cmd/compile: stop using fucomi* ops for 387 builds

The fucomi* opcodes were only introduced for the Pentium Pro.
They do not exist for an MMX Pentium.  Use the fucom* instructions
instead and move the condition codes from the fp flags register to
the integer flags register explicitly.

The use of fucomi* opcodes in ggen.go was introduced in 1.5 (CL 8738).
The bad ops were generated for 64-bit floating-point comparisons.

The use of fucomi* opcodes in gsubr.go dates back to at least 1.1.
The bad ops were generated for float{32,64} to uint64 conversions.

Fixes #13923

Change-Id: I5290599f5edea8abf8fb18036f44fa78bd1fc9e6
Reviewed-on: https://go-review.googlesource.com/18590
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -0,0 +1,102 @@
+// Copyright 2016 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.
+
+package gc
+
+import "testing"
+
+// For GO386=387, make sure fucomi* opcodes are not used
+// for comparison operations.
+// Note that this test will fail only on a Pentium MMX
+// processor (with GOARCH=386 GO386=387), as it just runs
+// some code and looks for an unimplemented instruction fault.
+
+//go:noinline
+func compare1(a, b float64) bool {
+	return a < b
+}
+
+//go:noinline
+func compare2(a, b float32) bool {
+	return a < b
+}
+
+func TestFloatCompare(t *testing.T) {
+	if !compare1(3, 5) {
+		t.Errorf("compare1 returned false")
+	}
+	if !compare2(3, 5) {
+		t.Errorf("compare2 returned false")
+	}
+}
+
+// For GO386=387, make sure fucomi* opcodes are not used
+// for float->int conversions.
+
+//go:noinline
+func cvt1(a float64) uint64 {
+	return uint64(a)
+}
+
+//go:noinline
+func cvt2(a float64) uint32 {
+	return uint32(a)
+}
+
+//go:noinline
+func cvt3(a float32) uint64 {
+	return uint64(a)
+}
+
+//go:noinline
+func cvt4(a float32) uint32 {
+	return uint32(a)
+}
+
+//go:noinline
+func cvt5(a float64) int64 {
+	return int64(a)
+}
+
+//go:noinline
+func cvt6(a float64) int32 {
+	return int32(a)
+}
+
+//go:noinline
+func cvt7(a float32) int64 {
+	return int64(a)
+}
+
+//go:noinline
+func cvt8(a float32) int32 {
+	return int32(a)
+}
+
+func TestFloatConvert(t *testing.T) {
+	if got := cvt1(3.5); got != 3 {
+		t.Errorf("cvt1 got %d, wanted 3", got)
+	}
+	if got := cvt2(3.5); got != 3 {
+		t.Errorf("cvt2 got %d, wanted 3", got)
+	}
+	if got := cvt3(3.5); got != 3 {
+		t.Errorf("cvt3 got %d, wanted 3", got)
+	}
+	if got := cvt4(3.5); got != 3 {
+		t.Errorf("cvt4 got %d, wanted 3", got)
+	}
+	if got := cvt5(3.5); got != 3 {
+		t.Errorf("cvt5 got %d, wanted 3", got)
+	}
+	if got := cvt6(3.5); got != 3 {
+		t.Errorf("cvt6 got %d, wanted 3", got)
+	}
+	if got := cvt7(3.5); got != 3 {
+		t.Errorf("cvt7 got %d, wanted 3", got)
+	}
+	if got := cvt8(3.5); got != 3 {
+		t.Errorf("cvt8 got %d, wanted 3", got)
+	}
+}