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

[Mingming Liu 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),
----------------
mingmingl-llvm wrote:

`drop_front` is clearer to me, despite `make_range` supersedes the ArrayRef usage in the updated patch.

I became interested in why both `drop` and `take` exists, and it turns out (amusingly) that `take_{front,back}` [handles](https://github.com/llvm/llvm-project/blob/22d4ff155aadf0f098dd5dc48d9038da15108937/llvm/include/llvm/ADT/ArrayRef.h#L414-L426) `N>=size()` by returning the full view while `drop_{front,back}` [asserts](https://github.com/llvm/llvm-project/blob/22d4ff155aadf0f098dd5dc48d9038da15108937/llvm/include/llvm/ADT/ArrayRef.h#L389-L398) `N <= size()` (and slices a bogus length otherwise). 

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