[CodeGen][NFC] Fix quadratic c-t for large jump tables

Deleting a basic block removes all references from jump tables, which
is O(n). When freeing a MachineFunction, all basic blocks are deleted
before the jump tables, causing O(n^2) runtime. Fix this by deallocating
the jump table first.

Test case generator:

    import sys

    n = int(sys.argv[1])
    print("define void @f(i64 %c, ptr %p) {")
    print("  switch i64 %c, label %d [")
    for i in range(n):
        print(f"    i64 {i}, label %h{i}")
    print(f"  ]")
    for i in range(n):
        print(f'h{i}:')
        print(f'  store i64 {i*i}, ptr %p')
        print(f'  ret void')
    print('d:')
    print('  ret void')
    print('}')

Improvement at 5000 entries:

    Benchmark 1: ./llc.pre -filetype=obj -O0 <switch5k.bc
      Time (mean ± σ):      49.7 ms ±   1.0 ms
      Range (min … max):    48.0 ms …  52.1 ms    57 runs

    Benchmark 2: ./llc.post -filetype=obj -O0 <switch5k.bc
      Time (mean ± σ):      39.4 ms ±   0.8 ms
      Range (min … max):    37.1 ms …  41.1 ms    72 runs

    Summary
      ./llc.post -filetype=obj -O0 <switch5k.bc ran
        1.26 ± 0.04 times faster than ./llc.pre -filetype=obj -O0 <switch5k.bc

Improvement at 20000 entries:

    Benchmark 1: ./llc.pre -filetype=obj -O0 <switch20k.bc
      Time (mean ± σ):     281.7 ms ±   1.0 ms
      Range (min … max):   280.2 ms … 283.0 ms    10 runs

    Benchmark 2: ./llc.post -filetype=obj -O0 <switch20k.bc
      Time (mean ± σ):     123.9 ms ±   1.5 ms
      Range (min … max):   121.4 ms … 129.2 ms    23 runs

    Summary
      ./llc.post -filetype=obj -O0 <switch20k.bc ran
        2.27 ± 0.03 times faster than ./llc.pre -filetype=obj -O0 <switch20k.bc

Pull Request: #144108

Author: aengelke

llvm/lib/CodeGen/MachineFunction.cpp

@@ -259,6 +259,15 @@ MachineFunction::~MachineFunction() {
 
 void MachineFunction::clear() {
   Properties.reset();
+
+  // Clear JumpTableInfo first. Otherwise, every MBB we delete would do a
+  // linear search over the jump table entries to find and erase itself.
+  if (JumpTableInfo) {
+    JumpTableInfo->~MachineJumpTableInfo();
+    Allocator.Deallocate(JumpTableInfo);
+    JumpTableInfo = nullptr;
+  }
+
   // Don't call destructors on MachineInstr and MachineOperand. All of their
   // memory comes from the BumpPtrAllocator which is about to be purged.
   //
@@ -287,11 +296,6 @@ void MachineFunction::clear() {
   ConstantPool->~MachineConstantPool();
   Allocator.Deallocate(ConstantPool);
 
-  if (JumpTableInfo) {
-    JumpTableInfo->~MachineJumpTableInfo();
-    Allocator.Deallocate(JumpTableInfo);
-  }
-
   if (WinEHInfo) {
     WinEHInfo->~WinEHFuncInfo();
     Allocator.Deallocate(WinEHInfo);