MC: Refactor FT_Align fragments when linker relaxation is enabled

Previously, two MCAsmBackend hooks were used, with
shouldInsertFixupForCodeAlign calling getWriter().recordRelocation
directly, bypassing generic code.

This patch:

* Introduces MCAsmBackend::relaxAlign to replace the two hooks.
* Tracks padding size using VarContentStart and VarContentEnd (content is arbitrary).
* Move setLinkerRelaxable from MCObjectStreamer::emitCodeAlignment to the backends.

Pull Request: llvm#149465

Author: MaskRay

llvm/include/llvm/MC/MCAsmBackend.h

@@ -103,20 +103,6 @@ class LLVM_ABI MCAsmBackend {
   /// Get information on a fixup kind.
   virtual MCFixupKindInfo getFixupKindInfo(MCFixupKind Kind) const;
 
-  /// Hook to check if extra nop bytes must be inserted for alignment directive.
-  /// For some targets this may be necessary in order to support linker
-  /// relaxation. The number of bytes to insert are returned in Size.
-  virtual bool shouldInsertExtraNopBytesForCodeAlign(const MCFragment &AF,
-                                                     unsigned &Size) {
-    return false;
-  }
-
-  /// Hook which indicates if the target requires a fixup to be generated when
-  /// handling an align directive in an executable section
-  virtual bool shouldInsertFixupForCodeAlign(MCAssembler &Asm, MCFragment &AF) {
-    return false;
-  }
-
   // Evaluate a fixup, returning std::nullopt to use default handling for
   // `Value` and `IsResolved`. Otherwise, returns `IsResolved` with the
   // expectation that the hook updates `Value`.
@@ -174,6 +160,10 @@ class LLVM_ABI MCAsmBackend {
   }
 
   // Defined by linker relaxation targets.
+
+  // Return false to use default handling. Otherwise, set `Size` to the number
+  // of padding bytes.
+  virtual bool relaxAlign(MCFragment &F, unsigned &Size) { return false; }
   virtual bool relaxDwarfLineAddr(MCFragment &, bool &WasRelaxed) const {
     return false;
   }

llvm/lib/MC/MCAssembler.cpp

@@ -196,6 +196,7 @@ uint64_t MCAssembler::computeFragmentSize(const MCFragment &F) const {
   switch (F.getKind()) {
   case MCFragment::FT_Data:
   case MCFragment::FT_Relaxable:
+  case MCFragment::FT_Align:
   case MCFragment::FT_LEB:
   case MCFragment::FT_Dwarf:
   case MCFragment::FT_DwarfFrame:
@@ -226,27 +227,6 @@ uint64_t MCAssembler::computeFragmentSize(const MCFragment &F) const {
   case MCFragment::FT_SymbolId:
     return 4;
 
