[BPF] Fix a bug for __builtin_preserve_field_info() processing

[yonghong-song]

Summary

Andrii discovered a problem where a simple case similar to below
will generate wrong relocation kind:

enum { FIELD_EXISTENCE = 2, };
struct s1 { int a1; };
int test() {
  struct s1 *v = 0;
  return __builtin_preserve_field_info(v[0], FIELD_EXISTENCE);
}

The expected relocation kind should be FIELD_EXISTENCE, but
recorded reloc kind in the final object file is FIELD_BYTE_OFFSET,
which is incorrect.

This exposed a bug in generating access strings from intrinsics.
The current access string generation has two steps:

step 1: find the base struct/union type,
step 2: traverse members in the base type.

The current implementation relies on at lease one member access
in step 2 to get the correct relocation kind, which is true
in typical cases. But if there is no member accesses, the current
implementation falls to the default info kind FIELD_BYTE_OFFSET.
This is incorrect, we should still record the reloc kind
based on the user input. This patch fixed this issue by properly
recording the reloc kind in such cases.

llvm/lib/Target/BPF/BPFAbstractMemberAccess.cpp

Value *BPFAbstractMemberAccess::computeBaseAndAccessKey(CallInst *Call,
 
   // Traverse the rest of access chain to complete offset calculation
   // and access key construction.
   while (CallStack.size()) {
     auto StackElem = CallStack.top();
     CInfo = StackElem.second;
     CallStack.pop();
 
-    if (CInfo.Kind == BPFPreserveFieldInfoAI)
+    if (CInfo.Kind == BPFPreserveFieldInfoAI) {
+      InfoKind = CInfo.AccessIndex;
       break;
+    }
 
     // If the next Call (the top of the stack) is a BPFPreserveFieldInfoAI,
     // the action will be extracting field info.
     if (CallStack.size()) {
       auto StackElem2 = CallStack.top();
       CallInfo CInfo2 = StackElem2.second;
       if (CInfo2.Kind == BPFPreserveFieldInfoAI) {
         InfoKind = CInfo2.AccessIndex;

llvm/test/CodeGen/BPF/CORE/intrinsic-array-2.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK,CHECK-ALU64 %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK,CHECK-ALU64 %s
+; RUN: llc -march=bpfel -mattr=+alu32 -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK,CHECK-ALU32 %s
+; RUN: llc -march=bpfeb -mattr=+alu32 -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK,CHECK-ALU32 %s
+; Source:
+;   enum { FIELD_EXISTENCE = 2, };
+;   struct s1 { int a1; };
+;   int test() {
+;     struct s1 *v = 0;
+;     return __builtin_preserve_field_info(v[0], FIELD_EXISTENCE);
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+%struct.s1 = type { i32 }
+
+; Function Attrs: nounwind readnone
+define dso_local i32 @test() local_unnamed_addr #0 !dbg !17 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.s1* null, metadata !21, metadata !DIExpression()), !dbg !22
+  %0 = tail call %struct.s1* @llvm.preserve.array.access.index.p0s_struct.s1s.p0s_struct.s1s(%struct.s1* null, i32 0, i32 0), !dbg !23, !llvm.preserve.access.index !8
+  %1 = tail call i32 @llvm.bpf.preserve.field.info.p0s_struct.s1s(%struct.s1* %0, i64 2), !dbg !24
+  ret i32 %1, !dbg !25
+}
+
+; CHECK:             .long   16                      # BTF_KIND_STRUCT(id = 4)
+
+; CHECK:             .ascii  ".text"                 # string offset=10
+; CHECK:             .ascii  "s1"                    # string offset=16
+; CHECK:             .byte   48                      # string offset=22
+
+; CHECK:             .long   16                      # FieldReloc
+; CHECK-NEXT:        .long   10                      # Field reloc section string offset=10
+; CHECK-NEXT:        .long   1
+; CHECK-NEXT:        .long   .Ltmp{{[0-9]+}}
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   22
+; CHECK-NEXT:        .long   2
+
+; Function Attrs: nounwind readnone
+declare %struct.s1* @llvm.preserve.array.access.index.p0s_struct.s1s.p0s_struct.s1s(%struct.s1*, i32 immarg, i32 immarg) #1
+
+; Function Attrs: nounwind readnone
+declare i32 @llvm.bpf.preserve.field.info.p0s_struct.s1s(%struct.s1*, i64 immarg) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind readnone "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"="true" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!13, !14, !15}
+!llvm.ident = !{!16}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0 (https://github.com/llvm/llvm-project.git 32791937d7aceb0a5e1eaabf1bb1a6dbe1639792)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !7, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/core")
+!2 = !{!3}
+!3 = !DICompositeType(tag: DW_TAG_enumeration_type, file: !1, line: 1, baseType: !4, size: 32, elements: !5)
+!4 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!5 = !{!6}
+!6 = !DIEnumerator(name: "FIELD_EXISTENCE", value: 2, isUnsigned: true)
+!7 = !{!8}
+!8 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !9, size: 64)
+!9 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s1", file: !1, line: 2, size: 32, elements: !10)
+!10 = !{!11}
+!11 = !DIDerivedType(tag: DW_TAG_member, name: "a1", scope: !9, file: !1, line: 2, baseType: !12, size: 32)
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !{i32 7, !"Dwarf Version", i32 4}
+!14 = !{i32 2, !"Debug Info Version", i32 3}
+!15 = !{i32 1, !"wchar_size", i32 4}
+!16 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git 32791937d7aceb0a5e1eaabf1bb1a6dbe1639792)"}
+!17 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 3, type: !18, scopeLine: 3, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !20)
+!18 = !DISubroutineType(types: !19)
+!19 = !{!12}
+!20 = !{!21}
+!21 = !DILocalVariable(name: "v", scope: !17, file: !1, line: 4, type: !8)
+!22 = !DILocation(line: 0, scope: !17)
+!23 = !DILocation(line: 5, column: 40, scope: !17)
+!24 = !DILocation(line: 5, column: 10, scope: !17)
+!25 = !DILocation(line: 5, column: 3, scope: !17)