[clang] [llvm] [MC] Emit a jump table size section (PR #101962)

[Saleem Abdulrasool via cfe-commits]

================
@@ -2764,10 +2771,59 @@ void AsmPrinter::emitJumpTableInfo() {
     for (const MachineBasicBlock *MBB : JTBBs)
       emitJumpTableEntry(MJTI, MBB, JTI);
   }
+
+  if (EmitJumpTableSizesSection)
+    emitJumpTableSizesSection(MJTI, F);
+
   if (!JTInDiffSection)
     OutStreamer->emitDataRegion(MCDR_DataRegionEnd);
 }
 
+void AsmPrinter::emitJumpTableSizesSection(const MachineJumpTableInfo *MJTI,
+                                           const Function &F) const {
+  const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
+
+  if (JT.empty())
+    return;
+
+  StringRef GroupName = F.hasComdat() ? F.getComdat()->getName() : "";
+  MCSection *JumpTableSizesSection = nullptr;
+  StringRef sectionName = ".llvm_jump_table_sizes";
+
+  if (TM.getTargetTriple().isOSBinFormatELF()) {
+    MCSymbolELF *LinkedToSym = dyn_cast<MCSymbolELF>(CurrentFnSym);
+    int Flags = F.hasComdat() ? ELF::SHF_GROUP : 0;
+
+    JumpTableSizesSection = OutContext.getELFSection(
+        sectionName, ELF::SHT_LLVM_JT_SIZES, Flags, 0, GroupName, 
F.hasComdat(),
+        MCSection::NonUniqueID, LinkedToSym);
+  } else if (TM.getTargetTriple().isOSBinFormatCOFF()) {
+    if (F.hasComdat()) {
+      MCSymbolCOFF *LinkedToSym = dyn_cast<MCSymbolCOFF>(CurrentFnSym);
+
+      JumpTableSizesSection = OutContext.getCOFFSection(
+          sectionName,
+          COFF::IMAGE_SCN_CNT_INITIALIZED_DATA | COFF::IMAGE_SCN_MEM_READ |
+              COFF::IMAGE_SCN_LNK_COMDAT | COFF::IMAGE_SCN_MEM_DISCARDABLE,
+          F.getComdat()->getName(), COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE);
+    } else
+      JumpTableSizesSection = OutContext.getCOFFSection(
+          sectionName, COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
+                           COFF::IMAGE_SCN_MEM_READ |
+                           COFF::IMAGE_SCN_MEM_DISCARDABLE);
+  } else {
+    return;
----------------
compnerd wrote:

Can you change this to an early return please?

https://github.com/llvm/llvm-project/pull/101962
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