[JITLink][LoongArch] Support R_LARCH_ALIGN relaxation

Linker relaxation is not implemented for jitlink now. But if
relaxation is enabled by clang, R_LARCH_RELAX and R_LARCH_ALIGN
relocations will be emitted.

This commit adapts lld's algorithm to jitlink. Currently, only
relaxing R_LARCH_ALIGN is implemented. Other relaxable relocs
can be implemented in the future.

Without this, interpreting C++ code using clang-repl or running
ir using lli when relaxation is enabled will occur error: `JIT
session error: Unsupported loongarch relocation:102: R_LARCH_ALIGN`.

Similar to 310473c but only implement align.

Author: zhaoqi5

llvm/include/llvm/ExecutionEngine/JITLink/ELF_loongarch.h

@@ -33,6 +33,10 @@ Expected<std::unique_ptr<LinkGraph>> createLinkGraphFromELFObject_loongarch(
 void link_ELF_loongarch(std::unique_ptr<LinkGraph> G,
                         std::unique_ptr<JITLinkContext> Ctx);
 
+/// Returns a pass that performs linker relaxation. Should be added to
+/// PostAllocationPasses.
+LinkGraphPassFunction createRelaxationPass_ELF_loongarch();
+
 } // end namespace jitlink
 } // end namespace llvm
 

llvm/include/llvm/ExecutionEngine/JITLink/loongarch.h

@@ -225,6 +225,13 @@ enum EdgeKind_loongarch : Edge::Kind {
   ///     out-of-range error will be returned.
   ///
   Call36PCRel,
+
+  /// Alignment requirement used by linker relaxation.
+  ///
+  /// Linker relaxation will use this to ensure all code sequences are properly
+  /// aligned and then remove these edges from the graph.
+  ///
+  AlignRelaxable,
 };
 
 /// Returns a string name for the given loongarch edge. For debugging purposes
@@ -362,6 +369,9 @@ inline Error applyFixup(LinkGraph &G, Block &B, const Edge &E) {
     *(little32_t *)(FixupPtr + 4) = Jirl | Lo16;
     break;
   }
+  case AlignRelaxable:
+    // Ignore when the relaxation pass did not run
+    break;
   default:
     return make_error<JITLinkError>(
         "In graph " + G.getName() + ", section " + B.getSection().getName() +

llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp

@@ -45,6 +45,238 @@ class ELFJITLinker_loongarch : public JITLinker<ELFJITLinker_loongarch> {
   }
 };
 
