[llvm] r326505 - bpf: introduce -mattr=dwarfris to disable DwarfUsesRelocationsAcrossSections

[Yonghong Song via llvm-commits]

Author: yhs
Date: Thu Mar  1 15:04:59 2018
New Revision: 326505

URL: http://llvm.org/viewvc/llvm-project?rev=326505&view=rev
Log:
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 at fb.com>

Modified:
    llvm/trunk/lib/Target/BPF/BPF.td
    llvm/trunk/lib/Target/BPF/BPFSubtarget.cpp
    llvm/trunk/lib/Target/BPF/BPFSubtarget.h
    llvm/trunk/lib/Target/BPF/BPFTargetMachine.cpp
    llvm/trunk/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h

Modified: llvm/trunk/lib/Target/BPF/BPF.td
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/BPF/BPF.td?rev=326505&r1=326504&r2=326505&view=diff
==============================================================================
--- llvm/trunk/lib/Target/BPF/BPF.td (original)
+++ llvm/trunk/lib/Target/BPF/BPF.td Thu Mar  1 15:04:59 2018
@@ -29,6 +29,9 @@ def DummyFeature : SubtargetFeature<"dum
 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;

Modified: llvm/trunk/lib/Target/BPF/BPFSubtarget.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/BPF/BPFSubtarget.cpp?rev=326505&r1=326504&r2=326505&view=diff
==============================================================================
--- llvm/trunk/lib/Target/BPF/BPFSubtarget.cpp (original)
+++ llvm/trunk/lib/Target/BPF/BPFSubtarget.cpp Thu Mar  1 15:04:59 2018
@@ -37,6 +37,7 @@ BPFSubtarget &BPFSubtarget::initializeSu
 void BPFSubtarget::initializeEnvironment() {
   HasJmpExt = false;
   HasAlu32 = false;
+  UseDwarfRIS = false;
 }
 
 void BPFSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {

Modified: llvm/trunk/lib/Target/BPF/BPFSubtarget.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/BPF/BPFSubtarget.h?rev=326505&r1=326504&r2=326505&view=diff
==============================================================================
--- llvm/trunk/lib/Target/BPF/BPFSubtarget.h (original)
+++ llvm/trunk/lib/Target/BPF/BPFSubtarget.h Thu Mar  1 15:04:59 2018
@@ -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 {

Modified: llvm/trunk/lib/Target/BPF/BPFTargetMachine.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/BPF/BPFTargetMachine.cpp?rev=326505&r1=326504&r2=326505&view=diff
==============================================================================
--- llvm/trunk/lib/Target/BPF/BPFTargetMachine.cpp (original)
+++ llvm/trunk/lib/Target/BPF/BPFTargetMachine.cpp Thu Mar  1 15:04:59 2018
@@ -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
       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.

Modified: llvm/trunk/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h?rev=326505&r1=326504&r2=326505&view=diff
==============================================================================
--- llvm/trunk/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h (original)
+++ llvm/trunk/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h Thu Mar  1 15:04:59 2018
@@ -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;
+  }
 };
 }