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[ABI] Handle C++20 [[no_unique_address]] attribute

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Many platform ABIs have special support for passing aggregates that
either just contain a single member of floatint-point type, or else
a homogeneous set of members of the same floating-point type.

When making this determination, any extra "empty" members of the
aggregate type will typically be ignored.  However, in C++ (at least
in all prior versions), no data member would actually count as empty,
even if it's type is an empty record -- it would still be considered
to take up at least one byte of space, and therefore make those ABI
special cases not apply.

This is now changing in C++20, which introduced the [[no_unique_address]]
attribute.  Members of empty record type, if they also carry this
attribute, now do *not* take up any space in the type, and therefore
the ABI special cases for single-element or homogeneous aggregates
should apply.

The C++ Itanium ABI has been updated accordingly, and GCC 10 has
added support for this new case.  This patch now adds support to
LLVM.  This is cross-platform; it affects all platforms that use
the single-element or homogeneous aggregate ABI special case and
implement this using any of the following common subroutines
in lib/CodeGen/TargetInfo.cpp:
  isEmptyField
  isEmptyRecord
  isSingleElementStruct
  isHomogeneousAggregate
This commit is contained in:
Ulrich Weigand
2020-07-10 13:52:47 +02:00
parent b69e0f674f
commit 4c5a93bd58
2 changed files with 50 additions and 1 deletions
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17
clang/lib/CodeGen/TargetInfo.cpp
View File

@@ -499,11 +499,15 @@ static bool isEmptyField(ASTContext &Context, const FieldDecl *FD,
// Constant arrays of empty records count as empty, strip them off.
// Constant arrays of zero length always count as empty.
bool WasArray = false;
if (AllowArrays)
while (const ConstantArrayType *AT = Context.getAsConstantArrayType(FT)) {
if (AT->getSize() == 0)
return true;
FT = AT->getElementType();
// The [[no_unique_address]] special case below does not apply to
// arrays of C++ empty records, so we need to remember this fact.
WasArray = true;
}
const RecordType *RT = FT->getAs<RecordType>();
@@ -514,7 +518,14 @@ static bool isEmptyField(ASTContext &Context, const FieldDecl *FD,
//
// FIXME: We should use a predicate for whether this behavior is true in the
// current ABI.
if (isa<CXXRecordDecl>(RT->getDecl()))
//
// The exception to the above rule are fields marked with the
// [[no_unique_address]] attribute (since C++20). Those do count as empty
// according to the Itanium ABI. The exception applies only to records,
// not arrays of records, so we must also check whether we stripped off an
// array type above.
if (isa<CXXRecordDecl>(RT->getDecl()) &&
(WasArray || !FD->hasAttr<NoUniqueAddressAttr>()))
return false;
return isEmptyRecord(Context, FT, AllowArrays);
@@ -7236,6 +7247,10 @@ QualType SystemZABIInfo::GetSingleElementType(QualType Ty) const {
if (getContext().getLangOpts().CPlusPlus &&
FD->isZeroLengthBitField(getContext()))
continue;
// Like isSingleElementStruct(), ignore C++20 empty data members.
if (FD->hasAttr<NoUniqueAddressAttr>() &&
isEmptyRecord(getContext(), FD->getType(), true))
continue;
// Unlike isSingleElementStruct(), arrays do not count.
// Nested structures still do though.

34
clang/test/CodeGen/systemz-abi.cpp
View File

@@ -23,3 +23,37 @@ struct agg_float_cpp pass_agg_float_cpp(struct agg_float_cpp arg) { return arg;
// CHECK-LABEL: define void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret align 4 %{{.*}}, float %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret align 4 %{{.*}}, i32 %{{.*}})
// A field member of empty class type in C++ makes the record nonhomogeneous,
// unless it is marked as [[no_unique_address]]. This does not apply to arrays.
struct empty { };
struct agg_nofloat_empty { float a; empty dummy; };
struct agg_nofloat_empty pass_agg_nofloat_empty(struct agg_nofloat_empty arg) { return arg; }
// CHECK-LABEL: define void @_Z22pass_agg_nofloat_empty17agg_nofloat_empty(%struct.agg_nofloat_empty* noalias sret align 4 %{{.*}}, i64 %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z22pass_agg_nofloat_empty17agg_nofloat_empty(%struct.agg_nofloat_empty* noalias sret align 4 %{{.*}}, i64 %{{.*}})
struct agg_float_empty { float a; [[no_unique_address]] empty dummy; };
struct agg_float_empty pass_agg_float_empty(struct agg_float_empty arg) { return arg; }
// CHECK-LABEL: define void @_Z20pass_agg_float_empty15agg_float_empty(%struct.agg_float_empty* noalias sret align 4 %{{.*}}, float %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z20pass_agg_float_empty15agg_float_empty(%struct.agg_float_empty* noalias sret align 4 %{{.*}}, i32 %{{.*}})
struct agg_nofloat_emptyarray { float a; [[no_unique_address]] empty dummy[3]; };
struct agg_nofloat_emptyarray pass_agg_nofloat_emptyarray(struct agg_nofloat_emptyarray arg) { return arg; }
// CHECK-LABEL: define void @_Z27pass_agg_nofloat_emptyarray22agg_nofloat_emptyarray(%struct.agg_nofloat_emptyarray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z27pass_agg_nofloat_emptyarray22agg_nofloat_emptyarray(%struct.agg_nofloat_emptyarray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
// And likewise for members of base classes.
struct noemptybase { empty dummy; };
struct agg_nofloat_emptybase : noemptybase { float a; };
struct agg_nofloat_emptybase pass_agg_nofloat_emptybase(struct agg_nofloat_emptybase arg) { return arg; }
// CHECK-LABEL: define void @_Z26pass_agg_nofloat_emptybase21agg_nofloat_emptybase(%struct.agg_nofloat_emptybase* noalias sret align 4 %{{.*}}, i64 %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z26pass_agg_nofloat_emptybase21agg_nofloat_emptybase(%struct.agg_nofloat_emptybase* noalias sret align 4 %{{.*}}, i64 %{{.*}})
struct emptybase { [[no_unique_address]] empty dummy; };
struct agg_float_emptybase : emptybase { float a; };
struct agg_float_emptybase pass_agg_float_emptybase(struct agg_float_emptybase arg) { return arg; }
// CHECK-LABEL: define void @_Z24pass_agg_float_emptybase19agg_float_emptybase(%struct.agg_float_emptybase* noalias sret align 4 %{{.*}}, float %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z24pass_agg_float_emptybase19agg_float_emptybase(%struct.agg_float_emptybase* noalias sret align 4 %{{.*}}, i32 %{{.*}})
struct noemptybasearray { [[no_unique_address]] empty dummy[3]; };
struct agg_nofloat_emptybasearray : noemptybasearray { float a; };
struct agg_nofloat_emptybasearray pass_agg_nofloat_emptybasearray(struct agg_nofloat_emptybasearray arg) { return arg; }
// CHECK-LABEL: define void @_Z31pass_agg_nofloat_emptybasearray26agg_nofloat_emptybasearray(%struct.agg_nofloat_emptybasearray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
// SOFT-FLOAT-LABEL: define void @_Z31pass_agg_nofloat_emptybasearray26agg_nofloat_emptybasearray(%struct.agg_nofloat_emptybasearray* noalias sret align 4 %{{.*}}, i64 %{{.*}})