[PATCH] skip testing time before epoch on mips

[Chen Li]

When execute libstdc++ testcases on mips, I notice that last_write_time
alawys failed, and the failed VERIFY is  "VERIFY(
approx_equal(last_write_time(f.path), time) );" where testing time before
than epoch.

Below is the minimal case:

```
// gcc a.c
int main()
{
        struct timespec times[2] = {{1, UTIME_OMIT}, {-1201, 985000000}};
        utimensat(AT_FDCWD, "test", times, 0);

}
```

$ touch test && gcc a.c && ./a.out && stat test
  File: test
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 805h/2053d      Inode: 1056841     Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/  deepin)   Gid: ( 1000/  deepin)
Access: 2021-03-08 13:52:55.966354501 +0800
Modify: 2106-02-07 14:08:15.985000000 +0800
Change: 2021-03-08 13:52:56.907782193 +0800
 Birth: -

Undoubtedly, mtime's type is unsigned somewhere on mips.

After debuging kernel, it turns out that mtime is always -1201 in
ext4_setattr, cp_new_stat, newlstat and etc, so the problem should not
occur in kernel space.

go back to user space via copy_to_user, I finally found mips used
"unsigned int st_mtime_sec;" in struct kernel_stat, which is used to
receive -1201 from kernel.

Maybe sparc also suffers from this problem, but I have no machine to
verify.
---
 .../testsuite/27_io/filesystem/operations/last_write_time.cc    | 2 ++
 .../experimental/filesystem/operations/last_write_time.cc       | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/libstdc++-v3/testsuite/27_io/filesystem/operations/last_write_time.cc b/libstdc++-v3/testsuite/27_io/filesystem/operations/last_write_time.cc
index 6ae64c482b6..72272d4e781 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/operations/last_write_time.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/operations/last_write_time.cc
@@ -200,11 +200,13 @@ test02()
   VERIFY( approx_equal(last_write_time(f.path), time) );

   ec = bad_ec;
+  #ifndef __mips__
   // A time before than the epoch
   time -= chrono::milliseconds(1000 * 60 * 20 + 15);
   last_write_time(f.path, time, ec);
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );
+  #endif
 }

 int
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/operations/last_write_time.cc b/libstdc++-v3/testsuite/experimental/filesystem/operations/last_write_time.cc
index 536ab1f28f6..6fb490a6bc3 100644
--- a/libstdc++-v3/testsuite/experimental/filesystem/operations/last_write_time.cc
+++ b/libstdc++-v3/testsuite/experimental/filesystem/operations/last_write_time.cc
@@ -165,11 +165,13 @@ test02()
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );

+  #ifndef __mips__
   ec = bad_ec;
   time -= chrono::milliseconds(1000 * 60 * 10 + 15);
   last_write_time(f.path, time, ec);
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );
+  #endif
 }

 int
--
2.30.0