cmd/compile/internal/ssa: BCE for induction variables

There are 5293 loop in the main go repository.
A survey of the top most common for loops:

     18 for __k__ := 0; i < len(sa.Addr); i++ {
     19 for __k__ := 0; ; i++ {
     19 for __k__ := 0; i < 16; i++ {
     25 for __k__ := 0; i < length; i++ {
     30 for __k__ := 0; i < 8; i++ {
     49 for __k__ := 0; i < len(s); i++ {
     67 for __k__ := 0; i < n; i++ {
    376 for __k__ := range __slice__ {
    685 for __k__, __v__ := range __slice__ {
   2074 for __, __v__ := range __slice__ {

The algorithm to find induction variables handles all cases
with an upper limit. It currently doesn't find related induction
variables such as c * ind or c + ind.

842 out of 22954 bound checks are removed for src/make.bash.
1957 out of 42952 bounds checks are removed for src/all.bash.

Things to do in follow-up CLs:
* Find the associated pointer for `for _, v := range a {}`
* Drop the NilChecks on the pointer.
* Replace the implicit induction variable by a loop over the pointer

Generated garbage can be reduced if we share the sdom between passes.

% benchstat old.txt new.txt
name       old time/op     new time/op     delta
Template       337ms ± 3%      333ms ± 3%    ~             (p=0.258 n=9+9)
GoTypes        1.11s ± 2%      1.10s ± 2%    ~           (p=0.912 n=10+10)
Compiler       5.25s ± 1%      5.29s ± 2%    ~             (p=0.077 n=9+9)
MakeBash       33.5s ± 1%      34.1s ± 2%  +1.85%          (p=0.011 n=9+9)

name       old alloc/op    new alloc/op    delta
Template      63.6MB ± 0%     63.9MB ± 0%  +0.52%         (p=0.000 n=10+9)
GoTypes        218MB ± 0%      219MB ± 0%  +0.59%         (p=0.000 n=10+9)
Compiler       978MB ± 0%      985MB ± 0%  +0.69%        (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        582k ± 0%       583k ± 0%  +0.10%        (p=0.000 n=10+10)
GoTypes        1.78M ± 0%      1.78M ± 0%  +0.12%        (p=0.000 n=10+10)
Compiler       7.68M ± 0%      7.69M ± 0%  +0.05%        (p=0.000 n=10+10)

name       old text-bytes  new text-bytes  delta
HelloSize       581k ± 0%       581k ± 0%  -0.08%        (p=0.000 n=10+10)
CmdGoSize      6.40M ± 0%      6.39M ± 0%  -0.08%        (p=0.000 n=10+10)

name       old data-bytes  new data-bytes  delta
HelloSize      3.66k ± 0%      3.66k ± 0%    ~     (all samples are equal)
CmdGoSize       134k ± 0%       134k ± 0%    ~     (all samples are equal)

name       old bss-bytes   new bss-bytes   delta
HelloSize       126k ± 0%       126k ± 0%    ~     (all samples are equal)
CmdGoSize       149k ± 0%       149k ± 0%    ~     (all samples are equal)

name       old exe-bytes   new exe-bytes   delta
HelloSize       947k ± 0%       946k ± 0%  -0.01%        (p=0.000 n=10+10)
CmdGoSize      9.92M ± 0%      9.91M ± 0%  -0.06%        (p=0.000 n=10+10)

Change-Id: Ie74bdff46fd602db41bb457333d3a762a0c3dc4d
Reviewed-on: https://go-review.googlesource.com/20517
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>

