[ARM] Remove a dead ADD during the creation of TBBs

During the optimisation of jump tables in the constant island pass,
an extra ADD could be left over, now dead but not removed.

Differential Revision: https://reviews.llvm.org/D31389

llvm-svn: 299634

lib/Target/ARM/ARMConstantIslandPass.cpp

@@ -2017,6 +2017,44 @@ static bool jumpTableFollowsTB(MachineInstr *JTMI, MachineInstr *CPEMI) {
          &*MBB->begin() == CPEMI;
 }
 
+static void RemoveDeadAddBetweenLEAAndJT(MachineInstr *LEAMI,
+                                         MachineInstr *JumpMI,
+                                         unsigned &DeadSize) {
+  // Remove a dead add between the LEA and JT, which used to compute EntryReg,
+  // but the JT now uses PC. Finds the last ADD (if any) that def's EntryReg
+  // and is not clobbered / used.
+  MachineInstr *RemovableAdd = nullptr;
+  unsigned EntryReg = JumpMI->getOperand(0).getReg();
+
+  // Find the last ADD to set EntryReg
+  MachineBasicBlock::iterator I(LEAMI);
+  for (++I; &*I != JumpMI; ++I) {
+    if (I->getOpcode() == ARM::t2ADDrs && I->getOperand(0).getReg() == EntryReg)
+      RemovableAdd = &*I;
+  }
+
+  if (!RemovableAdd)
+    return;
+
+  // Ensure EntryReg is not clobbered or used.
+  MachineBasicBlock::iterator J(RemovableAdd);
+  for (++J; &*J != JumpMI; ++J) {
+    for (unsigned K = 0, E = J->getNumOperands(); K != E; ++K) {
+      const MachineOperand &MO = J->getOperand(K);
+      if (!MO.isReg() || !MO.getReg())
+        continue;
+      if (MO.isDef() && MO.getReg() == EntryReg)
+        return;
+      if (MO.isUse() && MO.getReg() == EntryReg)
+        return;
+    }
+  }
+
+  DEBUG(dbgs() << "Removing Dead Add: " << *RemovableAdd);
+  RemovableAdd->eraseFromParent();
+  DeadSize += 4;
+}
+
 static bool registerDefinedBetween(unsigned Reg,
                                    MachineBasicBlock::iterator From,
                                    MachineBasicBlock::iterator To,
@@ -2172,7 +2210,10 @@ bool ARMConstantIslands::optimizeThumb2JumpTables() {
       NewJTMI->getOperand(0).setReg(ARM::PC);
       NewJTMI->getOperand(0).setIsKill(false);
 
-      if (CanDeleteLEA)  {
+      if (CanDeleteLEA) {
+        if (isThumb2)
+          RemoveDeadAddBetweenLEAAndJT(User.MI, MI, DeadSize);
+
         User.MI->eraseFromParent();
         DeadSize += isThumb2 ? 4 : 2;
 

test/CodeGen/Thumb2/tbb-removeadd.mir

@@ -0,0 +1,124 @@
+#RUN: llc -run-pass arm-cp-islands %s -o - | FileCheck %s
+
+--- |
+  ; ModuleID = 'test.ll'
+  source_filename = "test.c"
+  target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "thumbv8r-arm-none-eabi"
+  
+  define void @Func(i32 %i, i32* nocapture %p) local_unnamed_addr {
+  entry:
+    switch i32 %i, label %sw.epilog [
+      i32 0, label %sw.bb
+      i32 1, label %sw.bb1
+      i32 2, label %sw.epilog.sink.split
+      i32 4, label %sw.bb3
+    ]
+  
+  sw.bb:                                            ; preds = %entry
+    br label %sw.epilog.sink.split
+  
+  sw.bb1:                                           ; preds = %entry
+    store i32 0, i32* %p, align 4
+    br label %sw.epilog.sink.split
+  
+  sw.bb3:                                           ; preds = %entry
+    br label %sw.epilog.sink.split
+  
+  sw.epilog.sink.split:                             ; preds = %sw.bb3, %sw.bb1, %sw.bb, %entry
+    %.sink = phi i32 [ 2, %sw.bb3 ], [ 0, %sw.bb ], [ 1, %entry ], [ 1, %sw.bb1 ]
+    store i32 %.sink, i32* %p, align 4
+    br label %sw.epilog
+  
+  sw.epilog:                                        ; preds = %sw.epilog.sink.split, %entry
+    ret void
+  }
+
+...
+---
+name:            Func
+alignment:       1
+exposesReturnsTwice: false
+noVRegs:         true
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+liveins:         
+  - { reg: '%r0' }
+  - { reg: '%r1' }
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  maxCallFrameSize: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+jumpTable:       
+  kind:            inline
+  entries:         
+    - id:              0
+      blocks:          [ '%bb.2.sw.bb', '%bb.3.sw.bb1', '%bb.5.sw.epilog.sink.split', 
+                         '%bb.6.sw.epilog', '%bb.4.sw.bb3' ]
+# The ADD should be deleted along with the LEA
+# CHECK-NOT: t2LEApcrelJT
+# CHECK-NOT: t2ADDrs
+# CHECK: tMOVi8
+# CHECK: t2TBB_JT
+
+body:             |
+  bb.0.entry:
+    successors: %bb.6.sw.epilog(0x0ccccccb), %bb.1.entry(0x73333335)
+    liveins: %r0, %r1
+  
+    tCMPi8 %r0, 4, 14, _, implicit-def %cpsr
+    t2Bcc %bb.6.sw.epilog, 8, killed %cpsr
+  
+  bb.1.entry:
+    successors: %bb.2.sw.bb(0x1c71c71c), %bb.3.sw.bb1(0x1c71c71c), %bb.5.sw.epilog.sink.split(0x1c71c71c), %bb.6.sw.epilog(0x0e38e38e), %bb.4.sw.bb3(0x1c71c71c)
+    liveins: %r0, %r1
+  
+    %r2 = t2LEApcrelJT %jump-table.0, 14, _
+    %r3 = t2ADDrs killed %r2, %r0, 18, 14, _, _
+    %r2, dead %cpsr = tMOVi8 1, 14, _
+    t2BR_JT killed %r3, killed %r0, %jump-table.0
+  
+  bb.2.sw.bb:
+    successors: %bb.5.sw.epilog.sink.split(0x80000000)
+    liveins: %r1
+  
+    %r2, dead %cpsr = tMOVi8 0, 14, _
+    t2B %bb.5.sw.epilog.sink.split, 14, _
+  
+  bb.3.sw.bb1:
+    successors: %bb.5.sw.epilog.sink.split(0x80000000)
+    liveins: %r1
+  
+    %r0, dead %cpsr = tMOVi8 0, 14, _
+    %r2, dead %cpsr = tMOVi8 1, 14, _
+    tSTRi killed %r0, %r1, 0, 14, _ :: (store 4 into %ir.p)
+    t2B %bb.5.sw.epilog.sink.split, 14, _
+  
+  bb.4.sw.bb3:
+    successors: %bb.5.sw.epilog.sink.split(0x80000000)
+    liveins: %r1
+  
+    %r2, dead %cpsr = tMOVi8 2, 14, _
+  
+  bb.5.sw.epilog.sink.split:
+    successors: %bb.6.sw.epilog(0x80000000)
+    liveins: %r1, %r2
+  
+    tSTRi killed %r2, killed %r1, 0, 14, _ :: (store 4 into %ir.p)
+  
+  bb.6.sw.epilog:
+    tBX_RET 14, _
+
+...