<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/99644>99644</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
clang and GCC disagree about whether union containing empty struct can be HFA
</td>
</tr>
<tr>
<th>Labels</th>
<td>
clang
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
eleanor-arm
</td>
</tr>
</table>
<pre>
clang and GCC disagree about whether this union type is an HFA, and so whether it should be passed in floating-point or integer registers
**empty_struct.cpp**
```
struct empty {};
union maybe_hfa {
float a;
empty b;
};
float bar(float a, maybe_hfa b) { return b.a; }
```
The ABI isn't totally clear about what the right answer is here, but it seems more consistent to always treat empty as a type with size and alignment of 1 in all contexts in C++, so I think GCC's behaviour is more likely to be correct.
Note that the meaning of an empty struct varies between C and C++:
In C, it is technically undefined behaviour, but GCC defines it as having size zero as an extension, and I think clang tries to match that: https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Empty-Structures.html
In C++, an empty struct (or class, or union) is well-defined, and the Itanium C++ ABI says it has a size and alignment of 1 byte.
This is LLVM's codegen, for both AArch32 and AArch64, which shows it passing the maybe_hfa argument in s1, since it considers it to be an HFA:
```
clang --target=arm--none-eabi -march=armv8-a -O1 -c empty_struct.cpp -o - -S -mfloat-abi=hard
.text
.syntax unified
.eabi_attribute 67, "2.09" @ Tag_conformance
.eabi_attribute 6, 14 @ Tag_CPU_arch
.eabi_attribute 7, 65 @ Tag_CPU_arch_profile
.eabi_attribute 8, 1 @ Tag_ARM_ISA_use
.eabi_attribute 9, 2 @ Tag_THUMB_ISA_use
.fpu crypto-neon-fp-armv8
.eabi_attribute 12, 3 @ Tag_Advanced_SIMD_arch
.eabi_attribute 36, 1 @ Tag_FP_HP_extension
.eabi_attribute 42, 1 @ Tag_MPextension_use
.eabi_attribute 34, 1 @ Tag_CPU_unaligned_access
.eabi_attribute 68, 3 @ Tag_Virtualization_use
.eabi_attribute 17, 1 @ Tag_ABI_PCS_GOT_use
.eabi_attribute 20, 1 @ Tag_ABI_FP_denormal
.eabi_attribute 21, 0 @ Tag_ABI_FP_exceptions
.eabi_attribute 23, 3 @ Tag_ABI_FP_number_model
.eabi_attribute 24, 1 @ Tag_ABI_align_needed
.eabi_attribute 25, 1 @ Tag_ABI_align_preserved
.eabi_attribute 28, 1 @ Tag_ABI_VFP_args
.eabi_attribute 38, 1 @ Tag_ABI_FP_16bit_format
.eabi_attribute 18, 4 @ Tag_ABI_PCS_wchar_t
.eabi_attribute 26, 2 @ Tag_ABI_enum_size
.eabi_attribute 14, 0 @ Tag_ABI_PCS_R9_use
.file "empty_struct.cpp"
.globl _Z3barf9maybe_hfa @ -- Begin function _Z3barf9maybe_hfa
.p2align 2
.type _Z3barf9maybe_hfa,%function
.code 32 @ @_Z3barf9maybe_hfa
_Z3barf9maybe_hfa:
.fnstart
@ %bb.0: @ %entry
vmov.f32 s0, s1
bx lr
.Lfunc_end0:
.size _Z3barf9maybe_hfa, .Lfunc_end0-_Z3barf9maybe_hfa
.cantunwind
.fnend
@ -- End function
.ident "clang version 19.0.0git (git@github.com:llvm/llvm-project.git 79ce70b8033815b6abd3a9a5cc2335de70f1aaab)"
.section ".note.GNU-stack","",%progbits
.addrsig
.eabi_attribute 30, 1 @ Tag_ABI_optimization_goals
```
```
clang --target=aarch64--none-elf -march=armv8-a -O1 -c empty_struct.cpp -o - -S
.text
.file "empty_struct.cpp"
.globl _Z3barf9maybe_hfa // -- Begin function _Z3barf9maybe_hfa
.p2align 2
.type _Z3barf9maybe_hfa,@function
_Z3barf9maybe_hfa: // @_Z3barf9maybe_hfa
// %bb.0: // %entry
fmov s0, s1
ret
.Lfunc_end0:
.size _Z3barf9maybe_hfa, .Lfunc_end0-_Z3barf9maybe_hfa
// -- End function
.ident "clang version 19.0.0git (git@github.com:llvm/llvm-project.git 79ce70b8033815b6abd3a9a5cc2335de70f1aaab)"
.section ".note.GNU-stack","",@progbits
.addrsig
```
For comparison, this is gcc's output with the same options, showing that is passes the argument in r0/w0:
```
arm-none-eabi-gcc -march=armv8-a -mfpu=fp-armv8 -c empty_struct.cpp -O1 -o - -S -mfloat-abi=hard
.arch armv8-a
.fpu fp-armv8
.eabi_attribute 28, 1
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 1
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "empty_struct.cpp"
.text
.align 2
.global _Z3barf9maybe_hfa
.syntax unified
.arm
.type _Z3barf9maybe_hfa, %function
_Z3barf9maybe_hfa:
.fnstart
.LFB0:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
vmov s0, r0
bx lr
.cantunwind
.fnend
.size _Z3barf9maybe_hfa, .-_Z3barf9maybe_hfa
.ident "GCC: (fsf-trunk.3697) 15.0.0 20240514 (experimental)"
```
```
aarch64-none-elf-gcc -march=armv8-a -c -c empty_struct.cpp -O1 -o - -S
.arch armv8-a
.file "empty_struct.cpp"
.text
.align 2
.global _Z3barf9maybe_hfa
.type _Z3barf9maybe_hfa, %function
_Z3barf9maybe_hfa:
.LFB0:
.cfi_startproc
fmov s0, w0
ret
.cfi_endproc
.LFE0:
.size _Z3barf9maybe_hfa, .-_Z3barf9maybe_hfa
.ident "GCC: (fsf-trunk.3697) 15.0.0 20240514 (experimental)"
```
</pre>
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