Re: [PATCH 1/2] arm: correctly handle zero-sized bit-fields in AAPCS [PR102024]

[Richard Earnshaw via Gcc-patches]

On 29/03/2022 17:32, Jakub Jelinek via Gcc-patches wrote:

On Tue, Mar 29, 2022 at 04:32:10PM +0100, Richard Earnshaw wrote:

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.
---
  gcc/config/arm/arm.cc                      | 35 ++++++++++++++++++++--
  gcc/testsuite/gcc.target/arm/aapcs/vfp26.c | 31 +++++++++++++++++++
  2 files changed, 63 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/gcc.target/arm/aapcs/vfp26.c

diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
index e062361b985..43b775f6918 100644
--- a/gcc/config/arm/arm.cc
+++ b/gcc/config/arm/arm.cc
@@ -6274,6 +6274,7 @@ 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
@@ -6426,6 +6427,28 @@ 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);
@@ -6538,8 +6561,10 @@ 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#empty_base";

This should be
                = CHANGES_ROOT_URL "gcc-12/changes.html#zero_width_bitfields";
instead.

Otherwise LGTM.

        Jakub

Good catch. Thanks.  Updated and pushed both patches.

R.