[XRay][CodeGen] Use the current function symbol as the associated symbol for the instrumentation map

[dberris]

Summary

XRay had been assuming that the previous section is the "text" section
of the function when lowering the instrumentation map. Unfortunately
this is not a safe assumption, because we may be coming from lowering
debug type information for the function being lowered.

This fixes an issue with combining -gsplit-dwarf, -generate-type-units,
-debug-compile and -fxray-instrument for sole member functions. When the
split dwarf section is stripped, we're left with references from the
xray_instr_map to the debug section. The change now uses the function's
symbol instead of the previous section's start symbol.

We found the bug while attempting to strip the split debug sections off
an XRay-instrumented object file, which had a peculiar edge-case for
single-function classes where the single function is being lowered.
Because XRay had assocaited the instrumentation map for a function to
the debug types section instead of the function's section, the objcopy
call will fail due to the misplaced reference from the xray_instr_map
section.

[dberris]

Reworded description, added a test.

[pcc]

Perhaps simplify the test a little by requesting asm output and checking that the section directives are correct.

Lgtm otherwise.

[dberris]

• fixup: simplify test to use asm instead; remove some unnecessary attributes and directives

[dberris]

• fixup: still test the generated object file

[dberris]

Re-word to explain how we found the bug.

llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::emitXRayTable() {
   if (Sleds.empty())
     return;
 
   auto PrevSection = OutStreamer->getCurrentSectionOnly();
   auto Fn = MF->getFunction();
   MCSection *InstMap = nullptr;
   MCSection *FnSledIndex = nullptr;
   if (MF->getSubtarget().getTargetTriple().isOSBinFormatELF()) {
-    auto Associated = dyn_cast<MCSymbolELF>(PrevSection->getBeginSymbol());
+    auto Associated = dyn_cast<MCSymbolELF>(CurrentFnSym);
     assert(Associated != nullptr);
     auto Flags = ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_LINK_ORDER;
     std::string GroupName;
     if (Fn->hasComdat()) {
       Flags |= ELF::SHF_GROUP;
       GroupName = Fn->getComdat()->getName();
     }
 

llvm/trunk/test/CodeGen/Mips/xray-section-group.ll

 ; RUN: llc -filetype=asm -o - -mtriple=mips-unknown-linux-gnu -function-sections < %s | FileCheck %s
 ; RUN: llc -filetype=asm -o - -mtriple=mipsel-unknown-linux-gnu -function-sections < %s | FileCheck %s
 ; RUN: llc -filetype=obj -o %t -mtriple=mips-unknown-linux-gnu -function-sections < %s
 ; RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=CHECK-OBJ
 ; RUN: llc -filetype=obj -o %t -mtriple=mipsel-unknown-linux-gnu -function-sections < %s
 ; RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=CHECK-OBJ
 ; RUN: llc -filetype=asm -o - -mtriple=mips64-unknown-linux-gnu -function-sections < %s | FileCheck %s
 ; RUN: llc -filetype=asm -o - -mtriple=mips64el-unknown-linux-gnu -function-sections < %s | FileCheck %s
 ; RUN: llc -filetype=obj -o %t -mtriple=mips64-unknown-linux-gnu -function-sections < %s
 ; RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=CHECK-OBJ
 ; RUN: llc -filetype=obj -o %t -mtriple=mips64el-unknown-linux-gnu -function-sections < %s
 ; RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=CHECK-OBJ
 
 define i32 @foo() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK: .section .text.foo,"ax",@progbits
   ret i32 0
-; CHECK: .section xray_instr_map,"awo",@progbits,.text.foo,unique,1
+; CHECK: .section xray_instr_map,"awo",@progbits,foo,unique,1
 }
 
