[lld] [RISCV][LLD] Zcmt RISC-V extension in lld (PR #163142)

[Fangrui Song via llvm-commits]

================
@@ -1533,3 +1594,219 @@ template <class ELFT> void RISCV::scanSection1(InputSectionBase &sec) {
 void RISCV::scanSection(InputSectionBase &sec) {
   invokeELFT(scanSection1, sec);
 }
+
+TableJumpSection::TableJumpSection(Ctx &ctx)
+    : SyntheticSection(ctx, ".riscv.jvt", SHT_PROGBITS,
+                       SHF_ALLOC | SHF_EXECINSTR, tableAlign) {}
+
+void TableJumpSection::addCMJTEntryCandidate(const Symbol *symbol,
+                                             int csReduction) {
+  addEntry(symbol, CMJTEntryCandidates, csReduction);
+}
+
+int TableJumpSection::getCMJTEntryIndex(const Symbol *symbol) {
+  uint32_t index = getIndex(symbol, maxCMJTEntrySize, finalizedCMJTEntries);
+  return index < finalizedCMJTEntries.size() ? (int)(startCMJTEntryIdx + index)
+                                             : -1;
+}
+
+void TableJumpSection::addCMJALTEntryCandidate(const Symbol *symbol,
+                                               int csReduction) {
+  addEntry(symbol, CMJALTEntryCandidates, csReduction);
+}
+
+int TableJumpSection::getCMJALTEntryIndex(const Symbol *symbol) {
+  uint32_t index = getIndex(symbol, maxCMJALTEntrySize, finalizedCMJALTEntries);
+  return index < finalizedCMJALTEntries.size()
+             ? (int)(startCMJALTEntryIdx + index)
+             : -1;
+}
+
+void TableJumpSection::addEntry(
+    const Symbol *symbol, llvm::DenseMap<const Symbol *, int> &entriesList,
+    int csReduction) {
+  entriesList[symbol] += csReduction;
+}
+
+uint32_t TableJumpSection::getIndex(
+    const Symbol *symbol, uint32_t maxSize,
+    SmallVector<llvm::detail::DenseMapPair<const Symbol *, int>, 0>
+        &entriesList) {
+  // Find this symbol in the ordered list of entries if it exists.
+  assert(maxSize >= entriesList.size() &&
+         "Finalized vector of entries exceeds maximum");
+  auto idx = std::find_if(
+      entriesList.begin(), entriesList.end(),
+      [symbol](llvm::detail::DenseMapPair<const Symbol *, int> &e) {
+        return e.first == symbol;
+      });
+
+  if (idx == entriesList.end())
+    return entriesList.size();
+  return idx - entriesList.begin();
+}
+
+void TableJumpSection::scanTableJumpEntries(const InputSection &sec) const {
+  for (auto [i, r] : llvm::enumerate(sec.relocations)) {
+    Defined *definedSymbol = dyn_cast<Defined>(r.sym);
+    if (!definedSymbol)
+      continue;
+    if (i + 1 == sec.relocs().size() ||
+        sec.relocs()[i + 1].type != R_RISCV_RELAX)
+      continue;
+    switch (r.type) {
+    case R_RISCV_JAL:
+    case R_RISCV_CALL:
+    case R_RISCV_CALL_PLT: {
+      const uint32_t jalr =
+          read32le(sec.contentMaybeDecompress().data() + r.offset +
+                   (r.type == R_RISCV_JAL ? 0 : 4));
+      const uint8_t rd = extractBits(jalr, 11, 7);
+
+      int csReduction = 6;
+      if (sec.relaxAux->relocTypes[i] == R_RISCV_RVC_JUMP)
+        continue;
+      else if (sec.relaxAux->relocTypes[i] == R_RISCV_JAL)
+        csReduction = 2;
+
+      if (rd == 0)
+        ctx.in.riscvTableJumpSection->addCMJTEntryCandidate(r.sym, csReduction);
+      else if (rd == X_RA)
+        ctx.in.riscvTableJumpSection->addCMJALTEntryCandidate(r.sym,
+                                                              csReduction);
+    }
+    }
+  }
+}
+
+void TableJumpSection::finalizeContents() {
+  if (isFinalized)
+    return;
+  isFinalized = true;
+
+  finalizedCMJTEntries = finalizeEntry(CMJTEntryCandidates, maxCMJTEntrySize);
+  CMJTEntryCandidates.clear();
+  int32_t CMJTSizeReduction = getSizeReduction();
+  finalizedCMJALTEntries =
+      finalizeEntry(CMJALTEntryCandidates, maxCMJALTEntrySize);
+  CMJALTEntryCandidates.clear();
+
+  if (!finalizedCMJALTEntries.empty() &&
+      getSizeReduction() < CMJTSizeReduction) {
+    // In memory, the cm.jt table occupies the first 0x20 entries.
+    // To be able to use the cm.jalt table which comes afterwards
+    // it is necessary to pad out the cm.jt table.
+    // Remove cm.jalt entries if the code reduction of cm.jalt is
+    // smaller than the size of the padding.
+    finalizedCMJALTEntries.clear();
+  }
+  // if table jump still got negative effect, give up.
+  if (getSizeReduction() <= 0) {
+    warn("Table Jump Relaxation didn't got any reduction for code size.");
----------------
MaskRay wrote:

lld diagnostics don't capitalize the message and don't have the trailing fulls stop https://llvm.org/docs/CodingStandards.html#error-and-warning-messages

https://github.com/llvm/llvm-project/pull/163142