Set TARGET_OMIT_STRUCT_RETURN_REG to true

The reason this caught my eye on aarch64 is because
the return value register (x0) is not identical to the register in which
the hidden parameter for AArch64 is set (x8). Thus setting this to true
seems to be quite reasonable and shaves off 100 odd mov x0, x8's from
cc1 in a bootstrap build.

I don't expect this to make a huge impact on performance but as they
say every little counts.  The AAPCS64 is quite explicit about not
requiring that the contents of x8 be kept live.

Bootstrapped and regression tested on aarch64.

Ok to apply ?

Ramana

gcc/
* config/aarch64/aarch64.c (TARGET_OMIT_STRUCT_RETURN_REG): Set to
true.

gcc/testsuite

* gcc.target/aarch64/struct_return.c: New test.

From-SVN: r236197

gcc/ChangeLog

+2016-05-13  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
+
+	* config/aarch64/aarch64.c (TARGET_OMIT_STRUCT_RETURN_REG): Set
+	to true.
+
 2016-05-13  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	PR target/71080

gcc/config/aarch64/aarch64.c

@@ -14232,6 +14232,9 @@ aarch64_optab_supported_p (int op, machine_mode, machine_mode,
 #undef TARGET_OPTAB_SUPPORTED_P
 #define TARGET_OPTAB_SUPPORTED_P aarch64_optab_supported_p
 
+#undef TARGET_OMIT_STRUCT_RETURN_REG
+#define TARGET_OMIT_STRUCT_RETURN_REG true
+
 struct gcc_target targetm = TARGET_INITIALIZER;
 
 #include "gt-aarch64.h"

gcc/testsuite/ChangeLog

+2016-05-13  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
+
+	* gcc.target/aarch64/struct_return.c: New test.
+
 2016-05-12  Marek Polacek  <polacek@redhat.com>
 
 	PR c/70756

gcc/testsuite/gcc.target/aarch64/struct_return.c

+/* Test the absence of a spurious move from x8 to x0 for functions
+   return structures.  */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+struct s
+{
+  long x;
+  long y;
+  long z;
+};
+
+struct s __attribute__((noinline))
+foo (long a, long d, long c)
+{
+  struct s b;
+  b.x = a;
+  b.y = d;
+  b.z = c;
+  return b;
+}
+
+int
+main (void)
+{
+  struct s x;
+  x = foo ( 10, 20, 30);
+  return x.x + x.y + x.z;
+}
+
+/* { dg-final { scan-assembler-not "mov\tx0, x8" } } */