[tree-tailcall] Check if function returns it's argument

[Prathamesh Kulkarni]

Hi,
Consider following test-case:

void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
{
  __builtin_memcpy (a1, a2, a3);
  return a1;
}

return a1 can be considered equivalent to return value of memcpy,
and the call could be emitted as a tail-call.
gcc doesn't emit the above call to memcpy as a tail-call,
but if it is changed to:

void *t1 = __builtin_memcpy (a1, a2, a3);
return t1;

Then memcpy is emitted as a tail-call.
The attached patch tries to handle the former case.

Bootstrapped+tested on x86_64-unknown-linux-gnu.
Cross tested on arm*-*-*, aarch64*-*-*
Does this patch look OK ?

Thanks,
Prathamesh

2016-11-24  Prathamesh Kulkarni  <prathamesh.kulka...@linaro.org>

        * gimple.c (gimple_call_return_arg): New function.
        * gimple.h (gimple_call_return_arg): Declare.
        * tree-tailcall.c (find_tail_calls): Call gimple_call_return_arg.

testsuite/
        * gcc.dg/tree-ssa/tailcall-8.c: New test.

diff --git a/gcc/gimple.c b/gcc/gimple.c
index 0a3dc72..ec460fc 100644
--- a/gcc/gimple.c
+++ b/gcc/gimple.c
@@ -2561,6 +2561,23 @@ gimple_call_combined_fn (const gimple *stmt)
   return CFN_LAST;
 }
 
+/* Return arg, if function returns it's argument or NULL if it doesn't.  */
+tree
+gimple_call_return_arg (gcall *call_stmt)
+{
+  unsigned rf = gimple_call_return_flags (call_stmt);
+  if (rf & ERF_RETURNS_ARG)
+    {
+      unsigned argnum = rf & ERF_RETURN_ARG_MASK;
+      if (argnum < gimple_call_num_args (call_stmt))
+       {
+         tree arg = gimple_call_arg (call_stmt, argnum);
+         return arg;
+       }
+    }
+  return NULL_TREE;
+}
+
 /* Return true if STMT clobbers memory.  STMT is required to be a
    GIMPLE_ASM.  */
 
diff --git a/gcc/gimple.h b/gcc/gimple.h
index 0d0296e..ebccbe1 100644
--- a/gcc/gimple.h
+++ b/gcc/gimple.h
@@ -1520,6 +1520,7 @@ extern combined_fn gimple_call_combined_fn (const gimple 
*);
 extern bool gimple_call_builtin_p (const gimple *);
 extern bool gimple_call_builtin_p (const gimple *, enum built_in_class);
 extern bool gimple_call_builtin_p (const gimple *, enum built_in_function);
+extern tree gimple_call_return_arg (gcall *);
 extern bool gimple_asm_clobbers_memory_p (const gasm *);
 extern void dump_decl_set (FILE *, bitmap);
 extern bool nonfreeing_call_p (gimple *);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c 
b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c
new file mode 100644
index 0000000..b3fdc6c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-tailc-details" } */
+
+void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
+{
+  __builtin_memcpy (a1, a2, a3);
+  return a1;
+}
+
+/* { dg-final { scan-tree-dump-times "Found tail call" 1 "tailc" } } */ 
diff --git a/gcc/tree-tailcall.c b/gcc/tree-tailcall.c
index f97541d..3396473 100644
--- a/gcc/tree-tailcall.c
+++ b/gcc/tree-tailcall.c
@@ -422,6 +422,8 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
        {
          call = as_a <gcall *> (stmt);
          ass_var = gimple_call_lhs (call);
+         if (!ass_var)
+           ass_var = gimple_call_return_arg (call);
          break;
        }