[dev.regabi] cmd/compile: fix reporting of overflow

In the previous CL, I had incorrectly removed one of the error
messages from issue20232.go, because I thought go/constant was just
handling it. But actually the compiler was panicking in nodlit,
because it didn't handle constant.Unknown. So this CL makes it leave
n.Type == nil for unknown constant.Values.

While here, also address #42732 by making sure to report an error
message when origConst is called with an unknown constant.Value (as
can happen when multiplying two floating-point constants overflows).

Finally, add OXOR and OBITNOT to the list of operations to report
errors about, since they're also constant expressions that can produce
a constant with a greater bit length than their operands.

Fixes #42732.

Change-Id: I4a538fbae9b3ac4c553d7de5625dc0c87d9acce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/272928
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/gc/const.go

@@ -718,11 +718,14 @@ func square(x constant.Value) constant.Value {
 }
 
 // For matching historical "constant OP overflow" error messages.
+// TODO(mdempsky): Replace with error messages like go/types uses.
 var overflowNames = [...]string{
 	OADD:    "addition",
 	OSUB:    "subtraction",
 	OMUL:    "multiplication",
 	OLSH:    "shift",
+	OXOR:    "bitwise XOR",
+	OBITNOT: "bitwise complement",
 }
 
 // origConst returns an OLITERAL with orig n and value v.
@@ -732,32 +735,24 @@ func origConst(n *Node, v constant.Value) *Node {
 	lineno = lno
 
 	switch v.Kind() {
-	case constant.Unknown:
-		// If constant folding was attempted (we were called)
-		// but it produced an invalid constant value,
-		// mark n as broken and give up.
-		if Errors() == 0 {
-			Fatalf("should have reported an error")
-		}
-		n.Type = nil
-		return n
-
 	case constant.Int:
-		if constant.BitLen(v) > Mpprec {
+		if constant.BitLen(v) <= Mpprec {
+			break
+		}
+		fallthrough
+	case constant.Unknown:
 		what := overflowNames[n.Op]
 		if what == "" {
 			Fatalf("unexpected overflow: %v", n.Op)
 		}
-			yyerror("constant %v overflow", what)
+		yyerrorl(n.Pos, "constant %v overflow", what)
 		n.Type = nil
 		return n
 	}
-	}
 
 	orig := n
-	n = nod(OLITERAL, nil, nil)
+	n = nodl(orig.Pos, OLITERAL, nil, nil)
 	n.Orig = orig
-	n.Pos = orig.Pos
 	n.Type = orig.Type
 	n.SetVal(v)
 	return n
@@ -800,8 +795,10 @@ func origIntConst(n *Node, v int64) *Node {
 // nodlit returns a new untyped constant with value v.
 func nodlit(v constant.Value) *Node {
 	n := nod(OLITERAL, nil, nil)
-	n.Type = idealType(v.Kind())
+	if k := v.Kind(); k != constant.Unknown {
+		n.Type = idealType(k)
 		n.SetVal(v)
+	}
 	return n
 }
 

test/const2.go

@@ -19,3 +19,14 @@ const LargeB = LargeA * LargeA * LargeA
 const LargeC = LargeB * LargeB * LargeB // GC_ERROR "constant multiplication overflow"
 
 const AlsoLargeA = LargeA << 400 << 400 >> 400 >> 400 // GC_ERROR "constant shift overflow"
+
+// Issue #42732.
+
+const a = 1e+500000000
+const b = a * a // ERROR "constant multiplication overflow"
+const c = b * b
+
+const MaxInt512 = (1<<256 - 1) * (1<<256 + 1)
+const _ = MaxInt512 + 1  // ERROR "constant addition overflow"
+const _ = MaxInt512 ^ -1 // ERROR "constant bitwise XOR overflow"
+const _ = ^MaxInt512     // ERROR "constant bitwise complement overflow"

test/fixedbugs/issue20232.go

@@ -6,6 +6,7 @@
 
 package main
 
-const _ = 6e5518446744 // ERROR "malformed constant: 6e5518446744"
+const x = 6e5518446744 // ERROR "malformed constant: 6e5518446744"
+const _ = x * x
 const _ = 1e-1000000000
-const _ = 1e+1000000000
+const _ = 1e+1000000000 // ERROR "malformed constant: 1e\+1000000000"