[BPF] Support weak global variables for BTF

[yonghong-song]

Summary

Generate types for global variables with "weak" attribute.
Keep allocation scope the same for both weak and non-weak
globals as ELF symbol table can determine whether a global
symbol is weak or not.

[ast]

lgtm

llvm/lib/Target/BPF/BTFDebug.cpp

for (auto *GVE : GVs) {
         visitMapDefType(GVE->getVariable()->getType(), GVTypeId);
       else
         visitTypeEntry(GVE->getVariable()->getType(), GVTypeId, false, false);
       break;
     }
 
     // Only support the following globals:
     //  . static variables
-    //  . non-static global variables
+    //  . non-static weak or non-weak global variables
     // Essentially means:
     //  . .bcc/.data/.rodata DataSec entities only contain static data
     //  . Other DataSec entities contain static or initialized global data.
     //    Initialized global data are mostly used for finding map key/value type
     //    id's. Whether DataSec is readonly or not can be found from
     //    corresponding ELF section flags.
     auto Linkage = Global.getLinkage();
     if (Linkage != GlobalValue::InternalLinkage &&
-        Linkage != GlobalValue::ExternalLinkage)
+        Linkage != GlobalValue::ExternalLinkage &&
+        Linkage != GlobalValue::WeakAnyLinkage)
       continue;
 
-    uint32_t GVarInfo = Linkage == GlobalValue::ExternalLinkage
-                            ? BTF::VAR_GLOBAL_ALLOCATED
-                            : BTF::VAR_STATIC;
+    uint32_t GVarInfo = Linkage == GlobalValue::InternalLinkage
+                            ? BTF::VAR_STATIC
+                            : BTF::VAR_GLOBAL_ALLOCATED;
     auto VarEntry =
         std::make_unique<BTFKindVar>(Global.getName(), GVTypeId, GVarInfo);
     uint32_t VarId = addType(std::move(VarEntry));
 
     // Find or create a DataSec
     if (DataSecEntries.find(SecName) == DataSecEntries.end()) {
       DataSecEntries[SecName] = std::make_unique<BTFKindDataSec>(Asm, SecName);
     }

llvm/test/CodeGen/BPF/BTF/weak-global-2.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:
+;   char g __attribute__((weak)) = 2;
+;   int test() {
+;     return g;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+@g = weak dso_local local_unnamed_addr global i8 2, align 1, !dbg !0
+; Function Attrs: norecurse nounwind readonly
+define dso_local i32 @test() local_unnamed_addr #0 !dbg !11 {
+entry:
+  %0 = load i8, i8* @g, align 1, !dbg !15, !tbaa !16
+  %conv = sext i8 %0 to i32, !dbg !15
+  ret i32 %conv, !dbg !19
+}
+
+; CHECK:             .long   55                      # BTF_KIND_INT(id = 4)
+; CHECK-NEXT:        .long   16777216                # 0x1000000
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   16777224                # 0x1000008
+; CHECK-NEXT:        .long   60                      # BTF_KIND_VAR(id = 5)
+; CHECK-NEXT:        .long   234881024               # 0xe000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   62                      # BTF_KIND_DATASEC(id = 6)
+; CHECK-NEXT:        .long   251658241               # 0xf000001
+; CHECK-NEXT:        .long   0
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   g
+; CHECK-NEXT:        .long   1
+
+; CHECK:             .ascii  "char"                  # string offset=55
+; CHECK:             .byte   103                     # string offset=60
+; CHECK:             .ascii  ".data"                 # string offset=62
+
+
+attributes #0 = { norecurse 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" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!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 10.0.0 (https://github.com/llvm/llvm-project.git edf6717d8d30034da932b95350898e03c90a5082)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/global")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git edf6717d8d30034da932b95350898e03c90a5082)"}
+!11 = distinct !DISubprogram(name: "test", scope: !3, file: !3, line: 2, type: !12, scopeLine: 2, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !4)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!14}
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !DILocation(line: 3, column: 10, scope: !11)
+!16 = !{!17, !17, i64 0}
+!17 = !{!"omnipotent char", !18, i64 0}
+!18 = !{!"Simple C/C++ TBAA"}
+!19 = !DILocation(line: 3, column: 3, scope: !11)

llvm/test/CodeGen/BPF/BTF/weak-global.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:
+;   char g __attribute__((weak));
+;   int test() {
+;     return g;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+@g = weak dso_local local_unnamed_addr global i8 0, align 1, !dbg !0
+; Function Attrs: norecurse nounwind readonly
+define dso_local i32 @test() local_unnamed_addr #0 !dbg !11 {
+entry:
+  %0 = load i8, i8* @g, align 1, !dbg !15, !tbaa !16
+  %conv = sext i8 %0 to i32, !dbg !15
+  ret i32 %conv, !dbg !19
+}
+
+; CHECK:             .long   55                      # BTF_KIND_INT(id = 4)
+; CHECK-NEXT:        .long   16777216                # 0x1000000
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   16777224                # 0x1000008
+; CHECK-NEXT:        .long   60                      # BTF_KIND_VAR(id = 5)
+; CHECK-NEXT:        .long   234881024               # 0xe000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   62                      # BTF_KIND_DATASEC(id = 6)
+; CHECK-NEXT:        .long   251658241               # 0xf000001
+; CHECK-NEXT:        .long   0
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   g
+; CHECK-NEXT:        .long   1
+
+; CHECK:             .ascii  "char"                  # string offset=55
+; CHECK:             .byte   103                     # string offset=60
+; CHECK:             .ascii  ".bss"                  # string offset=62
+
+attributes #0 = { norecurse 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" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!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 10.0.0 (https://github.com/llvm/llvm-project.git edf6717d8d30034da932b95350898e03c90a5082)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/global")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git edf6717d8d30034da932b95350898e03c90a5082)"}
+!11 = distinct !DISubprogram(name: "test", scope: !3, file: !3, line: 2, type: !12, scopeLine: 2, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !4)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!14}
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !DILocation(line: 3, column: 10, scope: !11)
+!16 = !{!17, !17, i64 0}
+!17 = !{!"omnipotent char", !18, i64 0}
+!18 = !{!"Simple C/C++ TBAA"}
+!19 = !DILocation(line: 3, column: 3, scope: !11)