rs6000: Add psabi diagnostic for C++ zero-width bit field ABI change
Previously zero-width bit fields were removed from structs, so that otherwise
homogeneous aggregates were treated as such and passed in FPRs and VSRs.
This was incorrect behavior per the ELFv2 ABI. Now that these fields are no
longer being removed, we generate the correct parameter passing code. Alert
the unwary user in the rare cases where this behavior changes.
2021-09-23 Bill Schmidt <wschmidt@linux.ibm.com>
gcc/
PR target/102024
* config/rs6000/rs6000-call.c (rs6000_aggregate_candidate): Detect
zero-width bit fields and return indicator.
(rs6000_discover_homogeneous_aggregate): Diagnose when the
presence of a zero-width bit field changes parameter passing in
GCC 12.
gcc/testsuite/
PR target/102024
* g++.target/powerpc/pr102024.C: New.
diff --git a/gcc/config/rs6000/rs6000-call.c b/gcc/config/rs6000/rs6000-call.c
index 7d48548..2eceb2c7 100644
--- a/gcc/config/rs6000/rs6000-call.c
+++ b/gcc/config/rs6000/rs6000-call.c
@@ -6223,11 +6223,19 @@
or vector type. If a non-floating point or vector type is found, or
if a floating point or vector type that doesn't match a non-VOIDmode
*MODEP is found, then return -1, otherwise return the count in the
- sub-tree. */
+ sub-tree.
+
+ There have been some ABI snafus along the way with C++. Modify
+ EMPTY_BASE_SEEN to a nonzero value iff a C++ empty base class makes
+ an appearance; separate flag bits indicate whether or not such a
+ field is marked "no unique address". Modify ZERO_WIDTH_BF_SEEN
+ to 1 iff a C++ zero-length bitfield makes an appearance, but
+ in this case otherwise treat this as still being a homogeneous
+ aggregate. */
static int
rs6000_aggregate_candidate (const_tree type, machine_mode *modep,
- int *empty_base_seen)
+ int *empty_base_seen, int *zero_width_bf_seen)
{
machine_mode mode;
HOST_WIDE_INT size;
@@ -6298,7 +6306,8 @@
return -1;
count = rs6000_aggregate_candidate (TREE_TYPE (type), modep,
- empty_base_seen);
+ empty_base_seen,
+ zero_width_bf_seen);
if (count == -1
|| !index
|| !TYPE_MAX_VALUE (index)
@@ -6336,6 +6345,26 @@
if (TREE_CODE (field) != FIELD_DECL)
continue;
+ if (DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (field))
+ {
+ /* GCC 11 and earlier generated incorrect code in a rare
+ corner case for C++. When a RECORD_TYPE looks like a
+ homogeneous aggregate, except that it also contains
+ one or more zero-width bit fields, these earlier
+ compilers would incorrectly pass the fields in FPRs
+ or VSRs. This occurred because the front end wrongly
+ removed these bitfields from the RECORD_TYPE. In
+ GCC 12 and later, the front end flaw was corrected.
+ We want to diagnose this case. To do this, we pretend
+ that we don't see the zero-width bit fields (hence
+ the continue statement here), but pass back a flag
+ indicating what happened. The caller then diagnoses
+ the issue and rejects the RECORD_TYPE as a homogeneous
+ aggregate. */
+ *zero_width_bf_seen = 1;
+ continue;
+ }
+
if (DECL_FIELD_ABI_IGNORED (field))
{
if (lookup_attribute ("no_unique_address",
@@ -6347,7 +6376,8 @@
}
sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep,
- empty_base_seen);
+ empty_base_seen,
+ zero_width_bf_seen);
if (sub_count < 0)
return -1;
count += sub_count;
@@ -6381,7 +6411,8 @@
continue;
sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep,
- empty_base_seen);
+ empty_base_seen,
+ zero_width_bf_seen);
if (sub_count < 0)
return -1;
count = count > sub_count ? count : sub_count;
@@ -6423,8 +6454,10 @@
{
machine_mode field_mode = VOIDmode;
int empty_base_seen = 0;
+ int zero_width_bf_seen = 0;
int field_count = rs6000_aggregate_candidate (type, &field_mode,
- &empty_base_seen);
+ &empty_base_seen,
+ &zero_width_bf_seen);
if (field_count > 0)
{
@@ -6460,6 +6493,25 @@
last_reported_type_uid = uid;
}
}
+ if (zero_width_bf_seen && warn_psabi)
+ {
+ static unsigned last_reported_type_uid;
+ unsigned uid = TYPE_UID (TYPE_MAIN_VARIANT (type));
+ if (uid != last_reported_type_uid)
+ {
+ inform (input_location,
+ "ELFv2 parameter passing for an argument "
+ "containing zero-width bit fields but that is "
+ "otherwise a homogeneous aggregate was "
+ "corrected in GCC 12");
+ last_reported_type_uid = uid;
+ }
+ if (elt_mode)
+ *elt_mode = mode;
+ if (n_elts)
+ *n_elts = 1;
+ return false;
+ }
return true;
}
}
diff --git a/gcc/testsuite/g++.target/powerpc/pr102024.C b/gcc/testsuite/g++.target/powerpc/pr102024.C
new file mode 100644
index 0000000..7695850
--- /dev/null
+++ b/gcc/testsuite/g++.target/powerpc/pr102024.C
@@ -0,0 +1,23 @@
+// PR target/102024
+// { dg-do compile { target powerpc_elfv2 } }
+// { dg-options "-O2" }
+
+// Test that a zero-width bit field in an otherwise homogeneous aggregate
+// generates a psabi warning and passes arguments in GPRs.
+
+// { dg-final { scan-assembler-times {\mstd\M} 4 } }
+
+struct a_thing
+{
+ double x;
+ double y;
+ double z;
+ int : 0;
+ double w;
+};
+
+double
+foo (a_thing a) // { dg-message "ELFv2 parameter passing for an argument containing zero-width bit fields but that is otherwise a homogeneous aggregate was corrected in GCC 12" }
+{
+ return a.x * a.y + a.z - a.w;
+}
