cmd/compile: remove unused constant divide strength reduction code

Change list https://golang.org/cl/37015/ moved the optimization
of division by constants to the generic ssa backend.
This removes the old now unused code that was used
for this optimization outside of the ssa backend.

Change-Id: I86223e56742e48dbb372ba8d779681e66448c513
Reviewed-on: https://go-review.googlesource.com/37198
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/magic.go

-// Copyright 2009 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.
-
-package gc
-
-// Transmogrify slow integer division into fast multiplication using magic.
-
-// argument passing to/from
-// smagic and umagic
-type Magic struct {
-	W   int // input for both - width
-	S   int // output for both - shift
-	Bad int // output for both - unexpected failure
-
-	// magic multiplier for signed literal divisors
-	Sd int64 // input - literal divisor
-	Sm int64 // output - multiplier
-
-	// magic multiplier for unsigned literal divisors
-	Ud uint64 // input - literal divisor
-	Um uint64 // output - multiplier
-	Ua int    // output - adder
-}
-
-// magic number for signed division
-// see hacker's delight chapter 10
-func smagic(m *Magic) {
-	var mask uint64
-
-	m.Bad = 0
-	switch m.W {
-	default:
-		m.Bad = 1
-		return
-
-	case 8:
-		mask = 0xff
-
-	case 16:
-		mask = 0xffff
-
-	case 32:
-		mask = 0xffffffff
-
-	case 64:
-		mask = 0xffffffffffffffff
-	}
-
-	two31 := mask ^ (mask >> 1)
-
-	p := m.W - 1
-	ad := uint64(m.Sd)
-	if m.Sd < 0 {
-		ad = -uint64(m.Sd)
-	}
-
-	// bad denominators
-	if ad == 0 || ad == 1 || ad == two31 {
-		m.Bad = 1
-		return
-	}
-
-	t := two31
-	ad &= mask
-
-	anc := t - 1 - t%ad
-	anc &= mask
-
-	q1 := two31 / anc
-	r1 := two31 - q1*anc
-	q1 &= mask
-	r1 &= mask
-
-	q2 := two31 / ad
-	r2 := two31 - q2*ad
-	q2 &= mask
-	r2 &= mask
-
-	var delta uint64
-	for {
-		p++
-		q1 <<= 1
-		r1 <<= 1
-		q1 &= mask
-		r1 &= mask
-		if r1 >= anc {
-			q1++
-			r1 -= anc
-			q1 &= mask
-			r1 &= mask
-		}
-
-		q2 <<= 1
-		r2 <<= 1
-		q2 &= mask
-		r2 &= mask
-		if r2 >= ad {
-			q2++
-			r2 -= ad
-			q2 &= mask
-			r2 &= mask
-		}
-
-		delta = ad - r2
-		delta &= mask
-		if q1 < delta || (q1 == delta && r1 == 0) {
-			continue
-		}
-
-		break
-	}
-
-	m.Sm = int64(q2 + 1)
-	if uint64(m.Sm)&two31 != 0 {
-		m.Sm |= ^int64(mask)
-	}
-	m.S = p - m.W
-}
-
-// magic number for unsigned division
-// see hacker's delight chapter 10
-func umagic(m *Magic) {
-	var mask uint64
-
-	m.Bad = 0
-	m.Ua = 0
-
-	switch m.W {
-	default:
-		m.Bad = 1
-		return
-
-	case 8:
-		mask = 0xff
-
-	case 16:
-		mask = 0xffff
-
-	case 32:
-		mask = 0xffffffff
-
-	case 64:
-		mask = 0xffffffffffffffff
-	}
-
-	two31 := mask ^ (mask >> 1)
-
-	m.Ud &= mask
-	if m.Ud == 0 || m.Ud == two31 {
-		m.Bad = 1
-		return
-	}
-
-	nc := mask - (-m.Ud&mask)%m.Ud
-	p := m.W - 1
-
-	q1 := two31 / nc
-	r1 := two31 - q1*nc
-	q1 &= mask
-	r1 &= mask
-
-	q2 := (two31 - 1) / m.Ud
-	r2 := (two31 - 1) - q2*m.Ud
-	q2 &= mask
-	r2 &= mask
-
-	var delta uint64
-	for {
-		p++
-		if r1 >= nc-r1 {
-			q1 <<= 1
-			q1++
-			r1 <<= 1
-			r1 -= nc
-		} else {
-			q1 <<= 1
-			r1 <<= 1
-		}
-
-		q1 &= mask
-		r1 &= mask
-		if r2+1 >= m.Ud-r2 {
-			if q2 >= two31-1 {
-				m.Ua = 1
-			}
-
-			q2 <<= 1
-			q2++
-			r2 <<= 1
-			r2++
-			r2 -= m.Ud
-		} else {
-			if q2 >= two31 {
-				m.Ua = 1
-			}
-
-			q2 <<= 1
-			r2 <<= 1
-			r2++
-		}
-
-		q2 &= mask
-		r2 &= mask
-
-		delta = m.Ud - 1 - r2
-		delta &= mask
-
-		if p < m.W+m.W {
-			if q1 < delta || (q1 == delta && r1 == 0) {
-				continue
-			}
-		}
-
-		break
-	}
-
-	m.Um = q2 + 1
-	m.S = p - m.W
-}