tree-inline: Fix up __builtin_va_arg_pack handling [PR100898]

The following testcase ICEs, because gimple_call_arg_ptr (..., 0)
asserts that there is at least one argument, while we were using
it even if we didn't copy anything just to get a pointer from/to which
the zero arguments should be copied.

Fixed by guarding the memcpy calls.  Also, the code was calling
gimple_call_num_args too many times - 5 times instead of 2, so the patch
adds two temporaries for those.

2021-06-07  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/100898
	* tree-inline.c (copy_bb): Only use gimple_call_arg_ptr if memcpy
	should copy any arguments.  Don't call gimple_call_num_args
	on id->call_stmt or call_stmt more than once.

	* g++.dg/ext/va-arg-pack-3.C: New test.

gcc/testsuite/g++.dg/ext/va-arg-pack-3.C

@@ -0,0 +1,18 @@
+// PR middle-end/100898
+
+int a;
+int bar (int, ...);
+
+static inline __attribute__((always_inline)) int
+foo (...)
+{
+  while (a)
+    return bar (0, __builtin_va_arg_pack ());
+  return 0;
+}
+
+void
+baz (void)
+{
+  foo ();
+}

gcc/tree-inline.c

@@ -2090,27 +2090,29 @@ copy_bb (copy_body_data *id, basic_block bb,
 	      tree p;
 	      gcall *new_call;
 	      vec<tree> argarray;
-	      size_t nargs = gimple_call_num_args (id->call_stmt);
+	      size_t nargs_caller = gimple_call_num_args (id->call_stmt);
-	      size_t n;
+	      size_t nargs = nargs_caller;
 
 	      for (p = DECL_ARGUMENTS (id->src_fn); p; p = DECL_CHAIN (p))
 		nargs--;
 
 	      /* Create the new array of arguments.  */
-	      n = nargs + gimple_call_num_args (call_stmt);
+	      size_t nargs_callee = gimple_call_num_args (call_stmt);
+	      size_t n = nargs + nargs_callee;
 	      argarray.create (n);
 	      argarray.safe_grow_cleared (n, true);
 
 	      /* Copy all the arguments before '...'  */
+	      if (nargs_callee)
 		memcpy (argarray.address (),
 			gimple_call_arg_ptr (call_stmt, 0),
-		      gimple_call_num_args (call_stmt) * sizeof (tree));
+			nargs_callee * sizeof (tree));
 
 	      /* Append the arguments passed in '...'  */
-	      memcpy (argarray.address () + gimple_call_num_args (call_stmt),
+	      if (nargs)
+		memcpy (argarray.address () + nargs_callee,
 			gimple_call_arg_ptr (id->call_stmt, 0)
-		      + (gimple_call_num_args (id->call_stmt) - nargs),
+			+ (nargs_caller - nargs), nargs * sizeof (tree));
-		      nargs * sizeof (tree));
 
 	      new_call = gimple_build_call_vec (gimple_call_fn (call_stmt),
 						argarray);