cmd/6g: fix float32/64->uint64 conversion

    CVTSS2SQ's rounding mode is controlled by the RC field of MXCSR;
as we specifically need truncate semantic, we should use CVTTSS2SQ.

    Fixes #3804.

R=rsc, r
CC=golang-dev
https://golang.org/cl/6352079

src/cmd/6g/gsubr.c

@@ -796,9 +796,9 @@ gmove(Node *f, Node *t)
 		// algorithm is:
 		//	if small enough, use native float64 -> int64 conversion.
 		//	otherwise, subtract 2^63, convert, and add it back.
-		a = ACVTSS2SQ;
+		a = ACVTTSS2SQ;
 		if(ft == TFLOAT64)
-			a = ACVTSD2SQ;
+			a = ACVTTSD2SQ;
 		bignodes();
 		regalloc(&r1, types[ft], N);
 		regalloc(&r2, types[tt], t);

test/fixedbugs/bug447.go

+// runoutput
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 3804
+// test all possible float -> integer conversions
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+var (
+	intWidths = []int{8, 16, 32, 64} // int%d and uint%d
+	floatWidths = []int{32, 64} // float%d
+)
+
+func main() {
+
+	var names, funcs bytes.Buffer
+
+	for _, iWidth := range intWidths {
+		for _, typ := range []string{"int", "uint"} {
+			var segs bytes.Buffer
+			itype := fmt.Sprintf("%s%d", typ, iWidth)
+			names.WriteString("\ttest" + itype + ",\n")
+			for _, fWidth := range floatWidths {
+				ftype := fmt.Sprintf("float%d", fWidth)
+				seg := strings.Replace(testSegment, "$F", ftype, -1)
+				seg = strings.Replace(seg, "$I", itype, -1)
+				segs.WriteString(seg)
+			}
+			body := strings.Replace(testFunc, "$I", itype, -1)
+			if typ[0] == 'u' {
+				body = strings.Replace(body, "$TEST", " || i < 0", 1)
+			} else {
+				body = strings.Replace(body, "$TEST", "", 1)
+			}
+			body = strings.Replace(body, "$TESTSEGMENTS", segs.String(), 1)
+			funcs.WriteString(body)
+		}
+	}
+
+	program = strings.Replace(program, "$NAMES", names.String(), 1)
+	program = strings.Replace(program, "$FUNCS", funcs.String(), 1)
+	fmt.Print(program)
+}
+
+const testSegment = `
+	f$F := $F(f)
+	if math.Abs(float64(f$F) - f) < 0.05 {
+		if v := $I(f$F); v != $I(i) {
+			fmt.Printf("$I($F(%f)) = %v, expected %v\n", f, v, i)
+		}
+	}`
+
+const testFunc =
+`func test$I(f float64, i int64) {
+	if i != int64($I(i))$TEST {
+		return
+	}
+$TESTSEGMENTS
+}
+`
+
+var program =
+`package main
+
+import (
+	"fmt"
+	"math"
+)
+
+var tests = []struct {
+	f float64
+	i int64
+}{
+	{39.7, 39},
+	{-39.7, -39},
+	{258.6, 258},
+	{-258.6, -258},
+	{65538.9, 65538},
+	{-65538.9, -65538},
+	{4294967298.8, 4294967298},
+	{-4294967298.8, -4294967298},
+}
+
+var funcs = []func(float64, int64){
+$NAMES
+}
+
+$FUNCS
+func main() {
+	for _, t := range tests {
+		for _, f := range funcs {
+			f(t.f, t.i)
+		}
+	}
+}
+`