Sections containing instructions should be aligned to the architecture's instruction alignment

[pcc]

When assembling the following for AArch64:

        .section        .text.unlikely,"ax",@progbits
.Ltmp4:
        b       .Ltmp4

LLVM's section headers:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .strtab           STRTAB          0000000000000000 000078 000029 00      0   0  1
  [ 2] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
  [ 3] .text.unlikely    PROGBITS        0000000000000000 000040 000004 00  AX  0   0  1
  [ 4] .symtab           SYMTAB          0000000000000000 000048 000030 18      1   2  8

GNU as section headers:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  1
  [ 2] .data             PROGBITS        0000000000000000 000040 000000 00  WA  0   0  1
  [ 3] .bss              NOBITS          0000000000000000 000040 000000 00  WA  0   0  1
  [ 4] .text.unlikely    PROGBITS        0000000000000000 000040 000004 00  AX  0   0  4
  [ 5] .symtab           SYMTAB          0000000000000000 000048 000090 18      6   6  8
  [ 6] .strtab           STRTAB          0000000000000000 0000d8 000004 00      0   0  1
  [ 7] .shstrtab         STRTAB          0000000000000000 0000dc 00003b 00      0   0  1

As a result of the LLVM behavior, we can end up misaligning the .text.unlikely section (e.g. if a preceding section contains data of size not a multiple of 4) and causing a crash at runtime.

I think the GNU as behavior makes more sense, and we should adopt that behavior.

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: None (pcc)

When assembling the following for AArch64: ``` .section .text.unlikely,"ax",@progbits .Ltmp4: b .Ltmp4 ``` LLVM's section headers: ``` Section Headers: [Nr] Name Type Address Off Size ES Flg Lk Inf Al [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 [ 1] .strtab STRTAB 0000000000000000 000078 000029 00 0 0 1 [ 2] .text PROGBITS 0000000000000000 000040 000000 00 AX 0 0 4 [ 3] .text.unlikely PROGBITS 0000000000000000 000040 000004 00 AX 0 0 1 [ 4] .symtab SYMTAB 0000000000000000 000048 000030 18 1 2 8 ``` GNU as section headers: ``` Section Headers: [Nr] Name Type Address Off Size ES Flg Lk Inf Al [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 [ 1] .text PROGBITS 0000000000000000 000040 000000 00 AX 0 0 1 [ 2] .data PROGBITS 0000000000000000 000040 000000 00 WA 0 0 1 [ 3] .bss NOBITS 0000000000000000 000040 000000 00 WA 0 0 1 [ 4] .text.unlikely PROGBITS 0000000000000000 000040 000004 00 AX 0 0 4 [ 5] .symtab SYMTAB 0000000000000000 000048 000090 18 6 6 8 [ 6] .strtab STRTAB 0000000000000000 0000d8 000004 00 0 0 1 [ 7] .shstrtab STRTAB 0000000000000000 0000dc 00003b 00 0 0 1 ``` As a result of the LLVM behavior, we can end up misaligning the `.text.unlikely` section (e.g. if a preceding section contains data of size not a multiple of 4) and causing a crash at runtime.

I think the GNU as behavior makes more sense, and we should adopt that behavior.

[efriedma-quic]

See also #90358 . (Not a duplicate, though.)

[lenary]

I think that #114031 will fix this? Sorry for dropping the ball on finishing that review.

[lenary]

Ah, sorry, that fix is just aarch64, but you want this for all architectures.