[llvm] cd40b5a - BPF: set .BTF and .BTF.ext section alignment to 4

[Yonghong Song via llvm-commits]

Author: Yonghong Song
Date: 2021-10-19T16:26:01-07:00
New Revision: cd40b5a71290bab313cc431fb9a90ac3f9f3fa02

URL: https://github.com/llvm/llvm-project/commit/cd40b5a71290bab313cc431fb9a90ac3f9f3fa02
DIFF: https://github.com/llvm/llvm-project/commit/cd40b5a71290bab313cc431fb9a90ac3f9f3fa02.diff

LOG: 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

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

Modified: 
    llvm/lib/Target/BPF/BTFDebug.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/BPF/BTFDebug.cpp b/llvm/lib/Target/BPF/BTFDebug.cpp
index 738d472f6efec..987edcedf58be 100644
--- a/llvm/lib/Target/BPF/BTFDebug.cpp
+++ b/llvm/lib/Target/BPF/BTFDebug.cpp
@@ -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();

diff  --git a/llvm/test/CodeGen/BPF/BTF/align.ll b/llvm/test/CodeGen/BPF/BTF/align.ll
new file mode 100644
index 0000000000000..d842b17ca88de
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/align.ll
@@ -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)