cmd/compile: derive unsigned limits from signed limits in prove

This adds a few simple deductions to the prove pass' fact table to
derive unsigned concrete limits from signed concrete limits where
possible.

This tweak lets the pass prove 70 additional branch conditions in std
and cmd.

This is based on a comment from the recently-deleted factsTable.get:
"// TODO: also use signed data if lim.min >= 0".

Change-Id: Ib4340249e7733070f004a0aa31254adf5df8a392
Reviewed-on: https://go-review.googlesource.com/87479
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -248,6 +248,16 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 				lim.min = c
 				lim.max = c
 			}
+			if lim.min >= 0 {
+				// int(x) >= 0 && int(x) >= N  ⇒  uint(x) >= N
+				lim.umin = uint64(lim.min)
+			}
+			if lim.max != noLimit.max && old.min >= 0 && lim.max >= 0 {
+				// 0 <= int(x) <= N  ⇒  0 <= uint(x) <= N
+				// This is for a max update, so the lower bound
+				// comes from what we already know (old).
+				lim.umax = uint64(lim.max)
+			}
 		case unsigned:
 			var uc uint64
 			switch w.Op {
@@ -281,6 +291,9 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 				lim.umin = uc
 				lim.umax = uc
 			}
+			// We could use the contrapositives of the
+			// signed implications to derive signed facts,
+			// but it turns out not to matter.
 		}
 		ft.limitStack = append(ft.limitStack, limitFact{v.ID, old})
 		lim = old.intersect(lim)

test/prove.go

@@ -479,12 +479,33 @@ func sm1(b []int, x int) {
 	// Test constant argument to slicemask.
 	useSlice(b[2:8]) // ERROR "Proved slicemask not needed$"
 	// Test non-constant argument with known limits.
-	// Right now prove only uses the unsigned limit.
-	if uint(cap(b)) > 10 {
+	if cap(b) > 10 {
 		useSlice(b[2:]) // ERROR "Proved slicemask not needed$"
 	}
 }
 
+func lim1(x, y, z int) {
+	// Test relations between signed and unsigned limits.
+	if x > 5 {
+		if uint(x) > 5 { // ERROR "Proved Greater64U$"
+			return
+		}
+	}
+	if y >= 0 && y < 4 {
+		if uint(y) > 4 { // ERROR "Disproved Greater64U$"
+			return
+		}
+		if uint(y) < 5 { // ERROR "Proved Less64U$"
+			return
+		}
+	}
+	if z < 4 {
+		if uint(z) > 4 { // Not provable without disjunctions.
+			return
+		}
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }