cmd/compile: add Trunc support to known bits

Uniqued by LOC this adds 3 known bits hits when building the std.

Updates #78633

Change-Id: I44f2ae1ed31f798d7881691729cce5d9707e8edc
Reviewed-on: https://go-review.googlesource.com/c/go/+/766120
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

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

@@ -88,7 +88,10 @@ func (kb *knownBitsState) fold(v *Value) (value, known int64) {
 		}
 		return value, known
 	case OpCopy, OpCvtBoolToUint8,
-		OpSignExt8to16, OpSignExt8to32, OpSignExt8to64, OpSignExt16to32, OpSignExt16to64, OpSignExt32to64:
+		OpSignExt8to16, OpSignExt8to32, OpSignExt8to64, OpSignExt16to32, OpSignExt16to64, OpSignExt32to64,
+		// The defer block handles maintaining the sign-extension invariant using v.Type.Size()
+		// thus we can just pass Truncs as-is.
+		OpTrunc64to32, OpTrunc64to16, OpTrunc64to8, OpTrunc32to16, OpTrunc32to8, OpTrunc16to8:
 		return kb.fold(v.Args[0])
 	case OpEq64, OpEq32, OpEq16, OpEq8, OpEqB:
 		x, xk := kb.fold(v.Args[0])

test/known_bits.go

@@ -383,3 +383,51 @@ func unknownBitsRshRightSide(x int32, y int32) int32 {
 
 	return (x >> y) & 0b110
 }
+
+func knownBitsTrunc(x int64, cond bool) int32 {
+	x |= 2
+	if cond {
+		x = 3
+	}
+
+	return int32(x) & 2 // ERROR "known value of v[0-9]+ \(And32\): 2$"
+}
+
+func unknownBitsTrunc(x int64, cond bool) int32 {
+	x |= 2
+	if cond {
+		x = 3
+	}
+
+	return int32(x) & 1
+}
+
+func knownBitsSextAfterTrunc(x int64, cond1, cond2 bool) int64 {
+	x |= -1 << 31
+	if cond1 {
+		x = -3
+	}
+
+	truncated := int32(x)
+
+	if cond2 {
+		truncated |= 4
+	}
+
+	return int64(truncated) & (-1 << 63) // ERROR "known value of v[0-9]+ \(And64\): -9223372036854775808$"
+}
+
+func unknownBitsSextAfterTrunc(x int64, cond1, cond2 bool) int64 {
+	x |= 2
+	if cond1 {
+		x = 3
+	}
+
+	truncated := int32(x)
+
+	if cond2 {
+		truncated |= 4
+	}
+
+	return int64(truncated) & (-1 << 63)
+}