[PATCH] nptl: Add compiler barrier in nptl/tst-pthread-getattr

[Florian Weimer <fweimer at redhat dot com>]

Recent GCC versions warn about the attempt to return the address of a
local variable:

tst-pthread-getattr.c: In function ‘allocate_and_test’:
tst-pthread-getattr.c:54:10: error: function returns address of local variable [-Werror=return-local-addr]
   54 |   return mem;
      |          ^~~
In file included from ../include/alloca.h:3,
                 from tst-pthread-getattr.c:26:
../stdlib/alloca.h:35:23: note: declared here
   35 | # define alloca(size) __builtin_alloca (size)
      |                       ^~~~~~~~~~~~~~~~~~~~~~~
tst-pthread-getattr.c:51:9: note: in expansion of macro ‘alloca’
   51 |   mem = alloca ((size_t) (mem - target));
      |         ^~~~~~

2019-07-22  Florian Weimer  <fweimer@redhat.com>

	* nptl/tst-pthread-getattr.c (compiler_barrier): New function.
	(allocate_and_test): Use it.

diff --git a/nptl/tst-pthread-getattr.c b/nptl/tst-pthread-getattr.c
index a954778767..a11a25437d 100644
--- a/nptl/tst-pthread-getattr.c
+++ b/nptl/tst-pthread-getattr.c
@@ -41,6 +41,15 @@
 
 static size_t pagesize;
 
+/* The weak attribute marks this function as interposable, so the
+   compiler cannot assume that the pointer remains unchanged.  */
+__attribute__ ((noinline, noclone,  weak))
+volatile char *
+compiler_barrier (volatile char *ptr)
+{
+  return ptr;
+}
+
 /* Check if the page in which TARGET lies is accessible.  This will segfault
    if it fails.  */
 static volatile char *
@@ -51,7 +60,10 @@ allocate_and_test (char *target)
   mem = alloca ((size_t) (mem - target));
 
   *mem = 42;
-  return mem;
+
+  /* Insert the compiler barrier to hide the fact that mem is
+     stack-allocated.  The address is only used in a diagnostic.  */
+  return compiler_barrier (mem);
 }
 
 static int