RFC: Enhancing function alignment attributes

LLVM Project
17

To summarize the behavior difference

% cat a.ll
define void @foo3(i32 %x) {
  ret void
}
define i32 @foo4(i32 %x) {
  ret i32 %x
}

% myllc a.ll -function-sections -filetype=obj -o a.o && readelf -W -S a.o
...
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
...
  [ 3] .text.foo3        PROGBITS        0000000000000000 000040 000001 00  AX  0   0  1
  [ 4] .text.foo4        PROGBITS        0000000000000000 000041 000003 00  AX  0   0  4

# -fno-function-sections is unchanged https://github.com/llvm/llvm-project/pull/155529#issuecomment-3947151340
% myllc a.ll -filetype=obj -o a.o && objdump -dr a.o
...
0000000000000000 <foo3>:
   0:   c3                      ret
   1:   66 2e 0f 1f 84 00 00    cs nopw 0x0(%rax,%rax,1)
   8:   00 00 00
   b:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)

0000000000000010 <foo4>:
  10:   89 f8                   mov    %edi,%eax
  12:   c3                      ret
1 Like