[llvm] [MachineSink] Fix stable sort comparator (PR #116705)

[Ellis Hoag via llvm-commits]

https://github.com/ellishg created https://github.com/llvm/llvm-project/pull/116705

Fix the comparator in `stable_sort()` to satisfy the strict weak ordering requirement.

In https://github.com/llvm/llvm-project/pull/115367 this comparator was changed to use `getCycleDepth()` when `shouldOptimizeForSize()` is true. However, I mistakenly changed to logic so that we use `LHSFreq < RHSFreq` if **either** of them are zero. This causes us to fail the last requirment (https://en.cppreference.com/w/cpp/named_req/Compare).

> if comp(a, b) == true and comp(b, c) == true then comp(a, c) == true

>From e39a57d226e788f84713ea8d1897d95a43f05c8e Mon Sep 17 00:00:00 2001
From: Ellis Hoag <ellis.sparky.hoag at gmail.com>
Date: Mon, 18 Nov 2024 14:33:32 -0800
Subject: [PATCH] [MachineSink] Fix stable sort comparator

---
 llvm/lib/CodeGen/MachineSink.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index c470bd71dfb29f..0def107f6306d7 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -1227,7 +1227,8 @@ MachineSinking::GetAllSortedSuccessors(MachineInstr &MI, MachineBasicBlock *MBB,
       AllSuccs, [&](const MachineBasicBlock *L, const MachineBasicBlock *R) {
         uint64_t LHSFreq = MBFI ? MBFI->getBlockFreq(L).getFrequency() : 0;
         uint64_t RHSFreq = MBFI ? MBFI->getBlockFreq(R).getFrequency() : 0;
-        if (llvm::shouldOptimizeForSize(MBB, PSI, MBFI) || !LHSFreq || !RHSFreq)
+        if (llvm::shouldOptimizeForSize(MBB, PSI, MBFI) ||
+            (!LHSFreq && !RHSFreq))
           return CI->getCycleDepth(L) < CI->getCycleDepth(R);
         return LHSFreq < RHSFreq;
       });