cmd/compile: don't elide extension for LoadReg to FP register on MIPS64

For an extension operation like MOWWreg, if the operand is already
extended, we optimize the second extension out. Usually a LoadReg
of a proper type would come already extended, as a MOVW/MOVWU etc.
instruction does. But for a LoadReg to a floating point register,
the instruction does not do the extension. So we cannot elide the
extension.

Fixes #50671.

Change-Id: Id8991df78d5acdecd3fd6138c558428cbd5f6ba3
Reviewed-on: https://go-review.googlesource.com/c/go/+/379236
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/mips64/ssa.go

@@ -320,7 +320,10 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		for a.Op == ssa.OpCopy || a.Op == ssa.OpMIPS64MOVVreg {
 			a = a.Args[0]
 		}
-		if a.Op == ssa.OpLoadReg {
+		if a.Op == ssa.OpLoadReg && mips.REG_R0 <= a.Reg() && a.Reg() <= mips.REG_R31 {
+			// LoadReg from a narrower type does an extension, except loading
+			// to a floating point register. So only eliminate the extension
+			// if it is loaded to an integer register.
 			t := a.Type
 			switch {
 			case v.Op == ssa.OpMIPS64MOVBreg && t.Size() == 1 && t.IsSigned(),

test/fixedbugs/issue50671.go

@@ -0,0 +1,35 @@
+// run
+
+// Copyright 2022 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 50671: sign extension eliminated incorrectly on MIPS64.
+
+package main
+
+//go:noinline
+func F(x int32) (float64, int64) {
+	a := float64(x)
+	b := int64(x)
+	return a, b
+}
+
+var a, b, c float64
+
+// Poison some floating point registers with non-zero high bits.
+//
+//go:noinline
+func poison(x float64) {
+	a = x - 123.45
+	b = a * 1.2
+	c = b + 3.4
+}
+
+func main() {
+	poison(333.3)
+	_, b := F(123)
+	if b != 123 {
+		panic("FAIL")
+	}
+}