+namespace {
+
+struct SymbolAnchor {
+  uint64_t Offset;
+  Symbol *Sym;
+  bool End; // true for the anchor of getOffset() + getSize()
+};
+
+struct BlockRelaxAux {
+  // This records symbol start and end offsets which will be adjusted according
+  // to the nearest RelocDeltas element.
+  SmallVector<SymbolAnchor, 0> Anchors;
+  // All edges that either 1) are R_LARCH_ALIGN or 2) have a R_LARCH_RELAX edge
+  // at the same offset.
+  SmallVector<Edge *, 0> RelaxEdges;
+  // For RelaxEdges[I], the actual offset is RelaxEdges[I]->getOffset() - (I ?
+  // RelocDeltas[I - 1] : 0).
+  SmallVector<uint32_t, 0> RelocDeltas;
+  // For RelaxEdges[I], the actual type is EdgeKinds[I].
+  SmallVector<Edge::Kind, 0> EdgeKinds;
+  // List of rewritten instructions. Contains one raw encoded instruction per
+  // element in EdgeKinds that isn't Invalid or R_LARCH_ALIGN.
+  SmallVector<uint32_t, 0> Writes;
+};
+
+struct RelaxAux {
+  DenseMap<Block *, BlockRelaxAux> Blocks;
+};
+
+} // namespace
+
+static bool shouldRelax(const Section &S) {
+  return (S.getMemProt() & orc::MemProt::Exec) != orc::MemProt::None;
+}
+
+static bool isRelaxable(const Edge &E) {
+  switch (E.getKind()) {
+  default:
+    return false;
+  case AlignRelaxable:
+    return true;
+  }
+}
+
+static RelaxAux initRelaxAux(LinkGraph &G) {
+  RelaxAux Aux;
+  for (auto &S : G.sections()) {
+    if (!shouldRelax(S))
+      continue;
+    for (auto *B : S.blocks()) {
+      auto BlockEmplaceResult = Aux.Blocks.try_emplace(B);
+      assert(BlockEmplaceResult.second && "Block encountered twice");
+      auto &BlockAux = BlockEmplaceResult.first->second;
+
+      for (auto &E : B->edges())
+        if (isRelaxable(E))
+          BlockAux.RelaxEdges.push_back(&E);
+
+      if (BlockAux.RelaxEdges.empty()) {
+        Aux.Blocks.erase(BlockEmplaceResult.first);
+        continue;
+      }
+
+      const auto NumEdges = BlockAux.RelaxEdges.size();
+      BlockAux.RelocDeltas.resize(NumEdges, 0);
+      BlockAux.EdgeKinds.resize_for_overwrite(NumEdges);
+
+      // Store anchors (offset and offset+size) for symbols.
+      for (auto *Sym : S.symbols()) {
+        if (!Sym->isDefined() || &Sym->getBlock() != B)
+          continue;
+
+        BlockAux.Anchors.push_back({Sym->getOffset(), Sym, false});
+        BlockAux.Anchors.push_back(
+            {Sym->getOffset() + Sym->getSize(), Sym, true});
+      }
+    }
+  }
+
+  // Sort anchors by offset so that we can find the closest relocation
+  // efficiently. For a zero size symbol, ensure that its start anchor precedes
+  // its end anchor. For two symbols with anchors at the same offset, their
+  // order does not matter.
+  for (auto &BlockAuxIter : Aux.Blocks) {
+    llvm::sort(BlockAuxIter.second.Anchors, [](auto &A, auto &B) {
+      return std::make_pair(A.Offset, A.End) < std::make_pair(B.Offset, B.End);
+    });
+  }
+
+  return Aux;
+}
+
+static void relaxAlign(orc::ExecutorAddr Loc, const Edge &E, uint32_t &Remove,
+                       Edge::Kind &NewEdgeKind) {
+  const uint64_t Addend =
+      !E.getTarget().isDefined() ? Log2_64(E.getAddend()) + 1 : E.getAddend();
+  const uint64_t AllBytes = (1ULL << (Addend & 0xff)) - 4;
+  const uint64_t Align = 1ULL << (Addend & 0xff);
+  const uint64_t MaxBytes = Addend >> 8;
+  const uint64_t Off = Loc.getValue() & (Align - 1);
+  const uint64_t CurBytes = Off == 0 ? 0 : Align - Off;
+  // All bytes beyond the alignment boundary should be removed.
+  // If emit bytes more than max bytes to emit, remove all.
+  if (MaxBytes != 0 && CurBytes > MaxBytes)
+    Remove = AllBytes;
+  else
+    Remove = AllBytes - CurBytes;
+
+  assert(static_cast<int32_t>(Remove) >= 0 &&
+         "R_LARCH_ALIGN needs expanding the content");
+  NewEdgeKind = AlignRelaxable;
+}
+
+static bool relaxBlock(LinkGraph &G, Block &Block, BlockRelaxAux &Aux) {
