llvm: emit metadata for exported global variables

[tw4452852]

One use case is to genearte BTF information from global variable's metadata.

[squeek502]

By chance could this affect/fix #15095?

[tw4452852]

@squeek502 I think so. Because this patch adds the missing connection between variable declearation and its debug information.

[tw4452852]

Close by accident, reopen.

[Vexu]

Doesn't seem to do anything for #15095

[andrewrk]

Can you please show the difference in LLVM IR of this change as well as some clang generated IR for comparison?

[tw4452852]

@andrewrk Sure, here it is:

~/t/test/src> cat foo.zig 
export var bar: u32 = 0;

Before:

~/t/test/src> zig build-obj -target bpfel-freestanding -O ReleaseSafe foo.zig   -fno-emit-bin -femit-llvm-ir=- 
LLVM Emit Object... ; ModuleID = 'foo'
source_filename = "foo"
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "bpfel-unknown-unknown-eabi"

@bar = dso_local local_unnamed_addr global i32 0, align 4

!llvm.module.flags = !{!0, !1}
!llvm.dbg.cu = !{!2}

!0 = !{i32 2, !"Debug Info Version", i32 3}
!1 = !{i32 2, !"Dwarf Version", i32 4}
!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "zig 0.11.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !26)
!3 = !DIFile(filename: "foo", directory: "/home/tw/t/test/src")
!4 = !{!5, !20}
!5 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "builtin.CompilerBackend", scope: !6, file: !6, line: 1545, baseType: !7, size: 64, align: 64, elements: !8)
!6 = !DIFile(filename: "builtin.zig", directory: "/home/tw/code/zig/stage3/lib/zig/std")
!7 = !DIBasicType(name: "u64", size: 64, encoding: DW_ATE_unsigned)
!8 = !{!9, !10, !11, !12, !13, !14, !15, !16, !17, !18, !19}
!9 = !DIEnumerator(name: "other", value: 0, isUnsigned: true)
!10 = !DIEnumerator(name: "stage1", value: 1, isUnsigned: true)
!11 = !DIEnumerator(name: "stage2_llvm", value: 2, isUnsigned: true)
!12 = !DIEnumerator(name: "stage2_c", value: 3, isUnsigned: true)
!13 = !DIEnumerator(name: "stage2_wasm", value: 4, isUnsigned: true)
!14 = !DIEnumerator(name: "stage2_arm", value: 5, isUnsigned: true)
!15 = !DIEnumerator(name: "stage2_x86_64", value: 6, isUnsigned: true)
!16 = !DIEnumerator(name: "stage2_aarch64", value: 7, isUnsigned: true)
!17 = !DIEnumerator(name: "stage2_x86", value: 8, isUnsigned: true)
!18 = !DIEnumerator(name: "stage2_riscv64", value: 9, isUnsigned: true)
!19 = !DIEnumerator(name: "stage2_sparc64", value: 10, isUnsigned: true)
!20 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "builtin.OutputMode", scope: !6, file: !6, line: 642, baseType: !21, size: 8, align: 8, elements: !22)
!21 = !DIBasicType(name: "u2", size: 8, encoding: DW_ATE_unsigned)
!22 = !{!23, !24, !25}
!23 = !DIEnumerator(name: "Exe", value: 0, isUnsigned: true)
!24 = !DIEnumerator(name: "Lib", value: 1, isUnsigned: true)
!25 = !DIEnumerator(name: "Obj", value: 2, isUnsigned: true)
!26 = !{!27, !30, !34, !36}
!27 = !DIGlobalVariableExpression(var: !28, expr: !DIExpression())
!28 = distinct !DIGlobalVariable(name: "zig_backend", linkageName: "builtin.zig_backend", scope: !29, file: !29, line: 6, type: !5, isLocal: true, isDefinition: true)
!29 = !DIFile(filename: "builtin.zig", directory: "/home/tw/t/test/zig-cache/o/c1221be412c7bc2df05b39d0beeb4049")
!30 = !DIGlobalVariableExpression(var: !31, expr: !DIExpression())
!31 = distinct !DIGlobalVariable(name: "simplified_logic", linkageName: "start.simplified_logic", scope: !32, file: !32, line: 20, type: !33, isLocal: true, isDefinition: true)
!32 = !DIFile(filename: "start.zig", directory: "/home/tw/code/zig/stage3/lib/zig/std")
!33 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
!34 = !DIGlobalVariableExpression(var: !35, expr: !DIExpression())
!35 = distinct !DIGlobalVariable(name: "output_mode", linkageName: "builtin.output_mode", scope: !29, file: !29, line: 8, type: !20, isLocal: true, isDefinition: true)
!36 = !DIGlobalVariableExpression(var: !37, expr: !DIExpression())
!37 = distinct !DIGlobalVariable(name: "bar", linkageName: "bar", scope: !38, file: !38, line: 1, type: !39, isLocal: false, isDefinition: true)
!38 = !DIFile(filename: "foo.zig", directory: "/home/tw/t/test/src")
!39 = !DIBasicType(name: "u32", size: 32, encoding: DW_ATE_unsigned)

After:

