go/internal/gcimporter: adjust importer to match compiler importer

This is a port of CL 288632 to x/tools/go/internal/gcimporter. This
logic was ostensibly unported to avoid breaking build compatibility with
go1.12. Now that gopls no longer support 1.12, It is safe to make this
change.

Fixes golang/go#53803

Change-Id: Ic9b4d7a60511076a83d8fa72cf7c4a3bdcab3fce
Reviewed-on: https://go-review.googlesource.com/c/tools/+/417580
Reviewed-by: Jamal Carvalho <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Robert Findley <[email protected]>

Author: findleyr

go/internal/gcimporter/iimport.go

@@ -17,6 +17,7 @@ import (
 	"go/token"
 	"go/types"
 	"io"
+	"math/big"
 	"sort"
 	"strings"
 
@@ -512,7 +513,9 @@ func (r *importReader) value() (typ types.Type, val constant.Value) {
 		val = constant.MakeString(r.string())
 
 	case types.IsInteger:
-		val = r.mpint(b)
+		var x big.Int
+		r.mpint(&x, b)
+		val = constant.Make(&x)
 
 	case types.IsFloat:
 		val = r.mpfloat(b)
@@ -561,8 +564,8 @@ func intSize(b *types.Basic) (signed bool, maxBytes uint) {
 	return
 }
 
-func (r *importReader) mpint(b *types.Basic) constant.Value {
-	signed, maxBytes := intSize(b)
+func (r *importReader) mpint(x *big.Int, typ *types.Basic) {
+	signed, maxBytes := intSize(typ)
 
 	maxSmall := 256 - maxBytes
 	if signed {
@@ -581,7 +584,8 @@ func (r *importReader) mpint(b *types.Basic) constant.Value {
 				v = ^v
 			}
 		}
-		return constant.MakeInt64(v)
+		x.SetInt64(v)
+		return
 	}
 
 	v := -n
@@ -591,47 +595,23 @@ func (r *importReader) mpint(b *types.Basic) constant.Value {
 	if v < 1 || uint(v) > maxBytes {
 		errorf("weird decoding: %v, %v => %v", n, signed, v)
 	}
-
-	buf := make([]byte, v)
-	io.ReadFull(&r.declReader, buf)
-
-	// convert to little endian
-	// TODO(gri) go/constant should have a more direct conversion function
-	//           (e.g., once it supports a big.Float based implementation)
-	for i, j := 0, len(buf)-1; i < j; i, j = i+1, j-1 {
-		buf[i], buf[j] = buf[j], buf[i]
-	}
-
-	x := constant.MakeFromBytes(buf)
+	b := make([]byte, v)
+	io.ReadFull(&r.declReader, b)
+	x.SetBytes(b)
 	if signed && n&1 != 0 {
-		x = constant.UnaryOp(token.SUB, x, 0)
+		x.Neg(x)
 	}
-	return x
 }
 
-func (r *importReader) mpfloat(b *types.Basic) constant.Value {
-	x := r.mpint(b)
-	if constant.Sign(x) == 0 {
-		return x
-	}
-
-	exp := r.int64()
-	switch {
-	case exp > 0:
-		x = constant.Shift(x, token.SHL, uint(exp))
-		// Ensure that the imported Kind is Float, else this constant may run into
-		// bitsize limits on overlarge integers. Eventually we can instead adopt
-		// the approach of CL 288632, but that CL relies on go/constant APIs that
-		// were introduced in go1.13.
-		//
-		// TODO(rFindley): sync the logic here with tip Go once we no longer
-		// support go1.12.
-		x = constant.ToFloat(x)
-	case exp < 0:
-		d := constant.Shift(constant.MakeInt64(1), token.SHL, uint(-exp))
-		x = constant.BinaryOp(x, token.QUO, d)
+func (r *importReader) mpfloat(typ *types.Basic) constant.Value {
+	var mant big.Int
+	r.mpint(&mant, typ)
+	var f big.Float
+	f.SetInt(&mant)
+	if f.Sign() != 0 {
+		f.SetMantExp(&f, int(r.int64()))
 	}
-	return x
+	return constant.Make(&f)
 }
 
 func (r *importReader) ident() string {