Fix pipe syscall for MIPS

Author: LemonBoy

lib/std/os/linux.zig

@@ -328,7 +328,9 @@ pub fn faccessat(dirfd: i32, path: [*]const u8, mode: u32, flags: u32) usize {
 }
 
 pub fn pipe(fd: *[2]i32) usize {
-    if (@hasDecl(@This(), "SYS_pipe")) {
+    if (builtin.arch == .mipsel) {
+        return syscall_pipe(fd);
+    } else if (@hasDecl(@This(), "SYS_pipe")) {
         return syscall1(SYS_pipe, @ptrToInt(fd));
     } else {
         return syscall2(SYS_pipe2, @ptrToInt(fd), 0);

lib/std/os/linux/mipsel.zig

@@ -12,6 +12,25 @@ pub fn syscall0(number: usize) usize {
     );
 }
 
+pub fn syscall_pipe(fd: *[2]i32) usize {
+    return asm volatile (
+        \\ .set noat
+        \\ .set noreorder
+        \\ syscall
+        \\ blez $7, 1f
+        \\ nop
+        \\ b 2f
+        \\ subu $2, $0, $2
+        \\ 1:
+        \\ sw $2, 0($4)
+        \\ sw $3, 4($4)
+        \\ 2:
+        : [ret] "={$2}" (-> usize)
+        : [number] "{$2}" (usize(SYS_pipe))
+        : "memory", "cc", "$7"
+    );
+}
+
 pub fn syscall1(number: usize, arg1: usize) usize {
     return asm volatile (
         \\ syscall

lib/std/os/test.zig

@@ -219,3 +219,16 @@ test "gethostname" {
     const hostname = try os.gethostname(&buf);
     expect(hostname.len != 0);
 }
+
+test "pipe" {
+    if (os.windows.is_the_target)
+        return error.SkipZigTest;
+
+    var fds = try os.pipe();
+    try os.write(fds[1], "hello");
+    var buf: [16]u8 = undefined;
+    expect((try os.read(fds[0], buf[0..])) == 5);
+    testing.expectEqualSlices(u8, buf[0..5], "hello");
+    os.close(fds[1]);
+    os.close(fds[0]);
+}