During PHI elimination, split critical edges that move copies out of loops.

This prevents the behavior observed in llvm.org/PR22369. I am not sure
whether I am reading the code correctly, but the early exit based on
isLiveOutPastPHIs() seems to make the wrong assumption that
RegisterCoalescer won't be able to coalesce those copies later.

This change hides the new behavior behind -no-phi-elim-live-out-early-exit
as it currently breaks four tests:
 * Assertion in:
     CodeGen/Hexagon/hwloop-cleanup.ll
 * Worse code in:
     CodeGen/X86/coalescer-commute4.ll
     CodeGen/X86/phys_subreg_coalesce-2.ll
     CodeGen/X86/zlib-longest-match.ll
   The root cause here seems to be that the heuristic that determines
   the visitation order in RegisterCoalescer gets less lucky.

llvm-svn: 231064

1 files changed, 13 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/PHIElimination.cpp b/llvm/lib/CodeGen/PHIElimination.cpp
index def2e3d..7098057 100644
--- a/llvm/lib/CodeGen/PHIElimination.cpp
+++ b/llvm/lib/CodeGen/PHIElimination.cpp
@@ -46,6 +46,10 @@ SplitAllCriticalEdges("phi-elim-split-all-critical-edges", cl::init(false),
                       cl::Hidden, cl::desc("Split all critical edges during "
                                            "PHI elimination"));
 
+static cl::opt<bool> NoPhiElimLiveOutEarlyExit(
+    "no-phi-elim-live-out-early-exit", cl::init(false), cl::Hidden,
+    cl::desc("Do not use an early exit if isLiveOutPastPHIs returns true."));
+
 namespace {
   class PHIElimination : public MachineFunctionPass {
     MachineRegisterInfo *MRI; // Machine register information
@@ -573,12 +577,14 @@ bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
       // there is a risk it may not be coalesced away.
       //
       // If the copy would be a kill, there is no need to split the edge.
-      if (!isLiveOutPastPHIs(Reg, PreMBB) && !SplitAllCriticalEdges)
+      bool ShouldSplit = isLiveOutPastPHIs(Reg, PreMBB);
+      if (!ShouldSplit && !NoPhiElimLiveOutEarlyExit)
         continue;
-
-      DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
-                   << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
-                   << ": " << *BBI);
+      if (ShouldSplit) {
+        DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
+                     << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
+                     << ": " << *BBI);
+      }
 
       // If Reg is not live-in to MBB, it means it must be live-in to some
       // other PreMBB successor, and we can avoid the interference by splitting
@@ -588,7 +594,7 @@ bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
       // is likely to be left after coalescing. If we are looking at a loop
       // exiting edge, split it so we won't insert code in the loop, otherwise
       // don't bother.
-      bool ShouldSplit = !isLiveIn(Reg, &MBB) || SplitAllCriticalEdges;
+      ShouldSplit = ShouldSplit && !isLiveIn(Reg, &MBB);
 
       // Check for a loop exiting edge.
       if (!ShouldSplit && CurLoop != PreLoop) {
@@ -603,7 +609,7 @@ bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
         // Split unless this edge is entering CurLoop from an outer loop.
         ShouldSplit = PreLoop && !PreLoop->contains(CurLoop);
       }
-      if (!ShouldSplit)
+      if (!ShouldSplit && !SplitAllCriticalEdges)
         continue;
       if (!PreMBB->SplitCriticalEdge(&MBB, this)) {
         DEBUG(dbgs() << "Failed to split critical edge.\n");