[release-branch.go1.16] cmd/compile: don't elide extension for LoadReg to FP register on MIPS64

For an extension operation like MOVWreg, 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.

Updates #50671.
Fixes #50682.

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>
(cherry picked from commit d93ff73ae207763871bee38590242be968b2e743)
Reviewed-on: https://go-review.googlesource.com/c/go/+/379515

1 files changed, 35 insertions, 0 deletions

diff --git a/test/fixedbugs/issue50671.go b/test/fixedbugs/issue50671.go
new file mode 100644
index 0000000000..9f4742bfcd
--- /dev/null
+++ b/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")
+	}
+}