[gcc r12-7895] arm: correctly handle zero-sized bit-fields in AAPCS [PR102024]

[Richard Earnshaw <rearnsha@gcc.gnu.org>]

https://gcc.gnu.org/g:3032df28f2a1cc6514571558b76d9b80373b19c6

commit r12-7895-g3032df28f2a1cc6514571558b76d9b80373b19c6
Author: Richard Earnshaw <rearnsha@arm.com>
Date:   Tue Mar 29 10:24:49 2022 +0100

    arm: correctly handle zero-sized bit-fields in AAPCS [PR102024]
    
    On arm the AAPCS states that an HFA is determined by the 'shape' of
    the object after layout has been completed, so anything that adds no
    members and does not cause the layout to be modified should be ignored
    for the purposes of determining which registers are used for parameter
    passing.
    
    A zero-sized bit-field falls into this category.  This was not handled
    correctly for C structs and in G++-11 only handled correctly because
    such fields were eliminated early by the front end.
    
    gcc/ChangeLog:
    
            PR target/102024
            * config/arm/arm.cc (aapcs_vfp_sub_candidate): Handle zero-sized
            bit-fields.  Detect cases where a warning may be needed.
            (aapcs_vfp_is_call_or_return_candidate): Emit a note if
            a zero-sized bit-field has caused parameter passing to change.
    
    gcc/testsuite/ChangeLog:
    
            PR target/102024
            * gcc.target/arm/aapcs/vfp26.c: New test.

Diff:
---
 gcc/config/arm/arm.cc                      | 35 +++++++++++++++++++++++++++---
 gcc/testsuite/gcc.target/arm/aapcs/vfp26.c | 31 ++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 3 deletions(-)

diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
index 26ed7f97fc6..14e2fdfeafa 100644
--- a/gcc/config/arm/arm.cc
+++ b/gcc/config/arm/arm.cc
@@ -6283,6 +6283,7 @@ aapcs_vfp_cum_init (CUMULATIVE_ARGS *pcum  ATTRIBUTE_UNUSED,
       a HFA or HVA.  */
 const unsigned int WARN_PSABI_EMPTY_CXX17_BASE = 1U << 0;
 const unsigned int WARN_PSABI_NO_UNIQUE_ADDRESS = 1U << 1;
+const unsigned int WARN_PSABI_ZERO_WIDTH_BITFIELD = 1U << 2;
 
 /* Walk down the type tree of TYPE counting consecutive base elements.
    If *MODEP is VOIDmode, then set it to the first valid floating point
@@ -6435,6 +6436,28 @@ aapcs_vfp_sub_candidate (const_tree type, machine_mode *modep,
 		    continue;
 		  }
 	      }
+	    /* A zero-width bitfield may affect layout in some
+	       circumstances, but adds no members.  The determination
+	       of whether or not a type is an HFA is performed after
+	       layout is complete, so if the type still looks like an
+	       HFA afterwards, it is still classed as one.  This is
+	       potentially an ABI break for the hard-float ABI.  */
+	    else if (DECL_BIT_FIELD (field)
+		     && integer_zerop (DECL_SIZE (field)))
+	      {
+		/* Prior to GCC-12 these fields were striped early,
+		   hiding them from the back-end entirely and
+		   resulting in the correct behaviour for argument
+		   passing.  Simulate that old behaviour without
+		   generating a warning.  */
+		if (DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (field))
+		  continue;
+		if (warn_psabi_flags)
+		  {
+		    *warn_psabi_flags |= WARN_PSABI_ZERO_WIDTH_BITFIELD;
+		    continue;
+		  }
+	      }
 
 	    sub_count = aapcs_vfp_sub_candidate (TREE_TYPE (field), modep,
 						 warn_psabi_flags);
@@ -6547,8 +6570,10 @@ aapcs_vfp_is_call_or_return_candidate (enum arm_pcs pcs_variant,
 	      && ((alt = aapcs_vfp_sub_candidate (type, &new_mode, NULL))
 		  != ag_count))
 	    {
-	      const char *url
+	      const char *url10
 		= CHANGES_ROOT_URL "gcc-10/changes.html#empty_base";
+	      const char *url12
+		= CHANGES_ROOT_URL "gcc-12/changes.html#zero_width_bitfields";
 	      gcc_assert (alt == -1);
 	      last_reported_type_uid = uid;
 	      /* Use TYPE_MAIN_VARIANT to strip any redundant const
@@ -6557,12 +6582,16 @@ aapcs_vfp_is_call_or_return_candidate (enum arm_pcs pcs_variant,
 		inform (input_location, "parameter passing for argument of "
 			"type %qT with %<[[no_unique_address]]%> members "
 			"changed %{in GCC 10.1%}",
-			TYPE_MAIN_VARIANT (type), url);
+			TYPE_MAIN_VARIANT (type), url10);
 	      else if (warn_psabi_flags & WARN_PSABI_EMPTY_CXX17_BASE)
 		inform (input_location, "parameter passing for argument of "
 			"type %qT when C++17 is enabled changed to match "
 			"C++14 %{in GCC 10.1%}",
-			TYPE_MAIN_VARIANT (type), url);
+			TYPE_MAIN_VARIANT (type), url10);
+	      else if (warn_psabi_flags & WARN_PSABI_ZERO_WIDTH_BITFIELD)
+		inform (input_location, "parameter passing for argument of "
+			"type %qT changed %{in GCC 12.1%}",
+			TYPE_MAIN_VARIANT (type), url12);
 	    }
 	  *count = ag_count;
 	}
diff --git a/gcc/testsuite/gcc.target/arm/aapcs/vfp26.c b/gcc/testsuite/gcc.target/arm/aapcs/vfp26.c
new file mode 100644
index 00000000000..9b1e8aa39d6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/aapcs/vfp26.c
@@ -0,0 +1,31 @@
+/* Test AAPCS layout (VFP variant) */
+
+/* { dg-do run { target arm_eabi } } */
+/* { dg-require-effective-target arm_hard_vfp_ok } */
+/* { dg-require-effective-target arm32 } */
+/* { dg-options "-O -mfpu=vfp -mfloat-abi=hard" } */
+
+#ifndef IN_FRAMEWORK
+#define VFP
+#define TESTFILE "vfp26.c"
+
+/* Anonymous bitfields do not add members; if they do not change the layout
+   then the end result may still be an HFA.  */
+struct z
+{
+  float a;
+  int :0;
+  float b;
+};
+
+struct z a = { 5.0f, 6.0f };
+struct z b = { 9.0f, 10.0f };
+
+#define MYFUNCTYPE struct z
+
+#include "abitest.h"
+#else
+  ARG(int, 7, R0)
+  ARG(struct z, a, S0)
+  LAST_ARG(struct z, b, S2)
+#endif