math: implement arch{Floor, Ceil, Trunc} in hardware on loong64

benchmark:

goos: linux
goarch: loong64
pkg: math
cpu: Loongson-3A6000 @ 2500.00MHz
        │  bench.old   │              bench.new              │
        │    sec/op    │   sec/op     vs base                │
Ceil      10.810n ± 0%   2.578n ± 0%  -76.15% (p=0.000 n=20)
Floor     10.810n ± 0%   2.531n ± 0%  -76.59% (p=0.000 n=20)
Trunc      9.606n ± 0%   2.530n ± 0%  -73.67% (p=0.000 n=20)
geomean    10.39n        2.546n       -75.50%

goos: linux
goarch: loong64
pkg: math
cpu: Loongson-3A5000 @ 2500.00MHz
        │  bench.old   │              bench.new              │
        │    sec/op    │   sec/op     vs base                │
Ceil      13.220n ± 0%   7.703n ± 8%  -41.73% (p=0.000 n=20)
Floor     12.410n ± 0%   7.248n ± 2%  -41.59% (p=0.000 n=20)
Trunc     11.210n ± 0%   7.757n ± 4%  -30.80% (p=0.000 n=20)
geomean    12.25n        7.566n       -38.25%

Change-Id: I3af51e9852e9cf5f965fed895d68945a2e8675f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/612615
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/math/floor_asm.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build 386 || amd64 || arm64 || ppc64 || ppc64le || riscv64 || s390x || wasm
+//go:build 386 || amd64 || arm64 || loong64 || ppc64 || ppc64le || riscv64 || s390x || wasm
 
 package math
 

src/math/floor_loong64.s

@@ -0,0 +1,41 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// derived from math/floor_riscv64.s
+
+#include "textflag.h"
+
+#define ROUNDFN(NAME, FUNC)	\
+TEXT NAME(SB),NOSPLIT,$0;	\
+	MOVD	x+0(FP), F0;	\
+	MOVV	F0, R11;	\
+	/* 1023: bias of exponent, [-2^53, 2^53]: exactly integer represent range */;	\
+	MOVV	$1023+53, R12;	\
+	/* Drop all fraction bits */;	\
+	SRLV	$52, R11, R11;	\
+	/* Remove sign bit */;	\
+	AND	$0x7FF, R11, R11;	\
+	BLTU	R12, R11, isExtremum;	\
+normal:;	\
+	FUNC	F0, F2;	\
+	MOVV	F2, R10;	\
+	BEQ	R10, R0, is0;	\
+	FFINTDV	F2, F0;	\
+/* Return either input is +-Inf, NaN(0x7FF) or out of precision limitation */;	\
+isExtremum:;	\
+	MOVD	F0, ret+8(FP);	\
+	RET;	\
+is0:;	\
+	FCOPYSGD	F0, F2, F2;	\
+	MOVD	F2, ret+8(FP);	\
+	RET
+
+// func archFloor(x float64) float64
+ROUNDFN(·archFloor, FTINTRMVD)
+
+// func archCeil(x float64) float64
+ROUNDFN(·archCeil, FTINTRPVD)
+
+// func archTrunc(x float64) float64
+ROUNDFN(·archTrunc, FTINTRZVD)

src/math/floor_noasm.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !386 && !amd64 && !arm64 && !ppc64 && !ppc64le && !riscv64 && !s390x && !wasm
+//go:build !386 && !amd64 && !arm64 && !loong64 && !ppc64 && !ppc64le && !riscv64 && !s390x && !wasm
 
 package math