On Tue, Mar 04, 2025 at 09:10:12AM +0800, Menglong Dong wrote:

Hello, sorry that I forgot to add something to the changelog. In fact,
I don't add extra 5-bytes anymore, which you can see in the 3rd patch.

The thing is that we can't add extra 5-bytes if CFI is enabled. Without
CFI, we can make the padding space any value, such as 5-bytes, and
the layout will be like this:

__align:
  nop
  nop
  nop
  nop
  nop
foo: -- __align +5

However, the CFI will always make the cfi insn 16-bytes aligned. When
we set the FUNCTION_PADDING_BYTES to (11 + 5), the layout will be
like this:

__cfi_foo:
  nop (11)
  mov $0x12345678, %reg
  nop (16)
foo:

and the padding space is 32-bytes actually. So, we can just select
FUNCTION_ALIGNMENT_32B instead, which makes the padding
space 32-bytes too, and have the following layout:

__cfi_foo:
  mov $0x12345678, %reg
  nop (27)
foo:

*blink*, wtf is clang smoking.

I mean, you're right, this is what it is doing, but that is somewhat
unexpected. Let me go look at clang source, this is insane.