[PATCH] gcc/riscv: Correctly ignore empty C++ structs when flattening for ABI

[Andrew Burgess <andrew dot burgess at embecosm dot com>]

Jim,

Sorry for the delay in sending this patch.

Thanks,
Andrew

---

This fixes PR target/89627.

The RISC-V ABI document[1] says:

   For the purposes of this section, "struct" refers to a C struct
   with its hierarchy flattened, including any array fields. That is,
   struct { struct { float f[1]; } g[2]; } and struct { float f; float
   g; } are treated the same. Fields containing empty structs or
   unions are ignored while flattening, even in C++, unless they have
   nontrivial copy constructors or destructors.

However, this flattening only applies when one of the fields of the
flattened structure can be placed into a floating point register,
otherwise no flattening occurs.

Currently GCC fails to correctly consider that empty C++ structures
have a non-zero size when constructing the arguments from a flattened
structure, and as a result, trying to pass a C++ structure like this:

  struct sf { struct {} e; float f; };

Doesn't work correctly, GCC fails to take the offset of 'f' within
'sf' into account and will actually pass the space backing 'e' as the
contents of 'f'.

This patch fixes this so that 'f' will be passed correctly.  A couple
of new tests are added to cover this functionality.

[1] https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

gcc/ChangeLog:

	PR target/89627
	* config/riscv/riscv.c (riscv_pass_fpr_single): Add offset
	parameter, and make use of it.
	(riscv_get_arg_info): Pass offset to riscv_pass_fpr_single.

gcc/testsuite/ChangeLog:

	PR target/89627
	* g++.target/riscv/call-with-empty-struct-float.C: New file.
	* g++.target/riscv/call-with-empty-struct-int.C: New file.
	* g++.target/riscv/call-with-empty-struct.H: New file.
	* g++.target/riscv/riscv.exp: New file.
---
 gcc/ChangeLog                                      |  7 +++++
 gcc/config/riscv/riscv.c                           |  8 +++--
 gcc/testsuite/ChangeLog                            |  8 +++++
 .../riscv/call-with-empty-struct-float.C           |  6 ++++
 .../g++.target/riscv/call-with-empty-struct-int.C  |  6 ++++
 .../g++.target/riscv/call-with-empty-struct.H      | 19 ++++++++++++
 gcc/testsuite/g++.target/riscv/riscv.exp           | 34 ++++++++++++++++++++++
 7 files changed, 85 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.target/riscv/call-with-empty-struct-float.C
 create mode 100644 gcc/testsuite/g++.target/riscv/call-with-empty-struct-int.C
 create mode 100644 gcc/testsuite/g++.target/riscv/call-with-empty-struct.H
 create mode 100644 gcc/testsuite/g++.target/riscv/riscv.exp

diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c
index 8881f80e18f..8e78ab76375 100644
--- a/gcc/config/riscv/riscv.c
+++ b/gcc/config/riscv/riscv.c
@@ -2449,13 +2449,14 @@ riscv_pass_aggregate_in_fpr_and_gpr_p (const_tree type,
 
 static rtx
 riscv_pass_fpr_single (machine_mode type_mode, unsigned regno,
-		       machine_mode value_mode)
+		       machine_mode value_mode,
+		       HOST_WIDE_INT offset)
 {
   rtx x = gen_rtx_REG (value_mode, regno);
 
   if (type_mode != value_mode)
     {
-      x = gen_rtx_EXPR_LIST (VOIDmode, x, const0_rtx);
+      x = gen_rtx_EXPR_LIST (VOIDmode, x, GEN_INT (offset));
       x = gen_rtx_PARALLEL (type_mode, gen_rtvec (1, x));
     }
   return x;
@@ -2517,7 +2518,8 @@ riscv_get_arg_info (struct riscv_arg_info *info, const CUMULATIVE_ARGS *cum,
 	  {
 	  case 1:
 	    return riscv_pass_fpr_single (mode, fregno,
-					  TYPE_MODE (fields[0].type));
+					  TYPE_MODE (fields[0].type),
+					  fields[0].offset);
 
 	  case 2:
 	    return riscv_pass_fpr_pair (mode, fregno,
diff --git a/gcc/testsuite/g++.target/riscv/call-with-empty-struct-float.C b/gcc/testsuite/g++.target/riscv/call-with-empty-struct-float.C
new file mode 100644
index 00000000000..76d0dc6d93d
--- /dev/null
+++ b/gcc/testsuite/g++.target/riscv/call-with-empty-struct-float.C
@@ -0,0 +1,6 @@
+// { dg-do run }
+
+#include "call-with-empty-struct.H"
+
+MAKE_STRUCT_PASSING_TEST(float,2.5)
+
diff --git a/gcc/testsuite/g++.target/riscv/call-with-empty-struct-int.C b/gcc/testsuite/g++.target/riscv/call-with-empty-struct-int.C
new file mode 100644
index 00000000000..cc82912dc3e
--- /dev/null
+++ b/gcc/testsuite/g++.target/riscv/call-with-empty-struct-int.C
@@ -0,0 +1,6 @@
+/* { dg-do run } */
+
+#include "call-with-empty-struct.H"
+
+MAKE_STRUCT_PASSING_TEST(int,2)
+
diff --git a/gcc/testsuite/g++.target/riscv/call-with-empty-struct.H b/gcc/testsuite/g++.target/riscv/call-with-empty-struct.H
new file mode 100644
index 00000000000..d2a46e78619
--- /dev/null
+++ b/gcc/testsuite/g++.target/riscv/call-with-empty-struct.H
@@ -0,0 +1,19 @@
+#define MAKE_STRUCT_PASSING_TEST(type,val)                              \
+  static struct struct_ ## type ## _t                                   \
+  {                                                                     \
+    struct { } e;                                                       \
+    struct { type f; } s;                                               \
+  } global_struct_ ## type = { {}, { val } };                           \
+                                                                        \
+  static bool                                                           \
+  check_struct_ ## type (struct_ ## type ## _t obj)                     \
+  {                                                                     \
+    return (obj.s.f == global_struct_ ## type .s.f);                    \
+  }                                                                     \
+                                                                        \
+  int                                                                   \
+  main ()                                                               \
+  {                                                                     \
+    bool result = check_struct_ ## type ( global_struct_ ## type );     \
+    return result ? 0 : 1;                                              \
+  }
diff --git a/gcc/testsuite/g++.target/riscv/riscv.exp b/gcc/testsuite/g++.target/riscv/riscv.exp
new file mode 100644
index 00000000000..a339b5cc56d
--- /dev/null
+++ b/gcc/testsuite/g++.target/riscv/riscv.exp
@@ -0,0 +1,34 @@
+# Copyright (C) 2019 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# GCC testsuite that uses the `dg.exp' driver.
+
+# Exit immediately if this isn't a RISC-V target.
+if ![istarget riscv*-*-*] then {
+  return
+}
+
+# Load support procs.
+load_lib g++-dg.exp
+
+# Initialize `dg'.
+dg-init
+
+# Main loop.
+dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.C]] "" ""
+
+# All done.
+dg-finish
-- 
2.14.5