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/18590Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

+// 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)
+	}
+}

src/cmd/compile/internal/x86/ggen.go

@@ -764,9 +764,7 @@ func bgen_float(n *gc.Node, wantTrue bool, likely int, to *obj.Prog) {
 				gc.Cgen(nr, &tmp)
 				gc.Cgen(nl, &tmp)
 			}
-
-			gins(x86.AFUCOMIP, &tmp, &n2)
-			gins(x86.AFMOVDP, &tmp, &tmp) // annoying pop but still better than STSW+SAHF
+			gins(x86.AFUCOMPP, &tmp, &n2)
 		} else {
 			// TODO(rsc): The moves back and forth to memory
 			// here are for truncating the value to 32 bits.
@@ -783,9 +781,9 @@ func bgen_float(n *gc.Node, wantTrue bool, likely int, to *obj.Prog) {
 			gc.Cgen(nl, &t2)
 			gmove(&t2, &tmp)
 			gins(x86.AFCOMFP, &t1, &tmp)
+		}
 		gins(x86.AFSTSW, nil, &ax)
 		gins(x86.ASAHF, nil, nil)
-		}
 	} else {
 		// Not 387
 		if !nl.Addable {

src/cmd/compile/internal/x86/gsubr.go

@@ -1198,14 +1198,17 @@ func floatmove(f *gc.Node, t *gc.Node) {
 
 		// if 0 > v { answer = 0 }
 		gins(x86.AFMOVD, &zerof, &f0)
-
-		gins(x86.AFUCOMIP, &f0, &f1)
+		gins(x86.AFUCOMP, &f0, &f1)
+		gins(x86.AFSTSW, nil, &ax)
+		gins(x86.ASAHF, nil, nil)
 		p1 := gc.Gbranch(optoas(gc.OGT, gc.Types[tt]), nil, 0)
 
 		// if 1<<64 <= v { answer = 0 too }
 		gins(x86.AFMOVD, &two64f, &f0)
 
-		gins(x86.AFUCOMIP, &f0, &f1)
+		gins(x86.AFUCOMP, &f0, &f1)
+		gins(x86.AFSTSW, nil, &ax)
+		gins(x86.ASAHF, nil, nil)
 		p2 := gc.Gbranch(optoas(gc.OGT, gc.Types[tt]), nil, 0)
 		gc.Patch(p1, gc.Pc)
 		gins(x86.AFMOVVP, &f0, t) // don't care about t, but will pop the stack
@@ -1235,7 +1238,9 @@ func floatmove(f *gc.Node, t *gc.Node) {
 		// actual work
 		gins(x86.AFMOVD, &two63f, &f0)
 
-		gins(x86.AFUCOMIP, &f0, &f1)
+		gins(x86.AFUCOMP, &f0, &f1)
+		gins(x86.AFSTSW, nil, &ax)
+		gins(x86.ASAHF, nil, nil)
 		p2 = gc.Gbranch(optoas(gc.OLE, gc.Types[tt]), nil, 0)
 		gins(x86.AFMOVVP, &f0, t)
 		p3 := gc.Gbranch(obj.AJMP, nil, 0)

src/cmd/compile/internal/x86/prog.go

@@ -91,7 +91,11 @@ var progtable = [x86.ALAST]obj.ProgInfo{
 	x86.AFCOMDPP:   {Flags: gc.SizeD | gc.LeftAddr | gc.RightRead},
 	x86.AFCOMF:     {Flags: gc.SizeF | gc.LeftAddr | gc.RightRead},
 	x86.AFCOMFP:    {Flags: gc.SizeF | gc.LeftAddr | gc.RightRead},
-	x86.AFUCOMIP:   {Flags: gc.SizeF | gc.LeftAddr | gc.RightRead},
+	// NOTE(khr): don't use FUCOMI* instructions, not available
+	// on Pentium MMX.  See issue 13923.
+	//x86.AFUCOMIP:   {Flags: gc.SizeF | gc.LeftAddr | gc.RightRead},
+	x86.AFUCOMP:  {Flags: gc.SizeD | gc.LeftRead | gc.RightRead},
+	x86.AFUCOMPP: {Flags: gc.SizeD | gc.LeftRead | gc.RightRead},
 	x86.AFCHS:    {Flags: gc.SizeD | RightRdwr}, // also SizeF
 
 	x86.AFDIVDP:  {Flags: gc.SizeD | gc.LeftAddr | RightRdwr},