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

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>

diff --git a/src/math/erfc_s390x.s b/src/math/erfc_s390x.s
index 1a867ad9f4aef1b17c4b6864627a98cfc747cd7e..57710b254b38ef49ee709b67604bd7b109c2868a 100644 (file)
--- a/src/math/erfc_s390x.s
+++ b/src/math/erfc_s390x.s
@@ -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)