[mips][micromips] Fix how values in .gcc_except_table are calculated

[abeserminji]

Summary

When a landing pad is calculated in a program that is compiled for micromips,
it will point to an even address. Such an error will cause a segmentation fault,
as the instructions in micromips are aligned on odd addresses.
This patch sets the last bit of the offset where a landing pad is, to 1, which will
effectively be an odd address and point to the instruction exactly.

[atanasyan]

lib/MC/MCExpr.cpp

10 ↗

(On Diff #168649)

Is it really necessary to include MCAsmBackend.h and at the same time add forward declaration of class MCAsmBackend below?

532 ↗

(On Diff #168649)

Let's add a comment for these lines (like for lines above).

lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h

21 ↗

(On Diff #168649)

Is it really necessary to include MCSymbolELF.h and at the same time add forward declaration of class MCSymbolELF below?

[atanasyan]

And the test case failed:

test/CodeGen/Mips/micromips-gcc-except-table.ll:4:15: error: CHECK-NEXT: expected string not foundin input
; CHECK-NEXT: 0000 ff9b1501 0c010f00 000f1025 011f1e00
              ^
<stdin>:5:2: note: scanning from here
 0000 ff9b1501 0c011100 00110e1f 011f1800 ................
 ^

[abeserminji]

Comments addressed.
Test fixed. I have one other patch in progress which affects this test, and forgot to remove it before the submission.

[atanasyan]

LGTM

[abeserminji]

Removed class forward declaration from MCExpr.cpp. It is not needed.

[MaskRay]

The patch special cased .uleb128 A-B for both direct object file emission and assembling, but changing assembling is not correct.
GNU assembler does not special case .uleb128 A-B to set the least significant bit.

llvm/trunk/include/llvm/MC/MCAsmBackend.h

public:
   virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {}
 
   /// Generate the compact unwind encoding for the CFI instructions.
   virtual uint32_t
       generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const {
     return 0;
   }
 
+  /// Check whether a given symbol has been flagged with MICROMIPS flag.
+  virtual bool isMicroMips(const MCSymbol *Sym) const {
+    return false;
+  }
+
   /// Handles all target related code padding when starting to write a new
   /// basic block to an object file.
   ///
   /// \param OS The streamer used for writing the padding data and function.
   /// \param Context the context of the padding, Embeds the basic block's
   /// parameters.
   void handleCodePaddingBasicBlockStart(MCObjectStreamer *OS,
                                         const MCCodePaddingContext &Context);

llvm/trunk/lib/MC/MCExpr.cpp

if (SA.getFragment() == SB.getFragment() && !SA.isVariable() &&
       !SA.isUnset() && !SB.isVariable() && !SB.isUnset()) {
     Addend += (SA.getOffset() - SB.getOffset());
 
     // Pointers to Thumb symbols need to have their low-bit set to allow
     // for interworking.
     if (Asm->isThumbFunc(&SA))
       Addend |= 1;
 
+    // If symbol is labeled as micromips, we set low-bit to ensure
+    // correct offset in .gcc_except_table
+    if (Asm->getBackend().isMicroMips(&SA))
+      Addend |= 1;
+
     // Clear the symbol expr pointers to indicate we have folded these
     // operands.
     A = B = nullptr;
     return;
   }
 
   if (!Layout)
     return;

llvm/trunk/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h

 #include "llvm/MC/MCAsmBackend.h"
 
 namespace llvm {
 
 class MCAssembler;
 struct MCFixupKindInfo;
 class MCObjectWriter;
 class MCRegisterInfo;
+class MCSymbolELF;
 class Target;
 
 class MipsAsmBackend : public MCAsmBackend {
   Triple TheTriple;
   bool IsN32;
 
 public:
   MipsAsmBackend(const Target &T, const MCRegisterInfo &MRI, const Triple &TT,
public:
 
   /// @}
 
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target) override;
 
+  bool isMicroMips(const MCSymbol *Sym) const override;
 }; // class MipsAsmBackend
 
 } // namespace
 
 #endif

llvm/trunk/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp

bool MipsAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
   case Mips::fixup_MICROMIPS_TLS_GD:
   case Mips::fixup_MICROMIPS_TLS_LDM:
   case Mips::fixup_MICROMIPS_TLS_TPREL_HI16:
   case Mips::fixup_MICROMIPS_TLS_TPREL_LO16:
     return true;
   }
 }
 
+bool MipsAsmBackend::isMicroMips(const MCSymbol *Sym) const {
+  if (const auto *ElfSym = dyn_cast<const MCSymbolELF>(Sym)) {
+    if (ElfSym->getOther() & ELF::STO_MIPS_MICROMIPS)
+      return true;
+  }
+  return false;
+}
+
 MCAsmBackend *llvm::createMipsAsmBackend(const Target &T,
                                          const MCSubtargetInfo &STI,
                                          const MCRegisterInfo &MRI,
                                          const MCTargetOptions &Options) {
   MipsABIInfo ABI = MipsABIInfo::computeTargetABI(STI.getTargetTriple(), STI.getCPU(), Options);
   return new MipsAsmBackend(T, MRI, STI.getTargetTriple(), STI.getCPU(), ABI.IsN32());
 }

llvm/trunk/test/CodeGen/Mips/micromips-gcc-except-table.ll

+; RUN: llc -mtriple=mips-linux-gnu -mcpu=mips32r2 -mattr=+micromips -O3 -filetype=obj < %s | llvm-objdump -s -j .gcc_except_table - | FileCheck %s
+
+; CHECK: Contents of section .gcc_except_table:
+; CHECK-NEXT: 0000 ff9b1501 0c011100 00110e1f 011f1800
+; CHECK-NEXT: 0010 00010000 00000000
+
+@_ZTIi = external constant i8*
+
+define dso_local i32 @main() local_unnamed_addr norecurse personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+entry:
+  %exception.i = tail call i8* @__cxa_allocate_exception(i32 4) nounwind
+  %0 = bitcast i8* %exception.i to i32*
+  store i32 5, i32* %0, align 16
+  invoke void @__cxa_throw(i8* %exception.i, i8* bitcast (i8** @_ZTIi to i8*), i8* null) noreturn
+          to label %.noexc unwind label %return
+
+.noexc:
+  unreachable
+
+return:
+  %1 = landingpad { i8*, i32 }
+          catch i8* null
+  %2 = extractvalue { i8*, i32 } %1, 0
+  %3 = tail call i8* @__cxa_begin_catch(i8* %2) nounwind
+  tail call void @__cxa_end_catch()
+  ret i32 0
+}
+
+declare i32 @__gxx_personality_v0(...)
+
+declare i8* @__cxa_begin_catch(i8*) local_unnamed_addr
+
+declare void @__cxa_end_catch() local_unnamed_addr
+
+declare i8* @__cxa_allocate_exception(i32) local_unnamed_addr
+
+declare void @__cxa_throw(i8*, i8*, i8*) local_unnamed_addr