floor_riscv64.s

Documentation: math

     1// Copyright 2024 The Go Authors. All rights reserved.
     2// Use of this source code is governed by a BSD-style
     3// license that can be found in the LICENSE file.
     4
     5#include "textflag.h"
     6
     7// RISC-V offered floating-point (FP) rounding by FP conversion instructions (FCVT)
     8// with rounding mode field.
     9// As Go spec expects FP rounding result in FP, we have to use FCVT integer
    10// back to FP (fp -> int -> fp).
    11// RISC-V only set Inexact flag during invalid FP-integer conversion without changing any data,
    12// on the other hand, RISC-V sets out of integer represent range yet valid FP into NaN.
    13// When it comes to integer-FP conversion, invalid FP like NaN, +-Inf will be
    14// converted into the closest valid FP, for example:
    15//
    16// `Floor(-Inf) -> int64(0x7fffffffffffffff) -> float64(9.22e+18)`
    17// `Floor(18446744073709549568.0) -> int64(0x7fffffffffffffff) -> float64(9.22e+18)`
    18//
    19// This ISA conversion limitation requires we skip all invalid or out of range FP
    20// before any normal rounding operations.
    21
    22#define ROUNDFN(NAME, MODE) 	\
    23TEXT NAME(SB),NOSPLIT,$0; 	\
    24	MOVD	x+0(FP), F10; 	\
    25	FMVXD	F10, X10;	\
    26	/* Drop all fraction bits */;\
    27	SRL	$52, X10, X12;	\
    28	/* Remove sign bit */;	\
    29	AND	$0x7FF, X12, X12;\
    30	/* Return either input is +-Inf, NaN(0x7FF) or out of precision limitation */;\
    31	/* 1023: bias of exponent, [-2^53, 2^53]: exactly integer represent range */;\
    32	MOV	$1023+53, X11;	\
    33	BLTU	X11, X12, 4(PC);\
    34	FCVTLD.MODE F10, X11;	\
    35	FCVTDL	X11, F11;	\
    36	/* RISC-V rounds negative values to +0, restore original sign */;\
    37	FSGNJD	F10, F11, F10;	\
    38	MOVD	F10, ret+8(FP); \
    39	RET
    40
    41// func archFloor(x float64) float64
    42ROUNDFN(·archFloor, RDN)
    43
    44// func archCeil(x float64) float64
    45ROUNDFN(·archCeil, RUP)
    46
    47// func archTrunc(x float64) float64
    48ROUNDFN(·archTrunc, RTZ)
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