math: use Trunc to implement Modf

By implementing Modf using Trunc, rather than the other way round,
we can get a significant performance improvement on platforms where
Trunc is implemented as an intrinsic.

Trunc is implemented as an intrinsic on ppc64x and arm64 so the assembly
implementations of Modf are no longer needed (the compiler can generate
very similar code that can now potentially be inlined).

GOAMD64=v1

goos: linux
goarch: amd64
pkg: math
cpu: 12th Gen Intel(R) Core(TM) i7-12700T
        │       sec/op       │    sec/op     vs base                │
Gamma            4.257n ± 0%    3.890n ± 0%   -8.61% (p=0.000 n=10)
Modf            1.6110n ± 0%   0.4243n ± 0%  -73.67% (p=0.000 n=10)
geomean          2.619n         1.285n       -50.94%

GOAMD64=v2

goos: linux
goarch: amd64
pkg: math
cpu: 12th Gen Intel(R) Core(TM) i7-12700T
        │       sec/op       │    sec/op     vs base                │
Gamma            4.100n ± 1%    3.717n ± 0%   -9.35% (p=0.000 n=10)
Modf            1.6070n ± 0%   0.2158n ± 1%  -86.57% (p=0.000 n=10)
geomean          2.567n        0.8957n       -65.11%

Change-Id: I689a560c344cf1d39ef002b540749bacc3179786
Reviewed-on: https://go-review.googlesource.com/c/go/+/694896
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>

6 files changed, 13 insertions, 90 deletions

diff --git a/src/math/floor.go b/src/math/floor.go
index cb5856424b..20a7641124 100644
--- a/src/math/floor.go
+++ b/src/math/floor.go
@@ -66,11 +66,18 @@ func Trunc(x float64) float64 {
 }
 
 func trunc(x float64) float64 {
-	if x == 0 || IsNaN(x) || IsInf(x, 0) {
-		return x
+	if Abs(x) < 1 {
+		return Copysign(0, x)
 	}
-	d, _ := Modf(x)
-	return d
+
+	b := Float64bits(x)
+	e := uint(b>>shift)&mask - bias
+
+	// Keep the top 12+e bits, the integer part; clear the rest.
+	if e < 64-12 {
+		b &^= 1<<(64-12-e) - 1
+	}
+	return Float64frombits(b)
 }
 
 // Round returns the nearest integer, rounding half away from zero.
diff --git a/src/math/modf.go b/src/math/modf.go
index 613a75fc9a..ab73e2dc36 100644
--- a/src/math/modf.go
+++ b/src/math/modf.go
@@ -12,32 +12,7 @@ package math
 //	Modf(±Inf) = ±Inf, NaN
 //	Modf(NaN) = NaN, NaN
 func Modf(f float64) (int float64, frac float64) {
-	if haveArchModf {
-		return archModf(f)
-	}
-	return modf(f)
-}
-
-func modf(f float64) (int float64, frac float64) {
-	if f < 1 {
-		switch {
-		case f < 0:
-			int, frac = Modf(-f)
-			return -int, -frac
-		case f == 0:
-			return f, f // Return -0, -0 when f == -0
-		}
-		return 0, f
-	}
-
-	x := Float64bits(f)
-	e := uint(x>>shift)&mask - bias
-
-	// Keep the top 12+e bits, the integer part; clear the rest.
-	if e < 64-12 {
-		x &^= 1<<(64-12-e) - 1
-	}
-	int = Float64frombits(x)
-	frac = f - int
+	int = Trunc(f)
+	frac = Copysign(f-int, f)
 	return
 }
diff --git a/src/math/modf_arm64.s b/src/math/modf_arm64.s
deleted file mode 100644
index 1e4a329a4b..0000000000
--- a/src/math/modf_arm64.s
+++ /dev/null
@@ -1,18 +0,0 @@
-// Copyright 2016 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"
-
-// func archModf(f float64) (int float64, frac float64)
-TEXT ·archModf(SB),NOSPLIT,$0
-	MOVD	f+0(FP), R0
-	FMOVD	R0, F0
-	FRINTZD	F0, F1
-	FMOVD	F1, int+8(FP)
-	FSUBD	F1, F0
-	FMOVD	F0, R1
-	AND	$(1<<63), R0
-	ORR	R0, R1 // must have same sign
-	MOVD	R1, frac+16(FP)
-	RET
diff --git a/src/math/modf_asm.go b/src/math/modf_asm.go
deleted file mode 100644
index c63be6cf36..0000000000
--- a/src/math/modf_asm.go
+++ /dev/null
@@ -1,11 +0,0 @@
-// Copyright 2021 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.
-
-//go:build arm64 || ppc64 || ppc64le
-
-package math
-
-const haveArchModf = true
-
-func archModf(f float64) (int float64, frac float64)
diff --git a/src/math/modf_noasm.go b/src/math/modf_noasm.go
deleted file mode 100644
index 55c6a7f6e2..0000000000
--- a/src/math/modf_noasm.go
+++ /dev/null
@@ -1,13 +0,0 @@
-// Copyright 2021 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.
-
-//go:build !arm64 && !ppc64 && !ppc64le
-
-package math
-
-const haveArchModf = false
-
-func archModf(f float64) (int float64, frac float64) {
-	panic("not implemented")
-}
diff --git a/src/math/modf_ppc64x.s b/src/math/modf_ppc64x.s
deleted file mode 100644
index 410b523c0e..0000000000
--- a/src/math/modf_ppc64x.s
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright 2017 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.
-
-//go:build ppc64 || ppc64le
-
-#include "textflag.h"
-
-// func archModf(f float64) (int float64, frac float64)
-TEXT ·archModf(SB),NOSPLIT,$0
-	FMOVD	f+0(FP), F0
-	FRIZ	F0, F1
-	FMOVD	F1, int+8(FP)
-	FSUB	F1, F0, F2
-	FCPSGN	F2, F0, F2
-	FMOVD	F2, frac+16(FP)
-	RET