From f16acbd2f983adaa285f6d38c8a4302ebfc64c19 Mon Sep 17 00:00:00 2001
From: Matthias Braun <matze@braunis.de>
Date: Wed, 6 Jul 2016 18:55:10 +0000
Subject: [PATCH] TailDuplicator: Remove live-in updating logic

This logic was introduced in r157663 and does not make any sense to me.
The motivating example in rdar://11538365 looks like this:

This is the tail:
BB#16: derived from LLVM BB %if.end68
    Live Ins: %R0 %R4 %R5
    Predecessors according to CFG: BB#15 BB#5
        tBLXi pred:14, pred:%noreg, <ga:@CFRelease>, %R0<kill>, <regmask>, %LR<imp-def,dead>, %SP<imp-use>, %SP<imp-def>
        t2B <BB#20>, pred:14, pred:%noreg
    Successors according to CFG: BB#20

This is the predBB:
BB#5:
    Live Ins: %R5
    Predecessors according to CFG: BB#4
        %R4<def> = t2MOVi 0, pred:14, pred:%noreg, opt:%noreg
        t2B <BB#16>, pred:14, pred:%noreg
    Successors according to CFG: BB#16

However this is invalid machine code to begin with, if %R0 is live-in to
BB#16 then it must be live-in to BB#5 as well if BB#5 does not define
it.  We should not need logic to retroactively fix broken machine code
and in fact the example from r157663 passes cleanly with the code
removed and I do not see any (newly) failing tests with the machine
verifier enabled.

Differential Revision: http://reviews.llvm.org/D22031

llvm-svn: 274655
---
 llvm/include/llvm/CodeGen/TailDuplicator.h |  1 -
 llvm/lib/CodeGen/TailDuplicator.cpp        | 18 ------------------
 2 files changed, 19 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/TailDuplicator.h b/llvm/include/llvm/CodeGen/TailDuplicator.h
index 7c70fbfceca1..8e65199418a6 100644
--- a/llvm/include/llvm/CodeGen/TailDuplicator.h
+++ b/llvm/include/llvm/CodeGen/TailDuplicator.h
@@ -33,7 +33,6 @@ class TailDuplicator {
   const MachineBranchProbabilityInfo *MBPI;
   const MachineModuleInfo *MMI;
   MachineRegisterInfo *MRI;
-  std::unique_ptr<RegScavenger> RS;
   bool PreRegAlloc;
 
   // A list of virtual registers for which to update SSA form.
diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
index 05421ac64a45..3c122358fd0d 100644
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -67,10 +67,6 @@ void TailDuplicator::initMF(MachineFunction &MF, const MachineModuleInfo *MMIin,
   assert(MBPI != nullptr && "Machine Branch Probability Info required");
 
   PreRegAlloc = MRI->isSSA();
-  RS.reset();
-
-  if (MRI->tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
-    RS.reset(new RegScavenger());
 }
 
 static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
@@ -770,20 +766,6 @@ bool TailDuplicator::tailDuplicate(MachineFunction &MF, bool IsSimple,
     // Remove PredBB's unconditional branch.
     TII->RemoveBranch(*PredBB);
 
-    if (RS && !TailBB->livein_empty()) {
-      // Update PredBB livein.
-      RS->enterBasicBlock(*PredBB);
-      if (!PredBB->empty())
-        RS->forward(std::prev(PredBB->end()));
-      for (const auto &LI : TailBB->liveins()) {
-        if (!RS->isRegUsed(LI.PhysReg, false))
-          // If a register is previously livein to the tail but it's not live
-          // at the end of predecessor BB, then it should be added to its
-          // livein list.
-          PredBB->addLiveIn(LI);
-      }
-    }
-
     // Clone the contents of TailBB into PredBB.
     DenseMap<unsigned, RegSubRegPair> LocalVRMap;
     SmallVector<std::pair<unsigned, RegSubRegPair>, 4> CopyInfos;