Weird behavior of padding of `-target-abi experimental-mv`

[yhx-12243]

When compiling WebAssembly using -target-abi experimental-mv, the struct with padding will result in a very weird padding result: Use a lot of i32 to waste with. For example:

1.cpp:

struct Unit {};

struct A {
	int x;
	Unit c;
};

A foo() {
	return {};
}

Compiling command: clang++ 1.cpp -S -O3 --target=wasm32-unknown-unknown -Xclang -target-abi -Xclang experimental-mv

Result 1.s:

	.file	"1.cpp"
	.functype	_Z3foov () -> (i32, i32, i32, i32, i32)
	.section	.text._Z3foov,"",@
	.hidden	_Z3foov                         # -- Begin function _Z3foov
	.globl	_Z3foov
	.type	_Z3foov,@function
_Z3foov:                                # @_Z3foov
	.functype	_Z3foov () -> (i32, i32, i32, i32, i32)
	.local  	i32
# %bb.0:
	i32.const	0
	local.get	0
	local.get	0
	local.get	0
	local.get	0
                                        # fallthrough-return
	end_function
                                        # -- End function

Godbolt link: https://gcc.godbolt.org/z/s5Tb84qMz

However, if I change Unit into char, the behavior is as we expected.

I think it is a bug of experimental-mv ABI.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: None (yhx-12243)

When compiling WebAssembly using `-target-abi experimental-mv`, the struct with padding will result in a very weird padding result: Use a lot of `i32` to waste with. For example:

1.cpp:

struct Unit {};

struct A {
	int x;
	Unit c;
};

A foo() {
	return {};
}

Compiling command: clang++ 1.cpp -S -O3 --target=wasm32-unknown-unknown -Xclang -target-abi -Xclang experimental-mv

Result 1.s:

	.file	"1.cpp"
	.functype	_Z3foov () -> (i32, i32, i32, i32, i32)
	.section	.text._Z3foov,"",@
	.hidden	_Z3foov                         # -- Begin function _Z3foov
	.globl	_Z3foov
	.type	_Z3foov,@<!-- -->function
_Z3foov:                                # @<!-- -->_Z3foov
	.functype	_Z3foov () -&gt; (i32, i32, i32, i32, i32)
	.local  	i32
# %bb.0:
	i32.const	0
	local.get	0
	local.get	0
	local.get	0
	local.get	0
                                        # fallthrough-return
	end_function
                                        # -- End function

Godbolt link: https://gcc.godbolt.org/z/s5Tb84qMz

However, if I change Unit into char, the behavior is as we expected.

I think it is a bug of experimental-mv ABI.