bpf: introduce -mattr=dwarfris to disable DwarfUsesRelocationsAcrossSections

Commit e4507fb8c94b ("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>
llvm-svn: 326505

5 files changed, 16 insertions, 2 deletions

diff --git a/llvm/lib/Target/BPF/BPF.td b/llvm/lib/Target/BPF/BPF.td
index b8c2298584c..877bd15f4f2 100644
--- a/llvm/lib/Target/BPF/BPF.td
+++ b/llvm/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;
diff --git a/llvm/lib/Target/BPF/BPFSubtarget.cpp b/llvm/lib/Target/BPF/BPFSubtarget.cpp
index 2bb07032304..56780bd9d46 100644
--- a/llvm/lib/Target/BPF/BPFSubtarget.cpp
+++ b/llvm/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) {
diff --git a/llvm/lib/Target/BPF/BPFSubtarget.h b/llvm/lib/Target/BPF/BPFSubtarget.h
index 4fbd4b00181..067b22ad3ce 100644
--- a/llvm/lib/Target/BPF/BPFSubtarget.h
+++ b/llvm/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 {
diff --git a/llvm/lib/Target/BPF/BPFTargetMachine.cpp b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
index 0114d18599a..91ff64ba797 100644
--- a/llvm/lib/Target/BPF/BPFTargetMachine.cpp
+++ b/llvm/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.
diff --git a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h b/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
index fa06f31add9..171f7f607ff 100644
--- a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
+++ b/llvm/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;
+  }
 };
 }