BPF: generate proper BTF for globals with WeakODRLinkage

[yonghong-song]

Summary

For a global weak symbol defined as below:

char g __attribute__((weak)) = 2;

LLVM generates an allocated global with WeakAnyLinkage,
for which BPF backend generates proper BTF info.

For the above example, if a modifier "const" is added like

const char g __attribute__((weak)) = 2;

LLVM generates an allocated global with WeakODRLinkage,
for which BPF backend didn't generate any BTF as it
didn't handle WeakODRLinkage.

This patch addes support for WeakODRLinkage and proper
BTF info can be generated for weak symbol defined with
"const" modifier.

llvm/lib/Target/BPF/BTFDebug.cpp

for (const GlobalVariable &Global : M->globals()) {
     //  . weak or non-weak extern global variables
     // Whether DataSec is readonly or not can be found from corresponding ELF
     // section flags. Whether a BTF_KIND_VAR is a weak symbol or not
     // can be found from the corresponding ELF symbol table.
     auto Linkage = Global.getLinkage();
     if (Linkage != GlobalValue::InternalLinkage &&
         Linkage != GlobalValue::ExternalLinkage &&
         Linkage != GlobalValue::WeakAnyLinkage &&
+        Linkage != GlobalValue::WeakODRLinkage &&
         Linkage != GlobalValue::ExternalWeakLinkage)
       continue;
 
     uint32_t GVarInfo;
     if (Linkage == GlobalValue::InternalLinkage) {
       GVarInfo = BTF::VAR_STATIC;
     } else if (Global.hasInitializer()) {
       GVarInfo = BTF::VAR_GLOBAL_ALLOCATED;

llvm/test/CodeGen/BPF/BTF/weak-global-3.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+;
+; Source code:
+;   const volatile char g __attribute__((weak)) = 2;
+;   int test() {
+;     return g;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+@g = weak_odr dso_local constant i8 2, align 1, !dbg !0
+
+; Function Attrs: nofree norecurse nounwind willreturn
+define dso_local i32 @test() local_unnamed_addr #0 !dbg !13 {
+entry:
+  %0 = load volatile i8, i8* @g, align 1, !dbg !17, !tbaa !18
+  %conv = sext i8 %0 to i32, !dbg !17
+  ret i32 %conv, !dbg !21
+}
+
+; CHECK:             .long   0                               # BTF_KIND_FUNC_PROTO(id = 1)
+; CHECK-NEXT:        .long   218103808                       # 0xd000000
+; CHECK-NEXT:        .long   2
+; CHECK-NEXT:        .long   1                               # BTF_KIND_INT(id = 2)
+; CHECK-NEXT:        .long   16777216                        # 0x1000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   16777248                        # 0x1000020
+; CHECK-NEXT:        .long   5                               # BTF_KIND_FUNC(id = 3)
+; CHECK-NEXT:        .long   201326593                       # 0xc000001
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   0                               # BTF_KIND_CONST(id = 4)
+; CHECK-NEXT:        .long   167772160                       # 0xa000000
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   0                               # BTF_KIND_VOLATILE(id = 5)
+; CHECK-NEXT:        .long   150994944                       # 0x9000000
+; CHECK-NEXT:        .long   6
+; CHECK-NEXT:        .long   47                              # BTF_KIND_INT(id = 6)
+; CHECK-NEXT:        .long   16777216                        # 0x1000000
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   16777224                        # 0x1000008
+; CHECK-NEXT:        .long   52                              # BTF_KIND_VAR(id = 7)
+; CHECK-NEXT:        .long   234881024                       # 0xe000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   54                              # BTF_KIND_DATASEC(id = 8)
+; CHECK-NEXT:        .long   251658241                       # 0xf000001
+; CHECK-NEXT:        .long   0
+; CHECK-NEXT:        .long   7
+; CHECK-NEXT:        .long   g
+; CHECK-NEXT:        .long   1
+
+; CHECK:             .ascii  "int"                           # string offset=1
+; CHECK:             .ascii  "test"                          # string offset=5
+; CHECK:             .ascii  "char"                          # string offset=47
+; CHECK:             .byte   103                             # string offset=52
+; CHECK:             .ascii  ".rodata"                       # string offset=54
+
+attributes #0 = { nofree norecurse nounwind willreturn "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10, !11}
+!llvm.ident = !{!12}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "g", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 13.0.0 (https://github.com/llvm/llvm-project.git 9cc417cbca1cece0d55fa3d1e15682943a06139e)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/btf/tests")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !7)
+!7 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !8)
+!8 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!9 = !{i32 7, !"Dwarf Version", i32 4}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"wchar_size", i32 4}
+!12 = !{!"clang version 13.0.0 (https://github.com/llvm/llvm-project.git 9cc417cbca1cece0d55fa3d1e15682943a06139e)"}
+!13 = distinct !DISubprogram(name: "test", scope: !3, file: !3, line: 2, type: !14, scopeLine: 2, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !4)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!16}
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !DILocation(line: 3, column: 10, scope: !13)
+!18 = !{!19, !19, i64 0}
+!19 = !{!"omnipotent char", !20, i64 0}
+!20 = !{!"Simple C/C++ TBAA"}
+!21 = !DILocation(line: 3, column: 3, scope: !13)