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For C++ (but not C), empty structs should be passed to functions as if they are a 1 byte object with 1 byte alignment. This is defined in Arm's CPPABI32: https://github.com/ARM-software/abi-aa/blob/main/cppabi32/cppabi32.rst For the purposes of parameter passing in AAPCS32, a parameter whose type is an empty class shall be treated as if its type were an aggregate with a single member of type unsigned byte. The AArch64 equivalent of this has an exception for structs containing an array of size zero, I've kept that logic for ARM. I've not found a reason for this exception, but I've checked that GCC does have the same behaviour for ARM as it does for AArch64. The AArch64 version has an Apple ABI with different rules, which ignores empty structs in both C and C++. This is documented at https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms. The ARM equivalent of that appears to be AAPCS16_VFP, used for WatchOS, but I can't find any documentation for that ABI, so I'm not sure what rules it should follow. For now I've left it following the AArch64 Apple rules.
132 lines
4.6 KiB
C++
132 lines
4.6 KiB
C++
// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - -x c %s | FileCheck %s --check-prefixes=CHECK,C
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// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,CXX
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// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - %s -fclang-abi-compat=19 | FileCheck %s --check-prefixes=CHECK,CXXCLANG19
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// RUN: %clang_cc1 -triple thumbv7k-apple-watchos2.0 -target-abi aapcs16 -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,WATCHOS
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// Empty structs are ignored for PCS purposes on WatchOS and in C mode
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// elsewhere. In C++ mode they consume a register slot though. Functions are
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// slightly bigger than minimal to make confirmation against actual GCC
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// behaviour easier.
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#if __cplusplus
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#define EXTERNC extern "C"
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#else
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#define EXTERNC
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#endif
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struct Empty {};
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// C: define{{.*}} i32 @empty_arg(i32 noundef %a)
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// CXX: define{{.*}} i32 @empty_arg(i8 %e.coerce, i32 noundef %a)
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// CXXCLANG19: define{{.*}} i32 @empty_arg(i32 noundef %a)
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// WATCHOS: define{{.*}} i32 @empty_arg(i32 noundef %a)
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EXTERNC int empty_arg(struct Empty e, int a) {
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return a;
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}
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// C: define{{.*}} void @empty_ret()
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// CXX: define{{.*}} void @empty_ret()
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// CXXCLANG19: define{{.*}} void @empty_ret()
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// WATCHOS: define{{.*}} void @empty_ret()
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EXTERNC struct Empty empty_ret(void) {
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struct Empty e;
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return e;
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}
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// However, what counts as "empty" is a baroque mess. This is super-empty, it's
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// ignored even in C++ mode. It also has sizeof == 0, violating C++, but that's
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// legacy for you:
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struct SuperEmpty {
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int arr[0];
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};
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// C: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
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// CXX: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
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// CXXCLANG19: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
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// WATCHOS: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
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EXTERNC int super_empty_arg(struct SuperEmpty e, int a) {
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return a;
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}
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struct SortOfEmpty {
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struct SuperEmpty e;
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};
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// C: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
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// CXX: define{{.*}} i32 @sort_of_empty_arg(i8 %e.coerce, i32 noundef %a)
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// CXXCLANG19: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
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// WATCHOS: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
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EXTERNC int sort_of_empty_arg(struct Empty e, int a) {
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return a;
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}
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// C: define{{.*}} void @sort_of_empty_ret()
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// CXX: define{{.*}} void @sort_of_empty_ret()
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// CXXCLANG19: define{{.*}} void @sort_of_empty_ret()
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// WATCHOS: define{{.*}} void @sort_of_empty_ret()
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EXTERNC struct SortOfEmpty sort_of_empty_ret(void) {
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struct SortOfEmpty e;
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return e;
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}
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#include <stdarg.h>
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// va_arg matches the above rules, consuming an incoming argument in cases
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// where one would be passed, and not doing so when the argument should be
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// ignored.
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EXTERNC int empty_arg_variadic(int a, ...) {
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// CHECK-LABEL: @empty_arg_variadic(
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// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// C-NOT: {{ getelementptr }}
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// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXX: %argp.next2 = getelementptr inbounds i8, ptr %argp.cur1, i32 4
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// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXXCLANG19-NOT: {{ getelementptr }}
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// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// WATCHOS-NOT: {{ getelementptr }}
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va_list vl;
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va_start(vl, a);
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struct Empty b = va_arg(vl, struct Empty);
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int c = va_arg(vl, int);
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va_end(vl);
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return c;
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}
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EXTERNC int super_empty_arg_variadic(int a, ...) {
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// CHECK-LABEL: @super_empty_arg_variadic(
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// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// C-NOT: {{ getelementptr }}
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// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXX-NOT: {{ getelementptr }}
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// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXXCLANG19-NOT: {{ getelementptr }}
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// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// WATCHOS-NOT: {{ getelementptr }}
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va_list vl;
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va_start(vl, a);
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struct SuperEmpty b = va_arg(vl, struct SuperEmpty);
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int c = va_arg(vl, int);
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va_end(vl);
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return c;
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}
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EXTERNC int sort_of_empty_arg_variadic(int a, ...) {
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// CHECK-LABEL: @sort_of_empty_arg_variadic(
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// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// C-NOT: {{ getelementptr }}
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// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXX-NOT: {{ getelementptr }}
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// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// CXXCLANG19-NOT: {{ getelementptr }}
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// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
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// WATCHOS-NOT: {{ getelementptr }}
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va_list vl;
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va_start(vl, a);
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struct SortOfEmpty b = va_arg(vl, struct SortOfEmpty);
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int c = va_arg(vl, int);
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va_end(vl);
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return c;
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}
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