 ; CHECK-OBJ: Section {
 ; CHECK-OBJ:   Name: xray_instr_map
 
 $bar = comdat any
 define i32 @bar() nounwind noinline uwtable "function-instrument"="xray-always" comdat($bar) {
 ; CHECK: .section .text.bar,"axG",@progbits,bar,comdat
   ret i32 1
-; CHECK: .section xray_instr_map,"aGwo",@progbits,bar,comdat,.text.bar,unique,2
+; CHECK: .section xray_instr_map,"aGwo",@progbits,bar,comdat,bar,unique,2
 }
 
 ; CHECK-OBJ: Section {
 ; CHECK-OBJ:   Name: xray_instr_map

llvm/trunk/test/CodeGen/PowerPC/xray-attribute-instrumentation.ll

 ; CHECK:              blr
 ; CHECK-NEXT:         nop
 ; CHECK-NEXT:         std 0, -8(1)
 ; CHECK-NEXT:         mflr 0
 ; CHECK-NEXT:         bl __xray_FunctionExit
 ; CHECK-NEXT:         nop
 ; CHECK-NEXT:         mtlr 0
 }
-; CHECK-LABEL: xray_instr_map,"awo",@progbits,.text,unique,1
+; CHECK-LABEL: xray_instr_map,"awo",@progbits,foo,unique,1
 ; CHECK:      .Lxray_sleds_start0:
 ; CHECK-NEXT:         .quad   .Ltmp0
 ; CHECK-NEXT:         .quad   foo
 ; CHECK-NEXT:         .byte   0x00
 ; CHECK-NEXT:         .byte   0x01
 ; CHECK-NEXT:         .byte   0x00
 ; CHECK-NEXT:         .space  13
 ; CHECK-NEXT:         .quad   .Ltmp2
 ; CHECK-NEXT:         .quad   foo
 ; CHECK-NEXT:         .byte   0x01
 ; CHECK-NEXT:         .byte   0x01
 ; CHECK-NEXT:         .byte   0x00
 ; CHECK-NEXT:         .space  13
 ; CHECK-NEXT: .Lxray_sleds_end0:
-; CHECK-LABEL: xray_fn_idx,"awo",@progbits,.text,unique,1
+; CHECK-LABEL: xray_fn_idx,"awo",@progbits,foo,unique,1
 ; CHECK:              .p2align        4
 ; CHECK-NEXT:         .quad   .Lxray_sleds_start0
 ; CHECK-NEXT:         .quad   .Lxray_sleds_end0
 ; CHECK-NEXT:         .text

llvm/trunk/test/CodeGen/X86/xray-section-group.ll

 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu -function-sections < %s | FileCheck %s
 ; RUN: llc -filetype=obj -o %t -mtriple=x86_64-unknown-linux-gnu -function-sections < %s
 ; RUN: llvm-objdump -triple x86_64-unknown-linux-gnu -disassemble-all %t | FileCheck %s --check-prefix=CHECK-OBJ
 
 define i32 @foo() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK: .section .text.foo,"ax",@progbits
   ret i32 0
-; CHECK: .section xray_instr_map,"awo",@progbits,.text.foo,unique,1
+; CHECK: .section xray_instr_map,"awo",@progbits,foo,unique,1
 }
 
 $bar = comdat any
 define i32 @bar() nounwind noinline uwtable "function-instrument"="xray-always" comdat($bar) {
 ; CHECK: .section .text.bar,"axG",@progbits,bar,comdat
   ret i32 1
-; CHECK: .section xray_instr_map,"aGwo",@progbits,bar,comdat,.text.bar,unique,2
+; CHECK: .section xray_instr_map,"aGwo",@progbits,bar,comdat,bar,unique,2
 }
 
