BPF: set .BTF and .BTF.ext section alignment to 4

[yonghong-song]

Summary

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

[anakryiko]

Great, thank you!

llvm/lib/Target/BPF/BTFDebug.cpp

 }
 
 void BTFDebug::emitBTFSection() {
   // Do not emit section if no types and only "" string.
   if (!TypeEntries.size() && StringTable.getSize() == 1)
     return;
 
   MCContext &Ctx = OS.getContext();
-  OS.SwitchSection(Ctx.getELFSection(".BTF", ELF::SHT_PROGBITS, 0));
+  MCSectionELF *Sec = Ctx.getELFSection(".BTF", ELF::SHT_PROGBITS, 0);
+  Sec->setAlignment(Align(4));
+  OS.SwitchSection(Sec);
 
   // Emit header.
   emitCommonHeader();
   OS.emitInt32(BTF::HeaderSize);
 
   uint32_t TypeLen = 0, StrLen;
   for (const auto &TypeEntry : TypeEntries)
     TypeLen += TypeEntry->getSize();

 void BTFDebug::emitBTFExtSection() {
   // Do not emit section if empty FuncInfoTable and LineInfoTable
   // and FieldRelocTable.
   if (!FuncInfoTable.size() && !LineInfoTable.size() &&
       !FieldRelocTable.size())
     return;
 
   MCContext &Ctx = OS.getContext();
-  OS.SwitchSection(Ctx.getELFSection(".BTF.ext", ELF::SHT_PROGBITS, 0));
+  MCSectionELF *Sec = Ctx.getELFSection(".BTF.ext", ELF::SHT_PROGBITS, 0);
+  Sec->setAlignment(Align(4));
+  OS.SwitchSection(Sec);
 
   // Emit header.
   emitCommonHeader();
   OS.emitInt32(BTF::ExtHeaderSize);
 
   // Account for FuncInfo/LineInfo record size as well.
   uint32_t FuncLen = 4, LineLen = 4;
   // Do not account for optional FieldReloc.

llvm/test/CodeGen/BPF/BTF/align.ll

+; RUN: llc -march=bpfel -filetype=obj < %s | llvm-readelf -S - | FileCheck %s
+; RUN: llc -march=bpfeb -filetype=obj < %s | llvm-readelf -S - | FileCheck %s
+; Source:
+;   int foo() { return 0; }
+; Compilation flags:
+;   clang -target bpf -O2 -g -S -emit-llvm t.c
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn
+define dso_local i32 @foo() local_unnamed_addr #0 !dbg !7 {
+entry:
+  ret i32 0, !dbg !12
+}
+; CHECK:   Name              Type            Address          Off           Size          ES Flg Lk Inf Al
+; CHECK:   .BTF              PROGBITS        0000000000000000 {{[0-9a-f]+}} {{[0-9a-f]+}} 00      0   0  4
+; CHECK:   .BTF.ext          PROGBITS        0000000000000000 {{[0-9a-f]+}} {{[0-9a-f]+}} 00      0   0  4
+
+attributes #0 = { mustprogress nofree norecurse nosync nounwind readnone willreturn "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 14.0.0 (https://github.com/llvm/llvm-project.git b1ab2a57b83e4b7224c38b534532500cc90e5b9a)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "t.c", directory: "/tmp/home/yhs/work/tests/llvm/align")
+!2 = !{i32 7, !"Dwarf Version", i32 4}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 7, !"frame-pointer", i32 2}
+!6 = !{!"clang version 14.0.0 (https://github.com/llvm/llvm-project.git b1ab2a57b83e4b7224c38b534532500cc90e5b9a)"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{}
+!12 = !DILocation(line: 1, column: 13, scope: !7)