pru: Skip register save if function will not return

There is no need to store callee-saved registers in prologue if the
function would never return.  Size optimization is paramount for the
microcontroller-class PRU.

Some backends save some registers for noreturn functions.  But for PRU
debuggability is a less concern because GDB has not been ported yet
for PRU.

gcc/ChangeLog:

	* config/pru/pru.cc (prologue_saved_reg_p): Skip saving
	if function will not return.

gcc/testsuite/ChangeLog:

	* gcc.target/pru/noreturn-prologue-1.c: New test.
	* gcc.target/pru/noreturn-prologue-2.c: New test.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

3 files changed, 25 insertions, 0 deletions

diff --git a/gcc/config/pru/pru.cc b/gcc/config/pru/pru.cc
index a76451f4..e5ec398 100644
--- a/gcc/config/pru/pru.cc
+++ b/gcc/config/pru/pru.cc
@@ -443,6 +443,10 @@ prologue_saved_reg_p (int regno)
 {
   gcc_assert (GP_REG_P (regno));
 
+  /* Do not save the register if function will not return.  */
+  if (TREE_THIS_VOLATILE (current_function_decl))
+    return false;
+
   if (df_regs_ever_live_p (regno) && !call_used_or_fixed_reg_p (regno))
     return true;
 
diff --git a/gcc/testsuite/gcc.target/pru/noreturn-prologue-1.c b/gcc/testsuite/gcc.target/pru/noreturn-prologue-1.c
new file mode 100644
index 0000000..af69e52
--- /dev/null
+++ b/gcc/testsuite/gcc.target/pru/noreturn-prologue-1.c
@@ -0,0 +1,10 @@
+/* Ensure prologues are not generated for noreturn functions.  */
+/* { dg-do assemble } */
+/* { dg-options "-Os" } */
+/* { dg-final { object-size text == 0 } } */
+
+void test(void)
+{
+  asm volatile ("# \n\t" : : : "r5", "r10");
+  __builtin_unreachable ();
+}
diff --git a/gcc/testsuite/gcc.target/pru/noreturn-prologue-2.c b/gcc/testsuite/gcc.target/pru/noreturn-prologue-2.c
new file mode 100644
index 0000000..8d12a9c
--- /dev/null
+++ b/gcc/testsuite/gcc.target/pru/noreturn-prologue-2.c
@@ -0,0 +1,11 @@
+/* Ensure prologues are not generated for noreturn functions.  */
+/* { dg-do assemble } */
+/* { dg-options "-Os" } */
+/* { dg-final { object-size text == 4 } } */
+
+void test(void)
+{
+  asm volatile ("# \n\t" : : : "r0", "r9");
+  for (;;)
+    ;
+}