 ; CHECK-OBJ:      section xray_instr_map:

llvm/trunk/test/DebugInfo/X86/xray-split-dwarf-interaction.ll

+; RUN: %llc_dwarf -split-dwarf-file=input.dwo -O3 \
+; RUN:     -function-sections -data-sections \
+; RUN:     -relocation-model=pic -filetype=asm \
+; RUN:     -generate-type-units -debug-compile -o - %s | \
+; RUN:     FileCheck %s --check-prefix=CHECK-ASM
+; RUN: %llc_dwarf -split-dwarf-file=input.dwo -O3 \
+; RUN:     -function-sections -data-sections \
+; RUN:     -relocation-model=pic -filetype=obj \
+; RUN:     -generate-type-units -debug-compile -o - %s | \
+; RUN:     llvm-readelf -sections | \
+; RUN:     FileCheck %s --check-prefix=CHECK-ELF
+; Created from `clang++ -fxray-instrument -gsplit-dwarf -fdebug-types-section
+; -ffunction-sections -fdata-sections -emit-llvm -S input.cc`:
+; input.cc:
+;
+; class a {
+;   int b();
+; };
+; int a::b() {
+;   for (;;)
+;     ;
+; }
+;
+; In this test we want to make sure that the xray_instr_map section for
+; `a::b()` is actually associated with the function's symbol instead of the
+; .debug_types.dwo section.
+;
+; CHECK-ASM: xray_fn_idx,"awo",@progbits,_ZN1a1bEv,unique,1
+;
+; CHECK-ELF-DAG: [[FSECT:[0-9]+]]] .text._ZN1a1bEv PROGBITS
+; CHECK-ELF-DAG: [{{.*}}] .debug_types.dwo PROGBITS
+; CHECK-ELF-DAG: [{{.*}}] xray_instr_map PROGBITS {{.*}} {{.*}} {{.*}} {{.*}} WAL [[FSECT]]
+target triple = "x86_64-pc-linux"
+
+%class.a = type { i8 }
+
+; Function Attrs: nounwind readnone uwtable
+define i32 @_ZN1a1bEv(%class.a* nocapture readnone) local_unnamed_addr #0 align 2 !dbg !8 {
+  tail call void @llvm.dbg.value(metadata %class.a* %0, metadata !17, metadata !DIExpression()), !dbg !19
+  br label %2, !dbg !20
+
+; <label>:2:                                      ; preds = %2, %1
+  br label %2, !dbg !21, !llvm.loop !25
+}
+
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind readnone uwtable "xray-instruction-threshold"="200" }
+attributes #1 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version trunk (trunk r312634)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, debugInfoForProfiling: true)
+!1 = !DIFile(filename: "input.cc", directory: "/usr/local/google/home/dberris/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 7, !"PIC Level", i32 2}
+!6 = !{i32 7, !"PIE Level", i32 2}
+!7 = !{!"clang version trunk (trunk r312634)"}
+!8 = distinct !DISubprogram(name: "b", linkageName: "_ZN1a1bEv", scope: !9, file: !1, line: 4, type: !12, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: true, unit: !0, declaration: !11, variables: !16)
+!9 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "a", file: !1, line: 1, size: 8, elements: !10, identifier: "_ZTS1a")
+!10 = !{!11}
+!11 = !DISubprogram(name: "b", linkageName: "_ZN1a1bEv", scope: !9, file: !1, line: 2, type: !12, isLocal: false, isDefinition: false, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: true)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!14, !15}
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !9, size: 64, flags: DIFlagArtificial | DIFlagObjectPointer)
+!16 = !{!17}
+!17 = !DILocalVariable(name: "this", arg: 1, scope: !8, type: !18, flags: DIFlagArtificial | DIFlagObjectPointer)
+!18 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !9, size: 64)
+!19 = !DILocation(line: 0, scope: !8)
+!20 = !DILocation(line: 5, column: 3, scope: !8)
+!21 = !DILocation(line: 5, column: 3, scope: !22)
+!22 = !DILexicalBlockFile(scope: !23, file: !1, discriminator: 2)
+!23 = distinct !DILexicalBlock(scope: !24, file: !1, line: 5, column: 3)
+!24 = distinct !DILexicalBlock(scope: !8, file: !1, line: 5, column: 3)
+!25 = distinct !{!25, !26, !27}
+!26 = !DILocation(line: 5, column: 3, scope: !24)
+!27 = !DILocation(line: 6, column: 5, scope: !24)