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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
This commit is contained in:
Yonghong Song 2021-10-19 15:29:12 -07:00
parent b6b7fe60a4
commit cd40b5a712
2 changed files with 41 additions and 2 deletions
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8
llvm/lib/Target/BPF/BTFDebug.cpp
View File

@ -832,7 +832,9 @@ void BTFDebug::emitBTFSection() {
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();
@ -870,7 +872,9 @@ void BTFDebug::emitBTFExtSection() {
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();

35
llvm/test/CodeGen/BPF/BTF/align.ll Normal file
View File

@ -0,0 +1,35 @@
; 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)