[LiveIns] Improve recomputeLiveIns() (#88951)

Some small changes to recomputeLiveIns() to improve performance:

- Instead of copying the list of old live-ins, and then clearing
  them, a new method swaps the list for an empty one.
- getLiveIns() now returns a constant reference to the list

As result, the list-data is never copied. Depending on the
implementation
details of the vector container, it can also save calls to allocate
and deallocate memory.
I see a small improvement on CTMark with these changes.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index b2114c2..0bd5f09 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -1728,6 +1728,12 @@ void MachineBasicBlock::clearLiveIns() {
   LiveIns.clear();
 }
 
+void MachineBasicBlock::clearLiveIns(
+    std::vector<RegisterMaskPair> &OldLiveIns) {
+  assert(OldLiveIns.empty() && "Vector must be empty");
+  std::swap(LiveIns, OldLiveIns);
+}
+
 MachineBasicBlock::livein_iterator MachineBasicBlock::livein_begin() const {
   assert(getParent()->getProperties().hasProperty(
       MachineFunctionProperties::Property::TracksLiveness) &&