+  const auto BlockAddr = Block.getAddress();
+  bool Changed = false;
+  ArrayRef<SymbolAnchor> SA = ArrayRef(Aux.Anchors);
+  uint32_t Delta = 0;
+
+  Aux.EdgeKinds.assign(Aux.EdgeKinds.size(), Edge::Invalid);
+  Aux.Writes.clear();
+
+  for (auto [I, E] : llvm::enumerate(Aux.RelaxEdges)) {
+    const auto Loc = BlockAddr + E->getOffset() - Delta;
+    auto &Cur = Aux.RelocDeltas[I];
+    uint32_t Remove = 0;
+    switch (E->getKind()) {
+    case AlignRelaxable:
+      relaxAlign(Loc, *E, Remove, Aux.EdgeKinds[I]);
+      break;
+    default:
+      llvm_unreachable("Unexpected relaxable edge kind");
+    }
+
+    // For all anchors whose offsets are <= E->getOffset(), they are preceded by
+    // the previous relocation whose RelocDeltas value equals Delta.
+    // Decrease their offset and update their size.
+    for (; SA.size() && SA[0].Offset <= E->getOffset(); SA = SA.slice(1)) {
+      if (SA[0].End)
+        SA[0].Sym->setSize(SA[0].Offset - Delta - SA[0].Sym->getOffset());
+      else
+        SA[0].Sym->setOffset(SA[0].Offset - Delta);
+    }
+
+    Delta += Remove;
+    if (Delta != Cur) {
+      Cur = Delta;
+      Changed = true;
+    }
+  }
+
+  for (const SymbolAnchor &A : SA) {
+    if (A.End)
+      A.Sym->setSize(A.Offset - Delta - A.Sym->getOffset());
+    else
+      A.Sym->setOffset(A.Offset - Delta);
+  }
+
+  return Changed;
+}
+
+static bool relaxOnce(LinkGraph &G, RelaxAux &Aux) {
+  bool Changed = false;
+
+  for (auto &[B, BlockAux] : Aux.Blocks)
+    Changed |= relaxBlock(G, *B, BlockAux);
+
+  return Changed;
+}
+
+static void finalizeBlockRelax(LinkGraph &G, Block &Block, BlockRelaxAux &Aux) {
+  auto Contents = Block.getAlreadyMutableContent();
+  auto *Dest = Contents.data();
+  uint32_t Offset = 0;
+  uint32_t Delta = 0;
+
+  // Update section content: remove NOPs for R_LARCH_ALIGN and rewrite
+  // instructions for relaxed relocations.
+  for (auto [I, E] : llvm::enumerate(Aux.RelaxEdges)) {
+    uint32_t Remove = Aux.RelocDeltas[I] - Delta;
+    Delta = Aux.RelocDeltas[I];
+    if (Remove == 0 && Aux.EdgeKinds[I] == Edge::Invalid)
+      continue;
+
+    // Copy from last location to the current relocated location.
+    const auto Size = E->getOffset() - Offset;
+    std::memmove(Dest, Contents.data() + Offset, Size);
+    Dest += Size;
+    Offset = E->getOffset() + Remove;
+  }
+
+  std::memmove(Dest, Contents.data() + Offset, Contents.size() - Offset);
+
+  // Fixup edge offsets and kinds.
+  Delta = 0;
+  size_t I = 0;
+  for (auto &E : Block.edges()) {
+    E.setOffset(E.getOffset() - Delta);
+
+    if (I < Aux.RelaxEdges.size() && Aux.RelaxEdges[I] == &E) {
+      if (Aux.EdgeKinds[I] != Edge::Invalid)
+        E.setKind(Aux.EdgeKinds[I]);
+
+      Delta = Aux.RelocDeltas[I];
+      ++I;
+    }
+  }
+
+  // Remove AlignRelaxable edges: all other relaxable edges got modified and
+  // will be used later while linking. Alignment is entirely handled here so we
+  // don't need these edges anymore.
+  for (auto IE = Block.edges().begin(); IE != Block.edges().end();) {
+    if (IE->getKind() == AlignRelaxable)
+      IE = Block.removeEdge(IE);
+    else
+      ++IE;
+  }
+}
+
+static void finalizeRelax(LinkGraph &G, RelaxAux &Aux) {
+  for (auto &[B, BlockAux] : Aux.Blocks)
+    finalizeBlockRelax(G, *B, BlockAux);
+}
+
+static Error relax(LinkGraph &G) {
+  auto Aux = initRelaxAux(G);
+  while (relaxOnce(G, Aux)) {
+  }
+  finalizeRelax(G, Aux);
+  return Error::success();
+}
+
 template <typename ELFT>
 class ELFLinkGraphBuilder_loongarch : public ELFLinkGraphBuilder<ELFT> {
 private:
@@ -74,13 +306,20 @@ class ELFLinkGraphBuilder_loongarch : public ELFLinkGraphBuilder<ELFT> {
       return RequestGOTAndTransformToPageOffset12;
     case ELF::R_LARCH_CALL36:
       return Call36PCRel;
+    case ELF::R_LARCH_ALIGN:
+      return AlignRelaxable;
     }
 
