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>

diff --git a/src/cmd/compile/internal/mips64/ssa.go b/src/cmd/compile/internal/mips64/ssa.go
index 990b9788f765e39974873f07f544b218b171df6f..6e12c6cb942d6873d61f43aeba4efed0f20183cd 100644 (file)
--- a/src/cmd/compile/internal/mips64/ssa.go
+++ b/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(),

diff --git a/test/fixedbugs/issue50671.go b/test/fixedbugs/issue50671.go
new file mode 100644 (file)
index 0000000..9f4742b
--- /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")
+       }
+}