syscall: avoid convT2I allocs for common Windows error values

This is was already done for Unix in https://golang.org/cl/6701 +
https://golang.org/cl/8192. Do it for Windows also.

Fixes #16988

Change-Id: Ia7832b0d0d48566b0cd205652b85130df529592e
Reviewed-on: https://go-review.googlesource.com/28484
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/internal/syscall/windows/registry/zsyscall_windows.go

@@ -10,6 +10,31 @@ import (
 
 var _ unsafe.Pointer
 
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoWSAEINPROGRESS = 10036
+)
+
+var (
+	errWSAEINPROGRESS error = syscall.Errno(errnoWSAEINPROGRESS)
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case errnoWSAEINPROGRESS:
+		return errWSAEINPROGRESS
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
 var (
 	modadvapi32 = syscall.NewLazyDLL(sysdll.Add("advapi32.dll"))
 	modkernel32 = syscall.NewLazyDLL(sysdll.Add("kernel32.dll"))
@@ -76,7 +101,7 @@ func expandEnvironmentStrings(src *uint16, dst *uint16, size uint32) (n uint32,
 	n = uint32(r0)
 	if n == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}

src/internal/syscall/windows/zsyscall_windows.go

@@ -10,6 +10,31 @@ import (
 
 var _ unsafe.Pointer
 
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoWSAEINPROGRESS = 10036
+)
+
+var (
+	errWSAEINPROGRESS error = syscall.Errno(errnoWSAEINPROGRESS)
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case errnoWSAEINPROGRESS:
+		return errWSAEINPROGRESS
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
 var (
 	modiphlpapi = syscall.NewLazyDLL(sysdll.Add("iphlpapi.dll"))
 	modkernel32 = syscall.NewLazyDLL(sysdll.Add("kernel32.dll"))
@@ -33,7 +58,7 @@ func GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) {
 	r1, _, e1 := syscall.Syscall(procGetComputerNameExW.Addr(), 3, uintptr(nameformat), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(n)))
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -45,7 +70,7 @@ func MoveFileEx(from *uint16, to *uint16, flags uint32) (err error) {
 	r1, _, e1 := syscall.Syscall(procMoveFileExW.Addr(), 3, uintptr(unsafe.Pointer(from)), uintptr(unsafe.Pointer(to)), uintptr(flags))
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -64,7 +89,7 @@ func MultiByteToWideChar(codePage uint32, dwFlags uint32, str *byte, nstr int32,
 	nwrite = int32(r0)
 	if nwrite == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}

src/syscall/mksyscall_windows.go

@@ -281,7 +281,7 @@ func (r *Rets) SetReturnValuesCode() string {
 func (r *Rets) useLongHandleErrorCode(retvar string) string {
 	const code = `if %s {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = %sEINVAL
 		}
@@ -829,6 +829,31 @@ import (
 
 var _ unsafe.Pointer
 
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoWSAEINPROGRESS = 10036
+)
+
+var (
+	errWSAEINPROGRESS error = {{syscalldot}}Errno(errnoWSAEINPROGRESS)
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e {{syscalldot}}Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case errnoWSAEINPROGRESS:
+		return errWSAEINPROGRESS
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
 var (
 {{template "dlls" .}}
 {{template "funcnames" .}})