[AArch64] DWARF: do not generate AT_location for thread local

[lliu0]

Summary

AArch64 ELF ABI does not define a static relocation type for TLS offset within a module, which makes it impossible for compiler to generate a valid DW_AT_location content for thread local variables. Currently LLVM generates an invalid R_AARCH64_ABS64 relocation at the DW_AT_location field for a TLS variable. That causes trouble for linker because thread local variable does not have an absolute address at link time. AArch64 GCC solves the problem by not generating DW_AT_location for thread local variables. We should do the same in LLVM.

This fixes https://bugs.llvm.org/show_bug.cgi?id=21077.

[dblaikie]

Could the frontend just generate a description of the variable witohut a
location instead?

[lliu0]

The problem is really a limitation of AArch64 ELF ABI. I don't think
frontend should change its behavior based on such low level target information.

Thanks,
Lei

[lliu0]

Ping.

Could anyone help review this please?

Thanks,
Lei

[peter.smith]

I had a quick look at the PR last week after some people mentioned that they were seeing link errors in their project and were blaming the linker and not the compiler. I'm not a debug expert so it would be best to get someone else to look over the test case. I think the source code changes are probably the best that we can do right now.

To the best of my knowledge there is currently no way of expressing a relocation for the debug location in the AArch64 ABI. All the existing TLS relocations are with respect to instructions with small immediate fields, there is no equivalent to the DTPREL relocation for data that other Targets are using so there is no way of describing the location from debug data. At the moment I'm trying to work out if this is a limitation of the descriptor based TLS model that AArch64 uses or whether this is just an oversight in the design of the ABI. GCC for Arm does implement the descriptor based model with -mtls-model=gnu2, this uses the same relocation for debug as the non-descriptor based model; however I'm not confident that this is guaranteed to be correct.

I'm going to speak to some colleagues that work on GCC and will raise an issue on the AArch64 ABI if it is a missing feature. This might make it possible to remove this patch if linker and debugger support becomes available later.

[lliu0]

Ping.

I think making the compiler not generate DW_AT_location for TLS variables is the best we can do with the current AArch64 ABI.

Thanks,
Lei

[dblaikie]

test/DebugInfo/AArch64/tls-at-location.ll

11

I guess this is sufficient/avoids needing to pass a triple to llc directly?

& does that ensure ELF, not MachO?

22

Strip out these attributes if they're not needed.

[lliu0]

Update test case to pass -mtriple=aarch64--linux-gnu to llc.

Rebase to LLVM trunk.

[dblaikie]

@aprantl @JDevlieghere - This looks good to me, but could you take a glance & see if it's reasonable for AArch64 (though this is for ELF, not MachO - but figure it might cut close enough you might want to check it)

[aprantl]

@aprantl @JDevlieghere - This looks good to me, but could you take a glance & see if it's reasonable for AArch64 (though this is for ELF, not MachO - but figure it might cut close enough you might want to check it)

To be honest, I really don't know. @t.p.northover perhaps?

[t.p.northover]

I think it's reasonable too.

include/llvm/Target/TargetLoweringObjectFile.h

 protected:
   bool SupportIndirectSymViaGOTPCRel = false;
   bool SupportGOTPCRelWithOffset = true;
+  bool SupportDebugThreadLocalLocation = true;
 
   /// This section contains the static constructor pointer list.
   MCSection *StaticCtorSection = nullptr;

     return SupportGOTPCRelWithOffset;
   }
 
+  /// Target supports TLS offset relocation in debug section?
+  bool supportDebugThreadLocalLocation() const {
+    return SupportDebugThreadLocalLocation;
+  }
+
   /// Get the target specific PC relative GOT entry relocation
   virtual const MCExpr *getIndirectSymViaGOTPCRel(const MCSymbol *Sym,
                                                   const MCValue &MV,

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

     if (!Global && (!Expr || !Expr->isConstant()))
       continue;
 
+    if (Global && Global->isThreadLocal() &&
+        !Asm->getObjFileLowering().supportDebugThreadLocalLocation())
+      continue;
+
     if (!Loc) {
       addToAccelTable = true;
       Loc = new (DIEValueAllocator) DIELoc;

lib/Target/AArch64/AArch64TargetObjectFile.cpp

                                              const TargetMachine &TM) {
   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
   InitializeELF(TM.Options.UseInitArray);
+  // AARCH64 ELF ABI does not define static relocation type for TLS offset
+  // within a module.  Do not generate AT_location for TLS variables.
+  SupportDebugThreadLocalLocation = false;
 }
 
 AArch64_MachoTargetObjectFile::AArch64_MachoTargetObjectFile()

test/DebugInfo/AArch64/tls-at-location.ll

+; RUN: llc -filetype=obj -mtriple=aarch64--linux-gnu -o - %s | llvm-dwarfdump -v - | FileCheck %s
+;
+; CHECK: .debug_info contents:
+; CHECK: DW_TAG_variable
+; CHECK-NOT: DW_AT_location
+
+@var = thread_local global i32 0, align 4, !dbg !0
+
+; Function Attrs: noinline nounwind optnone
+define i32 @foo() #0 !dbg !11 {
+entry:
+  %0 = load i32, i32* @var, align 4, !dbg !14
+  ret i32 %0, !dbg !15
+}
+
+attributes #0 = { noinline nounwind optnone }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "var", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 7.0.0 (https://github.com/llvm-mirror/clang.git 43eac1f9d7d2c985831b485d9ccc807416d1cf29) (https://github.com/llvm-mirror/llvm.git d53cdbf4cc5414ea540174a036202c555ce8fc4b)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+!3 = !DIFile(filename: "tls-at-location.c", directory: "/home/lliu0/llvm/tls-at-location/DebugInfo/AArch64")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 7.0.0 (https://github.com/llvm-mirror/clang.git 43eac1f9d7d2c985831b485d9ccc807416d1cf29) (https://github.com/llvm-mirror/llvm.git d53cdbf4cc5414ea540174a036202c555ce8fc4b)"}
+!11 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 3, type: !12, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: false, unit: !2)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!6}
+!14 = !DILocation(line: 4, column: 10, scope: !11)
+!15 = !DILocation(line: 4, column: 3, scope: !11)