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