-  case MCFragment::FT_Align: {
-    unsigned Offset = F.Offset + F.getFixedSize();
-    unsigned Size = offsetToAlignment(Offset, F.getAlignment());
-
-    // Insert extra Nops for code alignment if the target define
-    // shouldInsertExtraNopBytesForCodeAlign target hook.
-    if (F.getParent()->useCodeAlign() && F.hasAlignEmitNops() &&
-        getBackend().shouldInsertExtraNopBytesForCodeAlign(F, Size))
-      return F.getFixedSize() + Size;
-
-    // If we are padding with nops, force the padding to be larger than the
-    // minimum nop size.
-    if (Size > 0 && F.hasAlignEmitNops()) {
-      while (Size % getBackend().getMinimumNopSize())
-        Size += F.getAlignment().value();
-    }
-    if (Size > F.getAlignMaxBytesToEmit())
-      Size = 0;
-    return F.getFixedSize() + Size;
-  }
-
   case MCFragment::FT_Org: {
     const MCOrgFragment &OF = cast<MCOrgFragment>(F);
     MCValue Value;
@@ -418,7 +398,6 @@ static void writeFragment(raw_ostream &OS, const MCAssembler &Asm,
   switch (F.getKind()) {
   case MCFragment::FT_Data:
   case MCFragment::FT_Relaxable:
-  case MCFragment::FT_Align:
   case MCFragment::FT_LEB:
   case MCFragment::FT_Dwarf:
   case MCFragment::FT_DwarfFrame:
@@ -431,42 +410,41 @@ static void writeFragment(raw_ostream &OS, const MCAssembler &Asm,
     const auto &EF = cast<MCFragment>(F);
     OS << StringRef(EF.getContents().data(), EF.getContents().size());
     OS << StringRef(EF.getVarContents().data(), EF.getVarContents().size());
-    if (F.getKind() == MCFragment::FT_Align) {
-      ++stats::EmittedAlignFragments;
-      assert(F.getAlignFillLen() &&
-             "Invalid virtual align in concrete fragment!");
-
-      uint64_t Count = (FragmentSize - F.getFixedSize()) / F.getAlignFillLen();
-      assert((FragmentSize - F.getFixedSize()) % F.getAlignFillLen() == 0 &&
-             "computeFragmentSize computed size is incorrect");
-
-      // See if we are aligning with nops, and if so do that first to try to
-      // fill the Count bytes.  Then if that did not fill any bytes or there are
-      // any bytes left to fill use the Value and ValueSize to fill the rest. If
-      // we are aligning with nops, ask that target to emit the right data.
-      if (F.hasAlignEmitNops()) {
-        if (!Asm.getBackend().writeNopData(OS, Count, F.getSubtargetInfo()))
-          report_fatal_error("unable to write nop sequence of " + Twine(Count) +
-                             " bytes");
-      } else {
-        // Otherwise, write out in multiples of the value size.
-        for (uint64_t i = 0; i != Count; ++i) {
-          switch (F.getAlignFillLen()) {
-          default:
-            llvm_unreachable("Invalid size!");
-          case 1:
-            OS << char(F.getAlignFill());
-            break;
-          case 2:
-            support::endian::write<uint16_t>(OS, F.getAlignFill(), Endian);
-            break;
-          case 4:
-            support::endian::write<uint32_t>(OS, F.getAlignFill(), Endian);
-            break;
-          case 8:
-            support::endian::write<uint64_t>(OS, F.getAlignFill(), Endian);
-            break;
-          }
+  } break;
+
+  case MCFragment::FT_Align: {
+    ++stats::EmittedAlignFragments;
+    OS << StringRef(F.getContents().data(), F.getContents().size());
+    assert(F.getAlignFillLen() &&
+           "Invalid virtual align in concrete fragment!");
+
+    uint64_t Count = (FragmentSize - F.getFixedSize()) / F.getAlignFillLen();
+    assert((FragmentSize - F.getFixedSize()) % F.getAlignFillLen() == 0 &&
+           "computeFragmentSize computed size is incorrect");
+
+    // In the nops mode, call the backend hook to write `Count` nops.
+    if (F.hasAlignEmitNops()) {
+      if (!Asm.getBackend().writeNopData(OS, Count, F.getSubtargetInfo()))
+        reportFatalInternalError("unable to write nop sequence of " +
+                                 Twine(Count) + " bytes");
+    } else {
+      // Otherwise, write out in multiples of the value size.
+      for (uint64_t i = 0; i != Count; ++i) {
+        switch (F.getAlignFillLen()) {
+        default:
+          llvm_unreachable("Invalid size!");
+        case 1:
+          OS << char(F.getAlignFill());
+          break;
+        case 2:
+          support::endian::write<uint16_t>(OS, F.getAlignFill(), Endian);
+          break;
+        case 4:
+          support::endian::write<uint32_t>(OS, F.getAlignFill(), Endian);
+          break;
+        case 8:
+          support::endian::write<uint64_t>(OS, F.getAlignFill(), Endian);
+          break;
         }
       }
     }
@@ -737,11 +715,6 @@ void MCAssembler::layout() {
           evaluateFixup(F, Fixup, Target, FixedValue,
                         /*RecordReloc=*/true, Contents);
         }
-      } else if (F.getKind() == MCFragment::FT_Align) {
-        // For RISC-V linker relaxation, an alignment relocation might be
-        // needed.
-        if (F.hasAlignEmitNops())
-          getBackend().shouldInsertFixupForCodeAlign(*this, F);
       }
     }
   }
@@ -992,7 +965,30 @@ void MCAssembler::layoutSection(MCSection &Sec) {
   uint64_t Offset = 0;
   for (MCFragment &F : Sec) {
     F.Offset = Offset;
-    Offset += computeFragmentSize(F);
+    if (F.getKind() == MCFragment::FT_Align) {
+      Offset += F.getFixedSize();
+      unsigned Size = offsetToAlignment(Offset, F.getAlignment());
+      // In the nops mode, RISC-V style linker relaxation might adjust the size
+      // and add a fixup, even if `Size` is originally 0.
+      bool AlignFixup = false;
+      if (F.hasAlignEmitNops()) {
+        AlignFixup = getBackend().relaxAlign(F, Size);
+        // If the backend does not handle the fragment specially, pad with nops,
+        // but ensure that the padding is larger than the minimum nop size.
+        if (!AlignFixup)
+          while (Size % getBackend().getMinimumNopSize())
+            Size += F.getAlignment().value();
+      }
+      if (!AlignFixup && Size > F.getAlignMaxBytesToEmit())
+        Size = 0;
+      // Update the variable tail size. The content is ignored.
+      F.VarContentEnd = F.VarContentStart + Size;
+      if (F.VarContentEnd > F.getParent()->ContentStorage.size())
+        F.getParent()->ContentStorage.resize(F.VarContentEnd);
+      Offset += Size;
+    } else {
+      Offset += computeFragmentSize(F);
+    }
   }
 }
 

