Do not remove implicit defs in BranchFolder

Branch folder removes implicit defs if they are the only non-branching
instructions in a block, and the branches do not use the defined registers.
The problem is that in some cases these implicit defs are required for
the liveness information to be correct.

Differential Revision: https://reviews.llvm.org/D25478

llvm-svn: 284036

llvm/lib/CodeGen/BranchFolding.cpp

@@ -145,59 +145,6 @@ void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
     MLI->removeBlock(MBB);
 }
 
-/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
-/// followed by terminators, and if the implicitly defined registers are not
-/// used by the terminators, remove those implicit_def's. e.g.
-/// BB1:
-///   r0 = implicit_def
-///   r1 = implicit_def
-///   br
-/// This block can be optimized away later if the implicit instructions are
-/// removed.
-bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
-  SmallSet<unsigned, 4> ImpDefRegs;
-  MachineBasicBlock::iterator I = MBB->begin();
-  while (I != MBB->end()) {
-    if (!I->isImplicitDef())
-      break;
-    unsigned Reg = I->getOperand(0).getReg();
-    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
-      for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
-           SubRegs.isValid(); ++SubRegs)
-        ImpDefRegs.insert(*SubRegs);
-    } else {
-      ImpDefRegs.insert(Reg);
-    }
-    ++I;
-  }
-  if (ImpDefRegs.empty())
-    return false;
-
-  MachineBasicBlock::iterator FirstTerm = I;
-  while (I != MBB->end()) {
-    if (!TII->isUnpredicatedTerminator(*I))
-      return false;
-    // See if it uses any of the implicitly defined registers.
-    for (const MachineOperand &MO : I->operands()) {
-      if (!MO.isReg() || !MO.isUse())
-        continue;
-      unsigned Reg = MO.getReg();
-      if (ImpDefRegs.count(Reg))
-        return false;
-    }
-    ++I;
-  }
-
-  I = MBB->begin();
-  while (I != FirstTerm) {
-    MachineInstr *ImpDefMI = &*I;
-    ++I;
-    MBB->erase(ImpDefMI);
-  }
-
-  return true;
-}
-
 /// OptimizeFunction - Perhaps branch folding, tail merging and other
 /// CFG optimizations on the given function.  Block placement changes the layout
 /// and may create new tail merging opportunities.
@@ -228,7 +175,6 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
     SmallVector<MachineOperand, 4> Cond;
     if (!TII->analyzeBranch(MBB, TBB, FBB, Cond, true))
       MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
-    MadeChange |= OptimizeImpDefsBlock(&MBB);
   }
 
   // Recalculate funclet membership.

llvm/lib/CodeGen/BranchFolding.h

@@ -156,7 +156,6 @@ namespace llvm {
     bool OptimizeBranches(MachineFunction &MF);
     bool OptimizeBlock(MachineBasicBlock *MBB);
     void RemoveDeadBlock(MachineBasicBlock *MBB);
-    bool OptimizeImpDefsBlock(MachineBasicBlock *MBB);
 
     bool HoistCommonCode(MachineFunction &MF);
     bool HoistCommonCodeInSuccs(MachineBasicBlock *MBB);

llvm/test/CodeGen/Hexagon/branchfolder-keep-impdef.ll

@@ -0,0 +1,29 @@
+; RUN: llc -march=hexagon -verify-machineinstrs < %s | FileCheck %s
+;
+; Check that the testcase compiles successfully. Expect that if-conversion
+; took place.
+; CHECK-LABEL: fred:
+; CHECK: if (!p0) r1 = memw(r0 + #0)
+
+target triple = "hexagon"
+
+define void @fred(i32 %p0) local_unnamed_addr align 2 {
+b0:
+  br i1 undef, label %b1, label %b2
+
+b1:                                               ; preds = %b0
+  %t0 = load i8*, i8** undef, align 4
+  br label %b2
+
+b2:                                               ; preds = %b1, %b0
+  %t1 = phi i8* [ %t0, %b1 ], [ undef, %b0 ]
+  %t2 = getelementptr inbounds i8, i8* %t1, i32 %p0
+  tail call void @llvm.memmove.p0i8.p0i8.i32(i8* undef, i8* %t2, i32 undef, i32 1, i1 false) #1
+  unreachable
+}
+
+declare void @llvm.memmove.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i32, i1) #0
+
+attributes #0 = { argmemonly nounwind }
+attributes #1 = { nounwind }
+

llvm/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll

@@ -100,6 +100,7 @@ while.end:                                        ; preds = %entry
 ; CHECK-NEXT: in Loop: Header
 ; CHECK-NEXT: incq
 ; CHECK-NEXT: %for.body3.us.i
+; CHECK-NEXT: Parent Loop
 ; CHECK-NEXT: Inner Loop
 ; CHECK: testb
 ; CHECK: je