[llvm] [CodeLayout] Do not rebuild chains with -apply-ext-tsp-for-size (PR #115934)

[Ellis Hoag via llvm-commits]

https://github.com/ellishg created https://github.com/llvm/llvm-project/pull/115934

https://github.com/llvm/llvm-project/pull/109711 disables `buildCFGChains()` when `-apply-ext-tsp-for-size` is used to improve codesize. Tail merging can change the layout and normally requires `buildCFGChains()` to be called again, but we want to prevent this when optimizing for codesize. We saw slight size improvement on large binaries with this change. If `-apply-ext-tsp-for-size` is not used, this should be a NFC.

>From 5f024a0c14cdb998953539ff8779b3ad5ebef9e7 Mon Sep 17 00:00:00 2001
From: Ellis Hoag <ellis.sparky.hoag at gmail.com>
Date: Tue, 12 Nov 2024 12:00:00 -0800
Subject: [PATCH] [CodeLayout] Do not rebuild chains when opt for size

---
 llvm/lib/CodeGen/MachineBlockPlacement.cpp | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index bdad63f368dfec..0f68313e64f542 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -3558,14 +3558,16 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
 
     if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(), MLI,
                             /*AfterPlacement=*/true)) {
-      // Redo the layout if tail merging creates/removes/moves blocks.
-      BlockToChain.clear();
-      ComputedEdges.clear();
       // Must redo the post-dominator tree if blocks were changed.
       if (MPDT)
         MPDT->recalculate(MF);
-      ChainAllocator.DestroyAll();
-      buildCFGChains();
+      if (!UseExtTspForSize) {
+        // Redo the layout if tail merging creates/removes/moves blocks.
+        BlockToChain.clear();
+        ComputedEdges.clear();
+        ChainAllocator.DestroyAll();
+        buildCFGChains();
+      }
     }
   }