[llvm-branch-commits] [llvm] [AsmPrinter][ELF] Support profile-guided section prefix for jump tables' (read-only) data sections (PR #122215)

[Ellis Hoag via llvm-branch-commits]

================
@@ -2876,42 +2875,101 @@ void AsmPrinter::emitJumpTableInfo() {
       MJTI->getEntryKind() == MachineJumpTableInfo::EK_LabelDifference32 ||
           MJTI->getEntryKind() == MachineJumpTableInfo::EK_LabelDifference64,
       F);
+
+  std::vector<unsigned> JumpTableIndices;
+  if (!TM.Options.EnableStaticDataPartitioning) {
+    for (unsigned JTI = 0, JTSize = JT.size(); JTI < JTSize; ++JTI)
+      JumpTableIndices.push_back(JTI);
+    emitJumpTables(JumpTableIndices, TLOF.getSectionForJumpTable(F, TM),
+                   JTInDiffSection, *MJTI);
+    return;
+  }
+
+  // When static data partitioning is enabled, collect jump table entries that
+  // go into the same section together to reduce the amount of section switch
+  // statements.
+  //
+  // Iterate all jump tables, put hot jump table indices towards the beginning
+  // of the vector, and cold jump table indices towards the end. Meanwhile
+  // retain the relative orders of original jump tables within a hot or 
unlikely
+  // section by reversing the cold jump table indices.
+  int NextHotJumpTableIndex = 0, NextColdJumpTableIndex = JT.size() - 1;
+  JumpTableIndices.resize(JT.size());
+  for (unsigned JTI = 0, JTSize = JT.size(); JTI < JTSize; ++JTI) {
+    if (JT[JTI].Hotness == MachineFunctionDataHotness::Cold)
+      JumpTableIndices[NextColdJumpTableIndex--] = JTI;
+    else
+      JumpTableIndices[NextHotJumpTableIndex++] = JTI;
+  }
+
+  if (NextHotJumpTableIndex != 0) {
+    emitJumpTables(
+        ArrayRef<unsigned>(JumpTableIndices).take_front(NextHotJumpTableIndex),
+        TLOF.getSectionForJumpTable(F, TM, &JT[0]), JTInDiffSection, *MJTI);
+  }
+
+  if (NextHotJumpTableIndex < (int)JT.size()) {
+    // Reverse the order of cold jump tables indices.
+    for (int L = NextHotJumpTableIndex, R = JT.size() - 1; L < R; ++L, --R)
+      std::swap(JumpTableIndices[L], JumpTableIndices[R]);
+  
+    emitJumpTables(
+        ArrayRef<unsigned>(JumpTableIndices)
+            .take_back(JT.size() - NextHotJumpTableIndex),
+        TLOF.getSectionForJumpTable(
+          F, TM, &JT[JumpTableIndices[NextHotJumpTableIndex]]),
+        JTInDiffSection, *MJTI);
+  }
+
+  return;
+}
+
+void AsmPrinter::emitJumpTables(ArrayRef<unsigned> JumpTableIndices,
+                                MCSection *JumpTableSection,
+                                bool JTInDiffSection,
+                                const MachineJumpTableInfo &MJTI) {
+  if (JumpTableIndices.empty())
+    return;
+
+  const DataLayout &DL = MF->getDataLayout();
   if (JTInDiffSection) {
-    // Drop it in the readonly section.
-    MCSection *ReadOnlySection = TLOF.getSectionForJumpTable(F, TM);
-    OutStreamer->switchSection(ReadOnlySection);
+    OutStreamer->switchSection(JumpTableSection);
   }
 
-  emitAlignment(Align(MJTI->getEntryAlignment(DL)));
+  emitAlignment(Align(MJTI.getEntryAlignment(MF->getDataLayout())));
 
   // Jump tables in code sections are marked with a data_region directive
   // where that's supported.
   if (!JTInDiffSection)
     OutStreamer->emitDataRegion(MCDR_DataRegionJT32);
 
-  for (unsigned JTI = 0, e = JT.size(); JTI != e; ++JTI) {
-    const std::vector<MachineBasicBlock*> &JTBBs = JT[JTI].MBBs;
+  const auto &JT = MJTI.getJumpTables();
+  for (unsigned Index = 0, e = JumpTableIndices.size(); Index != e; ++Index) {
----------------
ellishg wrote:

Then you can replace `JT[JumpTableIndices[Index]]` -> `JT[JTIndex]`
```suggestion
  for (auto JTIndex : JumpTableIndices) {
```

https://github.com/llvm/llvm-project/pull/122215
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