cmd/compile: move limit fact table in prove pass to dense encoding

Here begins a pretty major rewrite of the prove pass. The fundamental
observation is that although keeping facts about relations between
two SSA values could use O(n^2) space, keeping facts about relations
between an SSA value and constants needs only O(n) space. We can just
keep track of min/max for every SSA value at little cost.

Redo the limit table to just keep track of limits for all SSA values.
Use just a slice instead of a map. It may use more space (but still
just O(n) space), but accesses are a lot faster. And with the cache
in the compiler, that space will be reused quickly.

This is part of my planning to add lots more constant limits in the
prove pass.

Change-Id: Ie36819fad5631a8b79c3630fe0e819521796551a
Reviewed-on: https://go-review.googlesource.com/c/go/+/599255
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/_gen/allocators.go

@@ -46,14 +46,14 @@ func genAllocators() {
 			maxLog:   32,
 		},
 		{
-			name:     "Int64Slice",
-			typ:      "[]int64",
+			name:     "LimitSlice",
+			typ:      "[]limit", // the limit type is basically [4]uint64.
 			capacity: "cap(%s)",
-			mak:      "make([]int64, %s)",
+			mak:      "make([]limit, %s)",
 			resize:   "%s[:%s]",
-			clear:    "for i := range %[1]s {\n%[1]s[i] = 0\n}",
-			minLog:   5,
-			maxLog:   32,
+			clear:    "for i := range %[1]s {\n%[1]s[i] = limit{}\n}",
+			minLog:   3,
+			maxLog:   30,
 		},
 		{
 			name:     "SparseSet",
@@ -92,30 +92,35 @@ func genAllocators() {
 			typ:  "[]*Block",
 			base: "ValueSlice",
 		},
+		{
+			name: "Int64",
+			typ:  "[]int64",
+			base: "LimitSlice",
+		},
 		{
 			name: "IntSlice",
 			typ:  "[]int",
-			base: "Int64Slice",
+			base: "LimitSlice",
 		},
 		{
 			name: "Int32Slice",
 			typ:  "[]int32",
-			base: "Int64Slice",
+			base: "LimitSlice",
 		},
 		{
 			name: "Int8Slice",
 			typ:  "[]int8",
-			base: "Int64Slice",
+			base: "LimitSlice",
 		},
 		{
 			name: "BoolSlice",
 			typ:  "[]bool",
-			base: "Int64Slice",
+			base: "LimitSlice",
 		},
 		{
 			name: "IDSlice",
 			typ:  "[]ID",
-			base: "Int64Slice",
+			base: "LimitSlice",
 		},
 	}
 

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

@@ -34,7 +34,7 @@ type Cache struct {
 	// Free "headers" for use by the allocators in allocators.go.
 	// Used to put slices in sync.Pools without allocation.
 	hdrValueSlice []*[]*Value
-	hdrInt64Slice []*[]int64
+	hdrLimitSlice []*[]limit
 }
 
 func (c *Cache) Reset() {