amdgcn: Replace asm("s8") by __builtin_gcn_kernarg_ptr if existing

Check whether __builtin_gcn_kernarg_ptr is available and, if it is,
call it instead using the hard-coded 'asm("s8")' in:
* newlib/libc/machine/amdgcn/exit-value.h (exit_with_int)
* newlib/libc/machine/amdgcn/mlock.c (sbrk)
* newlib/libc/sys/amdgcn/write.c (write)

 newlib/libc/machine/amdgcn/exit-value.h |  6 ++++++
 newlib/libc/machine/amdgcn/mlock.c      | 10 +++++++---
 newlib/libc/sys/amdgcn/write.c          |  4 ++++
 3 files changed, 17 insertions(+), 3 deletions(-)

2 files changed, 13 insertions, 3 deletions

diff --git a/newlib/libc/machine/amdgcn/exit-value.h b/newlib/libc/machine/amdgcn/exit-value.h
index 6e88625b7..7aa2508bb 100644
--- a/newlib/libc/machine/amdgcn/exit-value.h
+++ b/newlib/libc/machine/amdgcn/exit-value.h
@@ -21,8 +21,14 @@ exit_with_int (int val)
 {
   /* Write the exit value to the conventional place.  */
   int *return_value;
+#if defined(__has_builtin) && __has_builtin(__builtin_gcn_kernarg_ptr)
+  asm ("s_load_dwordx2	%0, %1, 16 glc\n\t"
+       "s_waitcnt	0"
+       : "=Sg"(return_value) : "r"(__builtin_gcn_kernarg_ptr()));
+#else
   asm ("s_load_dwordx2	%0, s[8:9], 16 glc\n\t"
        "s_waitcnt	0" : "=Sg"(return_value));
+#endif
   *return_value = val;
 
   /* Terminate the current kernel.  */
diff --git a/newlib/libc/machine/amdgcn/mlock.c b/newlib/libc/machine/amdgcn/mlock.c
index 4848c978c..fbc4944e6 100644
--- a/newlib/libc/machine/amdgcn/mlock.c
+++ b/newlib/libc/machine/amdgcn/mlock.c
@@ -39,11 +39,15 @@ sbrk (ptrdiff_t nbytes)
 {
   if (__heap_ptr == (char *)-1)
     {
-      /* Find the heap from kernargs.
-         The kernargs pointer is in s[8:9].
-	 This will break if the enable_sgpr_* flags are ever changed.  */
+      /* Find the heap from kernargs.  */
       char *kernargs;
+#if defined(__has_builtin) && __has_builtin(__builtin_gcn_kernarg_ptr)
+      kernargs = __builtin_gcn_kernarg_ptr ();
+#else
+      /* The kernargs pointer is in s[8:9].
+	 This will break if the enable_sgpr_* flags are ever changed.  */
       asm ("s_mov_b64 %0, s[8:9]" : "=Sg"(kernargs));
+#endif
 
       /* The heap data is at kernargs[3].  */
       struct heap *heap = *(struct heap **)(kernargs + 24);