1 files changed, 176 insertions, 0 deletions

diff --git a/test/loopbce.go b/test/loopbce.go
new file mode 100644
index 0000000000..eb44092705
--- /dev/null
+++ b/test/loopbce.go
@@ -0,0 +1,176 @@
+// +build amd64
+// errorcheck -0 -d=ssa/loopbce/debug=3
+
+package main
+
+func f0a(a []int) int {
+	x := 0
+	for i := range a { // ERROR "Induction variable with minimum 0 and increment 1$"
+		x += a[i] // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func f0b(a []int) int {
+	x := 0
+	for i := range a { // ERROR "Induction variable with minimum 0 and increment 1$"
+		b := a[i:] // ERROR "Found redundant IsSliceInBounds$"
+		x += b[0]
+	}
+	return x
+}
+
+func f0c(a []int) int {
+	x := 0
+	for i := range a { // ERROR "Induction variable with minimum 0 and increment 1$"
+		b := a[:i+1] // ERROR "Found redundant IsSliceInBounds \(len promoted to cap\)$"
+		x += b[0]
+	}
+	return x
+}
+
+func f1(a []int) int {
+	x := 0
+	for _, i := range a { // ERROR "Induction variable with minimum 0 and increment 1$"
+		x += i
+	}
+	return x
+}
+
+func f2(a []int) int {
+	x := 0
+	for i := 1; i < len(a); i++ { // ERROR "Induction variable with minimum 1 and increment 1$"
+		x += a[i] // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func f4(a [10]int) int {
+	x := 0
+	for i := 0; i < len(a); i += 2 { // ERROR "Induction variable with minimum 0 and increment 2$"
+		x += a[i] // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func f5(a [10]int) int {
+	x := 0
+	for i := -10; i < len(a); i += 2 { // ERROR "Induction variable with minimum -10 and increment 2$"
+		x += a[i]
+	}
+	return x
+}
+
+func f6(a []int) {
+	for i := range a { // ERROR "Induction variable with minimum 0 and increment 1$"
+		b := a[0:i] // ERROR "Found redundant IsSliceInBounds \(len promoted to cap\)$"
+		f6(b)
+	}
+}
+
+func g0a(a string) int {
+	x := 0
+	for i := 0; i < len(a); i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		x += int(a[i]) // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func g0b(a string) int {
+	x := 0
+	for i := 0; len(a) > i; i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		x += int(a[i]) // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func g1() int {
+	a := "evenlength"
+	x := 0
+	for i := 0; i < len(a); i += 2 { // ERROR "Induction variable with minimum 0 and increment 2$"
+		x += int(a[i]) // ERROR "Found redundant IsInBounds$"
+	}
+	return x
+}
+
+func g2() int {
+	a := "evenlength"
+	x := 0
+	for i := 0; i < len(a); i += 2 { // ERROR "Induction variable with minimum 0 and increment 2$"
+		j := i
+		if a[i] == 'e' { // ERROR "Found redundant IsInBounds$"
+			j = j + 1
+		}
+		x += int(a[j])
+	}
+	return x
+}
+
+func g3a() {
+	a := "this string has length 25"
+	for i := 0; i < len(a); i += 5 { // ERROR "Induction variable with minimum 0 and increment 5$"
+		useString(a[i:]) // ERROR "Found redundant IsSliceInBounds$"
+		useString(a[:i+3])
+	}
+}
+
+func g3b(a string) {
+	for i := 0; i < len(a); i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		useString(a[i+1:]) // ERROR "Found redundant IsSliceInBounds$"
+	}
+}
+
+func g3c(a string) {
+	for i := 0; i < len(a); i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		useString(a[:i+1]) // ERROR "Found redundant IsSliceInBounds$"
+	}
+}
+
+func h1(a []byte) {
+	c := a[:128]
+	for i := range c { // ERROR "Induction variable with minimum 0 and increment 1$"
+		c[i] = byte(i) // ERROR "Found redundant IsInBounds$"
+	}
+}
+
+func h2(a []byte) {
+	for i := range a[:128] { // ERROR "Induction variable with minimum 0 and increment 1$"
+		a[i] = byte(i)
+	}
+}
+
+func nobce1() {
+	// tests overflow of max-min
+	a := int64(9223372036854774057)
+	b := int64(-1547)
+	z := int64(1337)
+
+	if a%z == b%z {
+		panic("invalid test: modulos should differ")
+	}
+
+	for i := b; i < a; i += z {
+		// No induction variable is possible because i will overflow a first iteration.
+		useString("foobar")
+	}
+}
+
+func nobce2(a string) {
+	for i := int64(0); i < int64(len(a)); i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		useString(a[i:]) // ERROR "Found redundant IsSliceInBounds$"
+	}
+	for i := int64(0); i < int64(len(a))-31337; i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		useString(a[i:]) // ERROR "Found redundant IsSliceInBounds$"
+	}
+	for i := int64(0); i < int64(len(a))+int64(-1<<63); i++ { // ERROR "Induction variable with minimum 0 and increment 1$"
+		// tests an overflow of StringLen-MinInt64
+		useString(a[i:])
+	}
+}
+
+//go:noinline
+func useString(a string) {
+}
+
+func main() {
+}