[all-commits] [llvm/llvm-project] cd40b5: BPF: set .BTF and .BTF.ext section alignment to 4

[yonghong-song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cd40b5a71290bab313cc431fb9a90ac3f9f3fa02
      https://github.com/llvm/llvm-project/commit/cd40b5a71290bab313cc431fb9a90ac3f9f3fa02
  Author: Yonghong Song <yhs at fb.com>
  Date:   2021-10-19 (Tue, 19 Oct 2021)

  Changed paths:
    M llvm/lib/Target/BPF/BTFDebug.cpp
    A llvm/test/CodeGen/BPF/BTF/align.ll

  Log Message:
  -----------
  BPF: set .BTF and .BTF.ext section alignment to 4

Currently, .BTF and .BTF.ext has default alignment of 1.
For example,
  $ cat t.c
    int foo() { return 0; }
  $ clang -target bpf -O2 -c -g t.c
  $ llvm-readelf -S t.o
    ...
    Section Headers:
    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
    ...
    [ 7] .BTF              PROGBITS        0000000000000000 000167 00008b 00      0   0  1
    [ 8] .BTF.ext          PROGBITS        0000000000000000 0001f2 000050 00      0   0  1

But to have no misaligned data access, .BTF and .BTF.ext
actually requires alignment of 4. Misalignment is not an issue
for architecture like x64/arm64 as it can handle it well. But
some architectures like mips may incur a trap if .BTF/.BTF.ext
is not properly aligned.

This patch explicitly forced .BTF and .BTF.ext alignment to be 4.
For the above example, we will have
    [ 7] .BTF              PROGBITS        0000000000000000 000168 00008b 00      0   0  4
    [ 8] .BTF.ext          PROGBITS        0000000000000000 0001f4 000050 00      0   0  4

Differential Revision: https://reviews.llvm.org/D112106