[ARM] Fix several state persistence bugs

Summary:
This fixes three bugs, in all of which state is not or incorrecly reset between
objects (i.e. when reusing the same pass manager to create multiple object
files):
1) AttributeSection needs to be reset to nullptr, because otherwise the backend
   will try to emit into the old object file's attribute section causing a
   segmentation fault.
2) MappingSymbolCounter needs to be reset, otherwise the second object file
   will start where the first one left off.
3) The MCStreamer base class resets the Streamer's e_flags settings. Since
   EF_ARM_EABI_VER5 is set on streamer creation, we need to set it again
   after the MCStreamer was rest.

Also rename Reset (uppser case) to EHReset to avoid confusion with
reset (lower case).

Reviewers: rengolin
Differential Revision: http://reviews.llvm.org/D15950

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257473 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCStreamer.h

@@ -131,6 +131,10 @@ public:
 
   void finish() override;
 
+  /// Reset any state between object emissions, i.e. the equivalent of
+  /// MCStreamer's reset method.
+  virtual void reset();
+
   /// Callback used to implement the ldr= pseudo.
   /// Add a new entry to the constant pool for the current section and return an
   /// MCExpr that can be used to refer to the constant pool location.

lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp

@@ -388,6 +388,9 @@ private:
 
   size_t calculateContentSize() const;
 
+  // Reset state between object emissions
+  void reset() override;
+
 public:
   ARMTargetELFStreamer(MCStreamer &S)
     : ARMTargetStreamer(S), CurrentVendor("aeabi"), FPU(ARM::FK_INVALID),
@@ -415,7 +418,7 @@ public:
                  MCCodeEmitter *Emitter, bool IsThumb)
       : MCELFStreamer(Context, TAB, OS, Emitter), IsThumb(IsThumb),
         MappingSymbolCounter(0), LastEMS(EMS_None) {
-    Reset();
+    EHReset();
   }
 
   ~ARMELFStreamer() {}
@@ -579,7 +582,10 @@ private:
   }
 
   // Helper functions for ARM exception handling directives
-  void Reset();
+  void EHReset();
+
+  // Reset state between object emissions
+  void reset() override;
 
   void EmitPersonalityFixup(StringRef Name);
   void FlushPendingOffset();
@@ -1040,6 +1046,8 @@ void ARMTargetELFStreamer::emitInst(uint32_t Inst, char Suffix) {
   getStreamer().emitInst(Inst, Suffix);
 }
 
+void ARMTargetELFStreamer::reset() { AttributeSection = nullptr; }
+
 void ARMELFStreamer::FinishImpl() {
   MCTargetStreamer &TS = *getTargetStreamer();
   ARMTargetStreamer &ATS = static_cast<ARMTargetStreamer &>(TS);
@@ -1048,6 +1056,18 @@ void ARMELFStreamer::FinishImpl() {
   MCELFStreamer::FinishImpl();
 }
 
+void ARMELFStreamer::reset() {
+  MCTargetStreamer &TS = *getTargetStreamer();
+  ARMTargetStreamer &ATS = static_cast<ARMTargetStreamer &>(TS);
+  ATS.reset();
+  MappingSymbolCounter = 0;
+  MCELFStreamer::reset();
+  // MCELFStreamer clear's the assembler's e_flags. However, for
+  // arm we manually set the ABI version on streamer creation, so
+  // do the same here
+  getAssembler().setELFHeaderEFlags(ELF::EF_ARM_EABI_VER5);
+}
+
 inline void ARMELFStreamer::SwitchToEHSection(const char *Prefix,
                                               unsigned Type,
                                               unsigned Flags,
@@ -1094,7 +1114,7 @@ void ARMELFStreamer::EmitFixup(const MCExpr *Expr, MCFixupKind Kind) {
                                               Kind));
 }
 
-void ARMELFStreamer::Reset() {
+void ARMELFStreamer::EHReset() {
   ExTab = nullptr;
   FnStart = nullptr;
   Personality = nullptr;
@@ -1164,7 +1184,7 @@ void ARMELFStreamer::emitFnEnd() {
   SwitchSection(&FnStart->getSection());
 
   // Clean exception handling frame information
-  Reset();
+  EHReset();
 }
 
 void ARMELFStreamer::emitCantUnwind() { CantUnwind = true; }

lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp

@@ -38,6 +38,9 @@ void ARMTargetStreamer::emitCurrentConstantPool() {
 // finish() - write out any non-empty assembler constant pools.
 void ARMTargetStreamer::finish() { ConstantPools->emitAll(Streamer); }
 
+// reset() - Reset any state
+void ARMTargetStreamer::reset() {}
+
 // The remaining callbacks should be handled separately by each
 // streamer.
 void ARMTargetStreamer::emitFnStart() {}

test/MC/ARM/twice.ll

@@ -0,0 +1,9 @@
+; Check for state persistence bugs in the ARM MC backend
+; This should neither fail (in the comparison that the second object
+; is bit-identical to the first) nor crash. Either failure would most
+; likely indicate some state that is not properly reset in the
+; appropriate ::reset method.
+; RUN: llc -compile-twice -filetype=obj %s -o -
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv4t-unknown-linux-gnueabi"