floor_riscv64.s

     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) 	\
    23  TEXT 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
    42  ROUNDFN(·archFloor, RDN)
    43  
    44  // func archCeil(x float64) float64
    45  ROUNDFN(·archCeil, RUP)
    46  
    47  // func archTrunc(x float64) float64
    48  ROUNDFN(·archTrunc, RTZ)
    49
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