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, 260 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/loopbce.go b/src/cmd/compile/internal/ssa/loopbce.go
new file mode 100644
index 0000000000..7fbb48a7fc
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/loopbce.go
@@ -0,0 +1,260 @@
+package ssa
+
+type indVar struct {
+	ind   *Value // induction variable
+	inc   *Value // increment, a constant
+	nxt   *Value // ind+inc variable
+	min   *Value // minimum value. inclusive,
+	max   *Value // maximum value. exclusive.
+	entry *Block // entry block in the loop.
+	// Invariants: for all blocks dominated by entry:
+	//	min <= ind < max
+	//	min <= nxt <= max
+}
+
+// findIndVar finds induction variables in a function.
+//
+// Look for variables and blocks that satisfy the following
+//
+// loop:
+//   ind = (Phi min nxt),
+//   if ind < max
+//     then goto enter_loop
+//     else goto exit_loop
+//
+//   enter_loop:
+//	do something
+//      nxt = inc + ind
+//	goto loop
+//
+// exit_loop:
+//
+//
+// TODO: handle 32 bit operations
+func findIndVar(f *Func, sdom sparseTree) []indVar {
+	var iv []indVar
+
+nextb:
+	for _, b := range f.Blocks {
+		if b.Kind != BlockIf || len(b.Preds) != 2 {
+			continue
+		}
+
+		var ind, max *Value // induction, and maximum
+		entry := -1         // which successor of b enters the loop
+
+		// Check thet the control if it either ind < max or max > ind.
+		// TODO: Handle Leq64, Geq64.
+		switch b.Control.Op {
+		case OpLess64:
+			entry = 0
+			ind, max = b.Control.Args[0], b.Control.Args[1]
+		case OpGreater64:
+			entry = 0
+			ind, max = b.Control.Args[1], b.Control.Args[0]
+		default:
+			continue nextb
+		}
+
+		// Check that the induction variable is a phi that depends on itself.
+		if ind.Op != OpPhi {
+			continue
+		}
+
+		// Extract min and nxt knowing that nxt is an addition (e.g. Add64).
+		var min, nxt *Value // minimum, and next value
+		if n := ind.Args[0]; n.Op == OpAdd64 && (n.Args[0] == ind || n.Args[1] == ind) {
+			min, nxt = ind.Args[1], n
+		} else if n := ind.Args[1]; n.Op == OpAdd64 && (n.Args[0] == ind || n.Args[1] == ind) {
+			min, nxt = ind.Args[0], n
+		} else {
+			// Not a recognized induction variable.
+			continue
+		}
+
+		var inc *Value
+		if nxt.Args[0] == ind { // nxt = ind + inc
+			inc = nxt.Args[1]
+		} else if nxt.Args[1] == ind { // nxt = inc + ind
+			inc = nxt.Args[0]
+		} else {
+			panic("unreachable") // one of the cases must be true from the above.
+		}
+
+		// Expect the increment to be a positive constant.
+		// TODO: handle negative increment.
+		if inc.Op != OpConst64 || inc.AuxInt <= 0 {
+			continue
+		}
+
+		// Up to now we extracted the induction variable (ind),
+		// the increment delta (inc), the temporary sum (nxt),
+		// the mininum value (min) and the maximum value (max).
+		//
+		// We also know that ind has the form (Phi min nxt) where
+		// nxt is (Add inc nxt) which means: 1) inc dominates nxt
+		// and 2) there is a loop starting at inc and containing nxt.
+		//
+		// We need to prove that the induction variable is incremented
+		// only when it's smaller than the maximum value.
+		// Two conditions must happen listed below to accept ind
+		// as an induction variable.
+
+		// First condition: loop entry has a single predecessor, which
+		// is the header block.  This implies that b.Succs[entry] is
+		// reached iff ind < max.
+		if len(b.Succs[entry].Preds) != 1 {
+			// b.Succs[1-entry] must exit the loop.
+			continue
+		}
+
+		// Second condition: b.Succs[entry] dominates nxt so that
+		// nxt is computed when inc < max, meaning nxt <= max.
+		if !sdom.isAncestorEq(b.Succs[entry], nxt.Block) {
+			// inc+ind can only be reached through the branch that enters the loop.
+			continue
+		}
+
+		// If max is c + SliceLen with c <= 0 then we drop c.
+		// Makes sure c + SliceLen doesn't overflow when SliceLen == 0.
+		// TODO: save c as an offset from max.
+		if w, c := dropAdd64(max); (w.Op == OpStringLen || w.Op == OpSliceLen) && 0 >= c && -c >= 0 {
+			max = w
+		}
+
+		// We can only guarantee that the loops runs withing limits of induction variable
+		// if the increment is 1 or when the limits are constants.