~/t/test/src> zig build-obj -target bpfel-freestanding -O ReleaseSafe foo.zig   -fno-emit-bin -femit-llvm-ir=- 
LLVM Emit Object... ; ModuleID = 'foo'
source_filename = "foo"
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "bpfel-unknown-unknown-eabi"

@bar = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0

!llvm.module.flags = !{!4, !5}
!llvm.dbg.cu = !{!6}

!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
!1 = distinct !DIGlobalVariable(name: "bar", linkageName: "bar", scope: !2, file: !2, line: 1, type: !3, isLocal: false, isDefinition: true)
!2 = !DIFile(filename: "foo.zig", directory: "/home/tw/t/test/src")
!3 = !DIBasicType(name: "u32", size: 32, encoding: DW_ATE_unsigned)
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = !{i32 2, !"Dwarf Version", i32 4}
!6 = distinct !DICompileUnit(language: DW_LANG_C99, file: !7, producer: "zig 0.11.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !8, globals: !30)
!7 = !DIFile(filename: "foo", directory: "/home/tw/t/test/src")
!8 = !{!9, !24}
!9 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "builtin.CompilerBackend", scope: !10, file: !10, line: 1545, baseType: !11, size: 64, align: 64, elements: !12)
!10 = !DIFile(filename: "builtin.zig", directory: "/home/tw/code/zig/stage3/lib/zig/std")
!11 = !DIBasicType(name: "u64", size: 64, encoding: DW_ATE_unsigned)
!12 = !{!13, !14, !15, !16, !17, !18, !19, !20, !21, !22, !23}
!13 = !DIEnumerator(name: "other", value: 0, isUnsigned: true)
!14 = !DIEnumerator(name: "stage1", value: 1, isUnsigned: true)
!15 = !DIEnumerator(name: "stage2_llvm", value: 2, isUnsigned: true)
!16 = !DIEnumerator(name: "stage2_c", value: 3, isUnsigned: true)
!17 = !DIEnumerator(name: "stage2_wasm", value: 4, isUnsigned: true)
!18 = !DIEnumerator(name: "stage2_arm", value: 5, isUnsigned: true)
!19 = !DIEnumerator(name: "stage2_x86_64", value: 6, isUnsigned: true)
!20 = !DIEnumerator(name: "stage2_aarch64", value: 7, isUnsigned: true)
!21 = !DIEnumerator(name: "stage2_x86", value: 8, isUnsigned: true)
!22 = !DIEnumerator(name: "stage2_riscv64", value: 9, isUnsigned: true)
!23 = !DIEnumerator(name: "stage2_sparc64", value: 10, isUnsigned: true)
!24 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "builtin.OutputMode", scope: !10, file: !10, line: 642, baseType: !25, size: 8, align: 8, elements: !26)
!25 = !DIBasicType(name: "u2", size: 8, encoding: DW_ATE_unsigned)
!26 = !{!27, !28, !29}
!27 = !DIEnumerator(name: "Exe", value: 0, isUnsigned: true)
!28 = !DIEnumerator(name: "Lib", value: 1, isUnsigned: true)
!29 = !DIEnumerator(name: "Obj", value: 2, isUnsigned: true)
!30 = !{!31, !34, !38, !0}
!31 = !DIGlobalVariableExpression(var: !32, expr: !DIExpression())
!32 = distinct !DIGlobalVariable(name: "zig_backend", linkageName: "builtin.zig_backend", scope: !33, file: !33, line: 6, type: !9, isLocal: true, isDefinition: true)
!33 = !DIFile(filename: "builtin.zig", directory: "/home/tw/t/test/zig-cache/o/95f34795906157beed77417e714d3875")
!34 = !DIGlobalVariableExpression(var: !35, expr: !DIExpression())
!35 = distinct !DIGlobalVariable(name: "simplified_logic", linkageName: "start.simplified_logic", scope: !36, file: !36, line: 20, type: !37, isLocal: true, isDefinition: true)
!36 = !DIFile(filename: "start.zig", directory: "/home/tw/code/zig/stage3/lib/zig/std")
!37 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
!38 = !DIGlobalVariableExpression(var: !39, expr: !DIExpression())
!39 = distinct !DIGlobalVariable(name: "output_mode", linkageName: "builtin.output_mode", scope: !33, file: !33, line: 8, type: !24, isLocal: true, isDefinition: true)

As you can see, the metadata for bar is appended after this change.

Clang:

~/t/test/src> cat foo.c
unsigned int bar = 0;
~/t/test/src> 
~/t/test/src> clang -g -O2 -target bpf -S foo.c -emit-llvm -o -
; ModuleID = 'foo.c'
source_filename = "foo.c"
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "bpf"

@bar = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0

!llvm.dbg.cu = !{!2}
!llvm.module.flags = !{!7, !8, !9}
!llvm.ident = !{!10}

!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
!1 = distinct !DIGlobalVariable(name: "bar", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 12.0.1", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
!3 = !DIFile(filename: "foo.c", directory: "/home/tw/t/test/src")
!4 = !{}
!5 = !{!0}
!6 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
!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 12.0.1"}

[andrewrk]

Thanks @tw4452852! Looks great.