cmd/compile: truncate constant arithmetic result with typed complex numbers

Fixes #33285

Change-Id: Idd125e3342058051216be3f105330aef987320c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/187697
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

2 files changed, 48 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/gc/const.go b/src/cmd/compile/internal/gc/const.go
index 504f8f0ec3..6d39417ba5 100644
--- a/src/cmd/compile/internal/gc/const.go
+++ b/src/cmd/compile/internal/gc/const.go
@@ -1042,9 +1042,15 @@ func setconst(n *Node, v Val) {
 	overflow(v, n.Type)
 	lineno = lno
 
-	// Truncate precision for non-ideal float.
-	if v.Ctype() == CTFLT && n.Type.Etype != TIDEAL {
-		n.SetVal(Val{truncfltlit(v.U.(*Mpflt), n.Type)})
+	if !n.Type.IsUntyped() {
+		switch v.Ctype() {
+		// Truncate precision for non-ideal float.
+		case CTFLT:
+			n.SetVal(Val{truncfltlit(v.U.(*Mpflt), n.Type)})
+		// Truncate precision for non-ideal complex.
+		case CTCPLX:
+			n.SetVal(Val{trunccmplxlit(v.U.(*Mpcplx), n.Type)})
+		}
 	}
 }
 
diff --git a/test/const.go b/test/const.go
index f8e0a753cb..3f4956497e 100644
--- a/test/const.go
+++ b/test/const.go
@@ -157,10 +157,49 @@ func interfaces() {
 		"for interface{}==int comipiler == runtime")
 }
 
+// Test that typed floating-point and complex arithmetic
+// is computed with correct precision.
+func truncate() {
+	const (
+		x30 = 1 << 30
+		x60 = 1 << 60
+
+		staticF32 = float32(x30) + 1 - x30
+		staticF64 = float64(x60) + 1 - x60
+		staticC64 = complex64(x30) + 1 - x30
+		staticC128 = complex128(x60) + 1 - x60
+	)
+	dynamicF32 := float32(x30)
+	dynamicF32 += 1
+	dynamicF32 -= x30
+
+	dynamicF64 := float64(x60)
+	dynamicF64 += 1
+	dynamicF64 -= x60
+
+	dynamicC64 := complex64(x30)
+	dynamicC64 += 1
+	dynamicC64 -= x30
+
+	dynamicC128 := complex128(x60)
+	dynamicC128 += 1
+	dynamicC128 -= x60
+
+	assert(staticF32 == 0, "staticF32 == 0")
+	assert(staticF64 == 0, "staticF64 == 0")
+	assert(dynamicF32 == 0, "dynamicF32 == 0")
+	assert(dynamicF64 == 0, "dynamicF64 == 0")
+	assert(staticC64 == 0, "staticC64 == 0")
+	assert(staticC128 == 0, "staticC128 == 0")
+	assert(dynamicC64 == 0, "dynamicC64 == 0")
+	assert(dynamicC128 == 0, "dynamicC128 == 0")
+}
+
 func main() {
 	ints()
 	floats()
 	interfaces()
+	truncate()
 
 	assert(ctrue == true, "ctrue == true")
 	assert(cfalse == false, "cfalse == false")