math: add round assembly implementations on riscv64

goos: linux
goarch: riscv64
pkg: math
            │ floor_old.bench │           floor_new.bench           │
            │     sec/op      │   sec/op     vs base                │
Ceil              54.12n ± 0%   22.05n ± 0%  -59.26% (p=0.000 n=10)
Floor             40.80n ± 0%   22.05n ± 0%  -45.96% (p=0.000 n=10)
Round             20.73n ± 0%   20.74n ± 0%        ~ (p=0.441 n=10)
RoundToEven       24.07n ± 0%   24.07n ± 0%        ~ (p=1.000 n=10)
Trunc             38.73n ± 0%   22.05n ± 0%  -43.07% (p=0.000 n=10)
geomean           33.58n        22.17n       -33.98%

Change-Id: I24fb9e3bbf8146da253b6791b21377bea1afbd16
Reviewed-on: https://go-review.googlesource.com/c/go/+/504737
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: M Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>

Author: mengzhuo

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 || s390x || wasm
+//go:build 386 || amd64 || arm64 || ppc64 || ppc64le || riscv64 || s390x || wasm
 
 package math
 

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 && !s390x && !wasm
+//go:build !386 && !amd64 && !arm64 && !ppc64 && !ppc64le && !riscv64 && !s390x && !wasm
 
 package math
 

src/math/floor_riscv64.s

@@ -0,0 +1,41 @@
+// Copyright 2023 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.
+
+#include "textflag.h"
+
+#define PosInf 0x7FF0000000000000
+
+// The rounding mode of RISC-V is different from Go spec.
+
+#define ROUNDFN(NAME, MODE) 	\
+TEXT NAME(SB),NOSPLIT,$0; 	\
+	MOVD	x+0(FP), F0; 	\
+	/* whether x is NaN */; \
+	FEQD	F0, F0, X6;	\
+	BNEZ	X6, 3(PC);	\
+	/* return NaN if x is NaN */; \
+	MOVD	F0, ret+8(FP); 	\
+	RET;			\
+	MOV	$PosInf, X6;	\
+	FMVDX	X6, F1;		\
+	FABSD	F0, F2;		\
+	/* if abs(x) > +Inf, return Inf instead of round(x) */; \
+	FLTD	F1, F2, X6;	\
+	/* Inf should keep same signed with x then return */;	\
+	BEQZ	X6, 3(PC); \
+	FCVTLD.MODE	F0, X6;	\
+	FCVTDL	X6, F1;		\
+	/* rounding will drop signed bit in RISCV, restore it */; \
+	FSGNJD	F0, F1, F0;	\
+	MOVD	F0, ret+8(FP); 	\
+	RET
+
+// func archFloor(x float64) float64
+ROUNDFN(·archFloor, RDN)
+
+// func archCeil(x float64) float64
+ROUNDFN(·archCeil, RUP)
+
+// func archTrunc(x float64) float64
+ROUNDFN(·archTrunc, RTZ)