llvm/lib/MC/MCExpr.cpp

@@ -370,7 +370,6 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
       }
 
       int64_t Num;
-      unsigned Count;
       if (DF) {
         Displacement += DF->getContents().size();
       } else if (F->getKind() == MCFragment::FT_Relaxable &&
@@ -380,9 +379,7 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
         // data fragment.
         Displacement += F->getSize();
       } else if (F->getKind() == MCFragment::FT_Align && Layout &&
-                 F->hasAlignEmitNops() &&
-                 !Asm->getBackend().shouldInsertExtraNopBytesForCodeAlign(
-                     *F, Count)) {
+                 F->isLinkerRelaxable()) {
         Displacement += Asm->computeFragmentSize(*F);
       } else if (auto *FF = dyn_cast<MCFillFragment>(F);
                  FF && FF->getNumValues().evaluateAsAbsolute(Num)) {

llvm/lib/MC/MCFragment.cpp

@@ -83,8 +83,13 @@ LLVM_DUMP_METHOD void MCFragment::dump() const {
     auto Fixed = getContents();
     auto Var = getVarContents();
     OS << " Size:" << Fixed.size();
-    if (getKind() != MCFragment::FT_Data)
+    if (getKind() != MCFragment::FT_Data) {
       OS << '+' << Var.size();
+      // FT_Align uses getVarContents to track the size, but the content is
+      // ignored and not useful.
+      if (getKind() == MCFragment::FT_Align)
+        Var = {};
+    }
     OS << " [";
     for (unsigned i = 0, e = Fixed.size(); i != e; ++i) {
       if (i) OS << ",";

llvm/lib/MC/MCObjectStreamer.cpp

@@ -555,14 +555,6 @@ void MCObjectStreamer::emitCodeAlignment(Align Alignment,
   emitValueToAlignment(Alignment, 0, 1, MaxBytesToEmit);
   auto *F = getCurrentFragment();
   F->setAlignEmitNops(true, STI);
-  // With RISC-V style linker relaxation, mark the section as linker-relaxable
-  // if the alignment is larger than the minimum NOP size.
-  unsigned Size;
-  if (getAssembler().getBackend().shouldInsertExtraNopBytesForCodeAlign(*F,
-                                                                        Size)) {
-    getCurrentSectionOnly()->setLinkerRelaxable();
-    newFragment();
-  }
 }
 
 void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset,

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp

@@ -177,74 +177,6 @@ void LoongArchAsmBackend::applyFixup(const MCFragment &F, const MCFixup &Fixup,
   }
 }
 
