[BPF] add "llvm." prefix to BPF internally created globals

[yonghong-song]

Summary

Currently, BPF backend creates some global variables with name like

<type_name>:<reloc_type>:<patch_imm>$<access_str>

to carry certain information to BPF backend.

With direct clang compilation, the following code in

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

is triggered and the above globals are emitted to the ELF file.
(clang enabled this as opt flag -faddrsig is on by default.)

 if (TM.Options.EmitAddrsig) {
  // Emit address-significance attributes for all globals.
  OutStreamer->EmitAddrsig();
  for (const GlobalValue &GV : M.global_values())
    if (!GV.use_empty() && !GV.isThreadLocal() &&
        !GV.hasDLLImportStorageClass() && !GV.getName().startswith("llvm.") &&
        !GV.hasAtLeastLocalUnnamedAddr())
      OutStreamer->EmitAddrsigSym(getSymbol(&GV));
}

...
10162: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND tcp_sock:0:2048$0:117
10163: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND tcp_sock:0:2112$0:126:0
10164: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND tcp_sock:1:8$0:31:6
...
While in llc, those globals are not emited since EmitAddrsig
default option is false for llc. The llc flag "-addrsig" can be used to
enable the above code.

This patch added "llvm." prefix to these internal globals so that
they can be ignored in the above codes and possible other
places.

llvm/lib/Target/BPF/BPFAbstractMemberAccess.cpp

while (CallStack.size()) {
     MDNode *MDN = CInfo.Metadata;
     uint32_t RecordAlignment = CInfo.RecordAlignment;
     // At this stage, it cannot be pointer type.
     auto *CTy = cast<DICompositeType>(stripQualifiers(cast<DIType>(MDN)));
     PatchImm = GetFieldInfo(InfoKind, CTy, AccessIndex, PatchImm,
                             RecordAlignment);
   }
 
-  // Access key is the type name + reloc type + patched imm + access string,
+  // Access key is the
+  //   "llvm." + type name + ":" + reloc type + ":" + patched imm + "$" +
+  //   access string,
   // uniquely identifying one relocation.
-  AccessKey = TypeName + ":" + std::to_string(InfoKind) + ":" +
+  // The prefix "llvm." indicates this is a temporary global, which should
+  // not be emitted to ELF file.
+  AccessKey = "llvm." + TypeName + ":" + std::to_string(InfoKind) + ":" +
               std::to_string(PatchImm) + "$" + AccessKey;
 
   return Base;
 }
 
 /// Call/Kind is the base preserve_*_access_index() call. Attempts to do
 /// transformation to a chain of relocable GEPs.
 bool BPFAbstractMemberAccess::transformGEPChain(Module &M, CallInst *Call,

llvm/test/CodeGen/BPF/CORE/no-elf-ama-symbol.ll

+; RUN: llc -march=bpfel -filetype=obj -o - %s | llvm-readelf -s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=obj -o - %s | llvm-readelf -s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfel -filetype=obj -addrsig -o - %s | llvm-readelf -s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=obj -addrsig -o - %s | llvm-readelf -s | FileCheck -check-prefixes=CHECK %s
+;
+; Source Code:
+;   struct tt { int a; } __attribute__((preserve_access_index));
+;   int test(struct tt *arg) {
+;     return arg->a;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm t.c
+
+%struct.tt = type { i32 }
+
+; Function Attrs: nounwind readonly
+define dso_local i32 @test(%struct.tt* readonly %arg) local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.tt* %arg, metadata !16, metadata !DIExpression()), !dbg !17
+  %0 = tail call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.tts(%struct.tt* %arg, i32 0, i32 0), !dbg !18, !llvm.preserve.access.index !12
+  %1 = load i32, i32* %0, align 4, !dbg !18, !tbaa !19
+  ret i32 %1, !dbg !24
+}
+
+; CHECK-NOT: llvm.tt:0:0$0:0
+
+; Function Attrs: nounwind readnone
+declare i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.tts(%struct.tt*, i32, i32) #1
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind readonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone }
+attributes #2 = { nounwind readnone speculatable}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (https://github.com/llvm/llvm-project.git 947f9692440836dcb8d88b74b69dd379d85974ce)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/bug")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git 947f9692440836dcb8d88b74b69dd379d85974ce)"}
+!7 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 2, type: !8, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !15)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !11}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "tt", file: !1, line: 1, size: 32, elements: !13)
+!13 = !{!14}
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !12, file: !1, line: 1, baseType: !10, size: 32)
+!15 = !{!16}
+!16 = !DILocalVariable(name: "arg", arg: 1, scope: !7, file: !1, line: 2, type: !11)
+!17 = !DILocation(line: 0, scope: !7)
+!18 = !DILocation(line: 3, column: 15, scope: !7)
+!19 = !{!20, !21, i64 0}
+!20 = !{!"tt", !21, i64 0}
+!21 = !{!"int", !22, i64 0}
+!22 = !{!"omnipotent char", !23, i64 0}
+!23 = !{!"Simple C/C++ TBAA"}
+!24 = !DILocation(line: 3, column: 3, scope: !7)