BPF: generate BTF info for LD_imm64 loaded function pointer

For an example like below,
    extern int do_work(int);
    long bpf_helper(void *callback_fn);
    long prog() {
        return bpf_helper(&do_work);
    }

The final generated codes look like:
    r1 = do_work ll
    call bpf_helper
    exit
where we have debuginfo for do_work() extern function:
    !17 = !DISubprogram(name: "do_work", ...)

This patch implemented to add additional checking
in processing LD_imm64 operands for possible function pointers
so BTF for bpf function do_work() can be properly generated.
The original llvm function name processReloc() is renamed to
processGlobalValue() to better reflect what the function is doing.

Differential Revision: https://reviews.llvm.org/D100568

Author: yonghong-song

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -1030,13 +1030,16 @@ void BTFDebug::generatePatchImmReloc(const MCSymbol *ORSym, uint32_t RootId,
   FieldRelocTable[SecNameOff].push_back(FieldReloc);
 }
 
-void BTFDebug::processReloc(const MachineOperand &MO) {
+void BTFDebug::processGlobalValue(const MachineOperand &MO) {
   // check whether this is a candidate or not
   if (MO.isGlobal()) {
     const GlobalValue *GVal = MO.getGlobal();
     auto *GVar = dyn_cast<GlobalVariable>(GVal);
-    if (!GVar)
+    if (!GVar) {
+      // Not a global variable. Maybe an extern function reference.
+      processFuncPrototypes(dyn_cast<Function>(GVal));
       return;
+    }
 
     if (!GVar->hasAttribute(BPFCoreSharedInfo::AmaAttr) &&
         !GVar->hasAttribute(BPFCoreSharedInfo::TypeIdAttr))
@@ -1087,12 +1090,12 @@ void BTFDebug::beginInstruction(const MachineInstr *MI) {
     //
     // If the insn is "r2 = LD_imm64 @<an TypeIdAttr global>",
     // The LD_imm64 result will be replaced with a btf type id.
-    processReloc(MI->getOperand(1));
+    processGlobalValue(MI->getOperand(1));
   } else if (MI->getOpcode() == BPF::CORE_MEM ||
              MI->getOpcode() == BPF::CORE_ALU32_MEM ||
              MI->getOpcode() == BPF::CORE_SHIFT) {
     // relocation insn is a load, store or shift insn.
-    processReloc(MI->getOperand(3));
+    processGlobalValue(MI->getOperand(3));
   } else if (MI->getOpcode() == BPF::JAL) {
     // check extern function references
     const MachineOperand &MO = MI->getOperand(0);

llvm/lib/Target/BPF/BTFDebug.h

@@ -320,8 +320,9 @@ class BTFDebug : public DebugHandlerBase {
   /// Populating unprocessed type on demand.
   unsigned populateType(const DIType *Ty);
 
-  /// Process relocation instructions.
-  void processReloc(const MachineOperand &MO);
+  /// Process global variables referenced by relocation instructions
+  /// and extern function references.
+  void processGlobalValue(const MachineOperand &MO);
 
   /// Emit common header of .BTF and .BTF.ext sections.
   void emitCommonHeader();

llvm/test/CodeGen/BPF/BTF/extern-func-ptr.ll

@@ -0,0 +1,76 @@
+; 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:
+;   extern int do_work(int) __attribute__((section(".callback_fn")));
+;   long bpf_helper(void *callback_fn);
+;   long prog() {
+;       return bpf_helper(&do_work);
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+; Function Attrs: nounwind
+define dso_local i64 @prog() local_unnamed_addr #0 !dbg !7 {
+entry:
+  %call = tail call i64 @bpf_helper(i8* bitcast (i32 (i32)* @do_work to i8*)) #2, !dbg !11
+  ret i64 %call, !dbg !12
+}
+
+; CHECK:             .long   0                               # BTF_KIND_FUNC_PROTO(id = 4)
+; CHECK-NEXT:        .long   218103809                       # 0xd000001
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   0
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   51                              # BTF_KIND_INT(id = 5)
+; CHECK-NEXT:        .long   16777216                        # 0x1000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   16777248                        # 0x1000020
+; CHECK-NEXT:        .long   55                              # BTF_KIND_FUNC(id = 6)
+; CHECK-NEXT:        .long   201326594                       # 0xc000002
+; CHECK-NEXT:        .long   4
+
+; CHECK:             .long   74                              # BTF_KIND_DATASEC(id = 10)
+; CHECK-NEXT:        .long   251658241                       # 0xf000001
+; CHECK-NEXT:        .long   0
+; CHECK-NEXT:        .long   6
+; CHECK-NEXT:        .long   do_work
+; CHECK-NEXT:        .long   0
+
+; CHECK:             .ascii  "int"                           # string offset=51
+; CHECK:             .ascii  "do_work"                       # string offset=55
+; CHECK:             .ascii  ".callback_fn"                  # string offset=74
+
+declare !dbg !13 dso_local i64 @bpf_helper(i8*) local_unnamed_addr #1
+
+declare !dbg !17 dso_local i32 @do_work(i32) #1 section ".callback_fn"
+
+attributes #0 = { nounwind "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #1 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #2 = { nounwind }
+
+!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 13.0.0 (https://github.com/llvm/llvm-project.git aa382ed8a38d5efa118e1b2617544f5c253658a9)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/btf/core")
+!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 13.0.0 (https://github.com/llvm/llvm-project.git aa382ed8a38d5efa118e1b2617544f5c253658a9)"}
+!7 = distinct !DISubprogram(name: "prog", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!11 = !DILocation(line: 4, column: 12, scope: !7)
+!12 = !DILocation(line: 4, column: 5, scope: !7)
+!13 = !DISubprogram(name: "bpf_helper", scope: !1, file: !1, line: 2, type: !14, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!10, !16}
+!16 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!17 = !DISubprogram(name: "do_work", scope: !1, file: !1, line: 1, type: !18, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+!18 = !DISubroutineType(types: !19)
+!19 = !{!20, !20}
+!20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)