-// Linker relaxation may change code size. We have to insert Nops
-// for .align directive when linker relaxation enabled. So then Linker
-// could satisfy alignment by removing Nops.
-// The function returns the total Nops Size we need to insert.
-bool LoongArchAsmBackend::shouldInsertExtraNopBytesForCodeAlign(
-    const MCFragment &AF, unsigned &Size) {
-  // Calculate Nops Size only when linker relaxation enabled.
-  if (!AF.getSubtargetInfo()->hasFeature(LoongArch::FeatureRelax))
-    return false;
-
-  // Ignore alignment if MaxBytesToEmit is less than the minimum Nop size.
-  const unsigned MinNopLen = 4;
-  if (AF.getAlignMaxBytesToEmit() < MinNopLen)
-    return false;
-  Size = AF.getAlignment().value() - MinNopLen;
-  return AF.getAlignment() > MinNopLen;
-}
-
-// We need to insert R_LARCH_ALIGN relocation type to indicate the
-// position of Nops and the total bytes of the Nops have been inserted
-// when linker relaxation enabled.
-// The function inserts fixup_loongarch_align fixup which eventually will
-// transfer to R_LARCH_ALIGN relocation type.
-// The improved R_LARCH_ALIGN requires symbol index. The lowest 8 bits of
-// addend represent alignment and the other bits of addend represent the
-// maximum number of bytes to emit. The maximum number of bytes is zero
-// means ignore the emit limit.
-bool LoongArchAsmBackend::shouldInsertFixupForCodeAlign(MCAssembler &Asm,
-                                                        MCFragment &AF) {
-  // Insert the fixup only when linker relaxation enabled.
-  if (!AF.getSubtargetInfo()->hasFeature(LoongArch::FeatureRelax))
-    return false;
-
-  // Calculate total Nops we need to insert. If there are none to insert
-  // then simply return.
-  unsigned InsertedNopBytes;
-  if (!shouldInsertExtraNopBytesForCodeAlign(AF, InsertedNopBytes))
-    return false;
-
-  MCSection *Sec = AF.getParent();
-  MCContext &Ctx = getContext();
-  const MCExpr *Dummy = MCConstantExpr::create(0, Ctx);
-  MCFixup Fixup = MCFixup::create(AF.getFixedSize(), Dummy, ELF::R_LARCH_ALIGN);
-  unsigned MaxBytesToEmit = AF.getAlignMaxBytesToEmit();
-
-  auto createExtendedValue = [&]() {
-    const MCSymbolRefExpr *MCSym = getSecToAlignSym()[Sec];
-    if (MCSym == nullptr) {
-      // Define a marker symbol at the section with an offset of 0.
-      MCSymbol *Sym = Ctx.createNamedTempSymbol("la-relax-align");
-      Sym->setFragment(&*Sec->getBeginSymbol()->getFragment());
-      Asm.registerSymbol(*Sym);
-      MCSym = MCSymbolRefExpr::create(Sym, Ctx);
-      getSecToAlignSym()[Sec] = MCSym;
-    }
-    return MCValue::get(&MCSym->getSymbol(), nullptr,
-                        MaxBytesToEmit << 8 | Log2(AF.getAlignment()));
-  };
-
-  uint64_t FixedValue = 0;
-  MCValue Value = MaxBytesToEmit >= InsertedNopBytes
-                      ? MCValue::get(InsertedNopBytes)
-                      : createExtendedValue();
-  Asm.getWriter().recordRelocation(AF, Fixup, Value, FixedValue);
-
-  return true;
-}
-
 bool LoongArchAsmBackend::shouldForceRelocation(const MCFixup &Fixup,
                                                 const MCValue &Target) {
   switch (Fixup.getKind()) {
@@ -279,6 +211,53 @@ getRelocPairForSize(unsigned Size) {
   }
 }
 