     return make_error<JITLinkError>(
         "Unsupported loongarch relocation:" + formatv("{0:d}: ", Type) +
         object::getELFRelocationTypeName(ELF::EM_LOONGARCH, Type));
   }
 
+  EdgeKind_loongarch getRelaxableRelocationKind(EdgeKind_loongarch Kind) {
+    // TODO: Implement more. Just ignore all relaxations now.
+    return Kind;
+  }
+
   Error addRelocations() override {
     LLVM_DEBUG(dbgs() << "Processing relocations:\n");
 
@@ -100,22 +339,17 @@ class ELFLinkGraphBuilder_loongarch : public ELFLinkGraphBuilder<ELFT> {
     using Base = ELFLinkGraphBuilder<ELFT>;
 
     uint32_t Type = Rel.getType(false);
-    // We do not implement linker relaxation for jitlink now, except what is
-    // required for alignment (see below).
-    if (Type == ELF::R_LARCH_RELAX)
-      return Error::success();
-
     int64_t Addend = Rel.r_addend;
-    if (Type == ELF::R_LARCH_ALIGN) {
-      uint64_t Alignment = PowerOf2Ceil(Addend);
-      // FIXME: Implement support for ensuring alignment together with linker
-      // relaxation. Addend is always 28 in the most common case when
-      // interpreting C++ code in clang-repl.
-      if (Alignment > 32)
-        return make_error<JITLinkError>(
-            formatv("Unsupported relocation R_LARCH_ALIGN with alignment {0} "
-                    "larger than 32 (addend: {1})",
-                    Alignment, Addend));
+
+    if (Type == ELF::R_LARCH_RELAX) {
+      if (BlockToFix.edges_empty())
+        return make_error<StringError>(
+            "R_LARCH_RELAX without preceding relocation",
+            inconvertibleErrorCode());
+
+      auto &PrevEdge = *std::prev(BlockToFix.edges().end());
+      auto Kind = static_cast<EdgeKind_loongarch>(PrevEdge.getKind());
+      PrevEdge.setKind(getRelaxableRelocationKind(Kind));
       return Error::success();
     }
 
@@ -227,6 +461,9 @@ void link_ELF_loongarch(std::unique_ptr<LinkGraph> G,
 
     // Add an in-place GOT/PLTStubs build pass.
     Config.PostPrunePasses.push_back(buildTables_ELF_loongarch);
+
+    // Add a linker relaxation pass.
+    Config.PostAllocationPasses.push_back(relax);
   }
 
   if (auto Err = Ctx->modifyPassConfig(*G, Config))
@@ -235,5 +472,7 @@ void link_ELF_loongarch(std::unique_ptr<LinkGraph> G,
   ELFJITLinker_loongarch::link(std::move(Ctx), std::move(G), std::move(Config));
 }
 
+LinkGraphPassFunction createRelaxationPass_ELF_loongarch() { return relax; }
+
 } // namespace jitlink
 } // namespace llvm

llvm/lib/ExecutionEngine/JITLink/loongarch.cpp

@@ -52,6 +52,7 @@ const char *getEdgeKindName(Edge::Kind K) {
     KIND_NAME_CASE(RequestGOTAndTransformToPage20)
     KIND_NAME_CASE(RequestGOTAndTransformToPageOffset12)
     KIND_NAME_CASE(Call36PCRel)
+    KIND_NAME_CASE(AlignRelaxable)
   default:
     return getGenericEdgeKindName(K);
   }