cmd/compile: minor comment improvements/corrections

Change-Id: Ie0934f1528d58d4971cdef726d3e2d23cf3935d3
Reviewed-on: https://go-review.googlesource.com/87475
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>

src/cmd/compile/internal/ssa/func.go

@@ -28,7 +28,7 @@ type Func struct {
 	Cache  *Cache      // re-usable cache
 	fe     Frontend    // frontend state associated with this Func, callbacks into compiler frontend
 	pass   *pass       // current pass information (name, options, etc.)
-	Name   string      // e.g. bytes·Compare
+	Name   string      // e.g. NewFunc or (*Func).NumBlocks (no package prefix)
 	Type   *types.Type // type signature of the function.
 	Blocks []*Block    // unordered set of all basic blocks (note: not indexable by ID)
 	Entry  *Block      // the entry basic block

src/cmd/compile/internal/ssa/prove.go

@@ -90,6 +90,12 @@ type limitFact struct {
 }
 
 // factsTable keeps track of relations between pairs of values.
+//
+// The fact table logic is sound, but incomplete. Outside of a few
+// special cases, it performs no deduction or arithmetic. While there
+// are known decision procedures for this, the ad hoc approach taken
+// by the facts table is effective for real code while remaining very
+// efficient.
 type factsTable struct {
 	facts map[pair]relation // current known set of relation
 	stack []fact            // previous sets of relations
@@ -433,8 +439,8 @@ var (
 		// TODO: OpIsInBounds actually test 0 <= a < b. This means
 		// that the positive branch learns signed/LT and unsigned/LT
 		// but the negative branch only learns unsigned/GE.
-		OpIsInBounds:      {unsigned, lt},
-		OpIsSliceInBounds: {unsigned, lt | eq},
+		OpIsInBounds:      {unsigned, lt},      // 0 <= arg0 < arg1
+		OpIsSliceInBounds: {unsigned, lt | eq}, // 0 <= arg0 <= arg1
 	}
 )
 
@@ -625,6 +631,7 @@ func updateRestrictions(parent *Block, ft *factsTable, t domain, v, w *Value, r
 // simplifyBlock simplifies block known the restrictions in ft.
 // Returns which branch must always be taken.
 func simplifyBlock(ft *factsTable, b *Block) branch {
+	// Replace OpSlicemask operations in b with constants where possible.
 	for _, v := range b.Values {
 		if v.Op != OpSlicemask {
 			continue