bpf: introduce -mattr=dwarfris to disable DwarfUsesRelocationsAcrossSections

Commit e4507fb8c9 ("bpf: disable DwarfUsesRelocationsAcrossSections")
disables MCAsmInfo DwarfUsesRelocationsAcrossSections unconditionally
so that dwarf will not use cross section (between dwarf and symbol table)
relocations. This new debug format enables pahole to dump structures
correctly as libdwarves.so does not have BPF backend support yet.

This new debug format, however, breaks bcc (https://github.com/iovisor/bcc)
source debug output as llvm in-memory Dwarf support has some issues to
handle it. More specifically, with DwarfUsesRelocationsAcrossSections
disabled, JIT compiler does not generate .debug_abbrev and Dwarf
DIE (debug info entry) processing is not happy about this.

This patch introduces a new flag -mattr=dwarfris
(dwarf relocation in section) to disable DwarfUsesRelocationsAcrossSections.
DwarfUsesRelocationsAcrossSections is true by default.

Signed-off-by: Yonghong Song <yhs@fb.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326505 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/BPF/BPF.td

@@ -29,6 +29,9 @@ def DummyFeature : SubtargetFeature<"dummy", "isDummyMode",
 def ALU32 : SubtargetFeature<"alu32", "HasAlu32", "true",
                              "Enable ALU32 instructions">;
 
+def DwarfRIS: SubtargetFeature<"dwarfris", "UseDwarfRIS", "true",
+                               "Disable MCAsmInfo DwarfUsesRelocationsAcrossSections">;
+
 def BPFInstPrinter : AsmWriter {
   string AsmWriterClassName  = "InstPrinter";
   bit isMCAsmWriter = 1;

lib/Target/BPF/BPFSubtarget.cpp

@@ -37,6 +37,7 @@ BPFSubtarget &BPFSubtarget::initializeSubtargetDependencies(StringRef CPU,
 void BPFSubtarget::initializeEnvironment() {
   HasJmpExt = false;
   HasAlu32 = false;
+  UseDwarfRIS = false;
 }
 
 void BPFSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {

lib/Target/BPF/BPFSubtarget.h

@@ -50,6 +50,9 @@ protected:
   // whether the cpu supports alu32 instructions.
   bool HasAlu32;
 
+  // whether we should enable MCAsmInfo DwarfUsesRelocationsAcrossSections
+  bool UseDwarfRIS;
+
 public:
   // This constructor initializes the data members to match that
   // of the specified triple.
@@ -63,6 +66,7 @@ public:
   void ParseSubtargetFeatures(StringRef CPU, StringRef FS);
   bool getHasJmpExt() const { return HasJmpExt; }
   bool getHasAlu32() const { return HasAlu32; }
+  bool getUseDwarfRIS() const { return UseDwarfRIS; }
 
   const BPFInstrInfo *getInstrInfo() const override { return &InstrInfo; }
   const BPFFrameLowering *getFrameLowering() const override {

lib/Target/BPF/BPFTargetMachine.cpp

@@ -13,6 +13,7 @@
 
 #include "BPFTargetMachine.h"
 #include "BPF.h"
+#include "MCTargetDesc/BPFMCAsmInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
@@ -68,6 +69,9 @@ BPFTargetMachine::BPFTargetMachine(const Target &T, const Triple &TT,
       TLOF(make_unique<TargetLoweringObjectFileELF>()),
       Subtarget(TT, CPU, FS, *this) {
   initAsmInfo();
+
+  BPFMCAsmInfo *MAI = static_cast<BPFMCAsmInfo *>(const_cast<MCAsmInfo *>(AsmInfo));
+  MAI->setDwarfUsesRelocationsAcrossSections(!Subtarget.getUseDwarfRIS());
 }
 namespace {
 // BPF Code Generator Pass Configuration Options.

lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h

@@ -37,8 +37,6 @@ public:
     ExceptionsType = ExceptionHandling::DwarfCFI;
     MinInstAlignment = 8;
 
-    DwarfUsesRelocationsAcrossSections = false;
-
     // the default is 4 and it only affects dwarf elf output
     // so if not set correctly, the dwarf data will be
     // messed up in random places by 4 bytes. .debug_line
@@ -46,6 +44,10 @@ public:
     // line numbers, etc.
     CodePointerSize = 8;
   }
+
+  void setDwarfUsesRelocationsAcrossSections(bool enable) {
+    DwarfUsesRelocationsAcrossSections = enable;
+  }
 };
 }