+		if inc.AuxInt != 1 {
+			ok := false
+			if min.Op == OpConst64 && max.Op == OpConst64 {
+				if max.AuxInt > min.AuxInt && max.AuxInt%inc.AuxInt == min.AuxInt%inc.AuxInt { // handle overflow
+					ok = true
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+
+		if f.pass.debug > 1 {
+			if min.Op == OpConst64 {
+				b.Func.Config.Warnl(b.Line, "Induction variable with minimum %d and increment %d", min.AuxInt, inc.AuxInt)
+			} else {
+				b.Func.Config.Warnl(b.Line, "Induction variable with non-const minimum and increment %d", inc.AuxInt)
+			}
+		}
+
+		iv = append(iv, indVar{
+			ind:   ind,
+			inc:   inc,
+			nxt:   nxt,
+			min:   min,
+			max:   max,
+			entry: b.Succs[entry],
+		})
+		b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)
+	}
+
+	return iv
+}
+
+// loopbce performs loop based bounds check elimination.
+func loopbce(f *Func) {
+	idom := dominators(f)
+	sdom := newSparseTree(f, idom)
+	ivList := findIndVar(f, sdom)
+
+	m := make(map[*Value]indVar)
+	for _, iv := range ivList {
+		m[iv.ind] = iv
+	}
+
+	removeBoundsChecks(f, sdom, m)
+}
+
+// removesBoundsChecks remove IsInBounds and IsSliceInBounds based on the induction variables.
+func removeBoundsChecks(f *Func, sdom sparseTree, m map[*Value]indVar) {
+	for _, b := range f.Blocks {
+		if b.Kind != BlockIf {
+			continue
+		}
+
+		v := b.Control
+
+		// Simplify:
+		// (IsInBounds ind max) where 0 <= const == min <= ind < max.
+		// (IsSliceInBounds ind max) where 0 <= const == min <= ind < max.
+		// Found in:
+		//	for i := range a {
+		//		use a[i]
+		//		use a[i:]
+		//		use a[:i]
+		//	}
+		if v.Op == OpIsInBounds || v.Op == OpIsSliceInBounds {
+			ind, add := dropAdd64(v.Args[0])
+			if ind.Op != OpPhi {
+				goto skip1
+			}
+			if v.Op == OpIsInBounds && add != 0 {
+				goto skip1
+			}
+			if v.Op == OpIsSliceInBounds && (0 > add || add > 1) {
+				goto skip1
+			}
+
+			if iv, has := m[ind]; has && sdom.isAncestorEq(iv.entry, b) && isNonNegative(iv.min) {
+				if v.Args[1] == iv.max {
+					if f.pass.debug > 0 {
+						f.Config.Warnl(b.Line, "Found redundant %s", v.Op)
+					}
+					goto simplify
+				}
+			}
+		}
+	skip1:
+
+		// Simplify:
+		// (IsSliceInBounds ind (SliceCap a)) where 0 <= min <= ind < max == (SliceLen a)
+		// Found in:
+		//	for i := range a {
+		//		use a[:i]
+		//		use a[:i+1]
+		//	}
+		if v.Op == OpIsSliceInBounds {
+			ind, add := dropAdd64(v.Args[0])
+			if ind.Op != OpPhi {
+				goto skip2
+			}
+			if 0 > add || add > 1 {
+				goto skip2
+			}
+
+			if iv, has := m[ind]; has && sdom.isAncestorEq(iv.entry, b) && isNonNegative(iv.min) {
+				if v.Args[1].Op == OpSliceCap && iv.max.Op == OpSliceLen && v.Args[1].Args[0] == iv.max.Args[0] {
+					if f.pass.debug > 0 {
+						f.Config.Warnl(b.Line, "Found redundant %s (len promoted to cap)", v.Op)
+					}
+					goto simplify
+				}
+			}
+		}
+	skip2:
+
+		continue
+
+	simplify:
+		f.Logf("removing bounds check %v at %v in %s\n", b.Control, b, f.Name)
+		b.Kind = BlockFirst
+		b.SetControl(nil)
+	}
+}
+
+func dropAdd64(v *Value) (*Value, int64) {
+	if v.Op == OpAdd64 && v.Args[0].Op == OpConst64 {
+		return v.Args[1], v.Args[0].AuxInt
+	}
+	if v.Op == OpAdd64 && v.Args[1].Op == OpConst64 {
+		return v.Args[0], v.Args[1].AuxInt
+	}
+	return v, 0
+}