+// Check if an R_LARCH_ALIGN relocation is needed for an alignment directive.
+// If conditions are met, compute the padding size and create a fixup encoding
+// the padding size in the addend. If MaxBytesToEmit is smaller than the padding
+// size, the fixup encodes MaxBytesToEmit in the higher bits and references a
+// per-section marker symbol.
+bool LoongArchAsmBackend::relaxAlign(MCFragment &F, unsigned &Size) {
+  // Use default handling unless linker relaxation is enabled and the
+  // MaxBytesToEmit >= the nop size.
+  if (!F.getSubtargetInfo()->hasFeature(LoongArch::FeatureRelax))
+    return false;
+  const unsigned MinNopLen = 4;
+  unsigned MaxBytesToEmit = F.getAlignMaxBytesToEmit();
+  if (MaxBytesToEmit < MinNopLen)
+    return false;
+
+  Size = F.getAlignment().value() - MinNopLen;
+  if (F.getAlignment() <= MinNopLen)
+    return false;
+
+  MCContext &Ctx = getContext();
+  const MCExpr *Expr = nullptr;
+  if (MaxBytesToEmit >= Size) {
+    Expr = MCConstantExpr::create(Size, getContext());
+  } else {
+    MCSection *Sec = F.getParent();
+    const MCSymbolRefExpr *SymRef = getSecToAlignSym()[Sec];
+    if (SymRef == nullptr) {
+      // Define a marker symbol at the section with an offset of 0.
+      MCSymbol *Sym = Ctx.createNamedTempSymbol("la-relax-align");
+      Sym->setFragment(&*Sec->getBeginSymbol()->getFragment());
+      Asm->registerSymbol(*Sym);
+      SymRef = MCSymbolRefExpr::create(Sym, Ctx);
+      getSecToAlignSym()[Sec] = SymRef;
+    }
+    Expr = MCBinaryExpr::createAdd(
+        SymRef,
+        MCConstantExpr::create((MaxBytesToEmit << 8) | Log2(F.getAlignment()),
+                               Ctx),
+        Ctx);
+  }
+  MCFixup Fixup =
+      MCFixup::create(0, Expr, FirstLiteralRelocationKind + ELF::R_LARCH_ALIGN);
+  F.setVarFixups({Fixup});
+  F.getParent()->setLinkerRelaxable();
+  return true;
+}
+
 std::pair<bool, bool> LoongArchAsmBackend::relaxLEB128(MCFragment &F,
                                                        int64_t &Value) const {
   const MCExpr &Expr = F.getLEBValue();

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h

@@ -45,19 +45,13 @@ class LoongArchAsmBackend : public MCAsmBackend {
                   MutableArrayRef<char> Data, uint64_t Value,
                   bool IsResolved) override;
 
-  // Return Size with extra Nop Bytes for alignment directive in code section.
-  bool shouldInsertExtraNopBytesForCodeAlign(const MCFragment &AF,
-                                             unsigned &Size) override;
-
-  // Insert target specific fixup type for alignment directive in code section.
-  bool shouldInsertFixupForCodeAlign(MCAssembler &Asm, MCFragment &AF) override;
-
   bool shouldForceRelocation(const MCFixup &Fixup, const MCValue &Target);
 
   std::optional<MCFixupKind> getFixupKind(StringRef Name) const override;
 
   MCFixupKindInfo getFixupKindInfo(MCFixupKind Kind) const override;
 
+  bool relaxAlign(MCFragment &F, unsigned &Size) override;
   bool relaxDwarfLineAddr(MCFragment &F, bool &WasRelaxed) const override;
   bool relaxDwarfCFA(MCFragment &F, bool &WasRelaxed) const override;
   std::pair<bool, bool> relaxLEB128(MCFragment &F,