD $0xB3CD0010 //lgdr %r1, %f0
FMOVD F0, F2
SRAD $48, R1
MOVH $0x3FFF, R3
MOVH R1, R2
ANDW $0x7FFF, R1
MOVH $Pos15, R3
CMPW R1, R3
BGT usego
MOVH $0x3FFF, R3
MOVW R1, R6
MOVW R3, R7
CMPBGT R6, R7, L2
@ -523,8 +527,5 @@ L18:
L25:
FMOVD 568(R9), F2
BR L1
erfcIsPosInf:
FMOVD $(2.0), F1
FMOVD F1, ret+8(FP)
RET
usego:
BR ·erfc(SB)