[ARM][LowOverheadLoops] Use tBcc when reverting

Check the branch target ranges and use a tBcc instead of t2Bcc when
we can.

Differential Revision: https://reviews.llvm.org/D67796

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372557 91177308-0d34-0410-b5e6-96231b3b80d8

Author: sparker-arm

lib/Target/ARM/ARMLowOverheadLoops.cpp

@@ -354,8 +354,7 @@ bool ARMLowOverheadLoops::ProcessLoop(MachineLoop *ML) {
 
 // WhileLoopStart holds the exit block, so produce a cmp lr, 0 and then a
 // beq that branches to the exit branch.
-// FIXME: Need to check that we're not trashing the CPSR when generating the
-// cmp. We could also try to generate a cbz if the value in LR is also in
+// TODO: We could also try to generate a cbz if the value in LR is also in
 // another low register.
 void ARMLowOverheadLoops::RevertWhile(MachineInstr *MI) const {
   LLVM_DEBUG(dbgs() << "ARM Loops: Reverting to cmp: " << *MI);
@@ -366,9 +365,12 @@ void ARMLowOverheadLoops::RevertWhile(MachineInstr *MI) const {
   MIB.addImm(0);
   MIB.addImm(ARMCC::AL);
   MIB.addReg(ARM::NoRegister);
+  
+  MachineBasicBlock *DestBB = MI->getOperand(1).getMBB();
+  unsigned BrOpc = BBUtils->isBBInRange(MI, DestBB, 254) ?
+    ARM::tBcc : ARM::t2Bcc;
 
-  // TODO: Try to use tBcc instead
-  MIB = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(ARM::t2Bcc));
+  MIB = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(BrOpc));
   MIB.add(MI->getOperand(1));   // branch target
   MIB.addImm(ARMCC::EQ);        // condition code
   MIB.addReg(ARM::CPSR);
@@ -391,8 +393,6 @@ void ARMLowOverheadLoops::RevertLoopDec(MachineInstr *MI) const {
 }
 
 // Generate a subs, or sub and cmp, and a branch instead of an LE.
-// FIXME: Need to check that we're not trashing the CPSR when generating
-// the cmp.
 void ARMLowOverheadLoops::RevertLoopEnd(MachineInstr *MI) const {
   LLVM_DEBUG(dbgs() << "ARM Loops: Reverting to cmp, br: " << *MI);
 
@@ -405,9 +405,12 @@ void ARMLowOverheadLoops::RevertLoopEnd(MachineInstr *MI) const {
   MIB.addImm(ARMCC::AL);
   MIB.addReg(ARM::NoRegister);
 
-  // TODO Try to use tBcc instead.
+  MachineBasicBlock *DestBB = MI->getOperand(1).getMBB();
+  unsigned BrOpc = BBUtils->isBBInRange(MI, DestBB, 254) ?
+    ARM::tBcc : ARM::t2Bcc;
+
   // Create bne
-  MIB = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(ARM::t2Bcc));
+  MIB = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(BrOpc));
   MIB.add(MI->getOperand(1));   // branch target
   MIB.addImm(ARMCC::NE);        // condition code
   MIB.addReg(ARM::CPSR);

test/CodeGen/Thumb2/LowOverheadLoops/branch-targets.ll

@@ -16,7 +16,7 @@
 ; CHECK-END: .LBB0_2:
 ; CHECK-END:   sub.w lr, lr, #1
 ; CHECK-END:   cmp.w lr, #0
-; CHECK-END:   bne.w .LBB0_3
+; CHECK-END:   bne .LBB0_3
 ; CHECK-END:   b .LBB0_4
 ; CHECK-END: .LBB0_3:
 ; CHECK-END:   b .LBB0_2

test/CodeGen/Thumb2/LowOverheadLoops/end-positive-offset.mir

@@ -4,7 +4,7 @@
 # CHECK-NOT: DLS
 # CHECK: bb.1.for.body:
 # CHECK:   t2CMPri $lr, 0, 14, $noreg, implicit-def $cpsr
-# CHECK:   t2Bcc %bb.3, 1, $cpsr
+# CHECK:   tBcc %bb.3, 1, $cpsr
 # CHECK:   tB %bb.2, 14, $noreg
 # CHECK: bb.2.for.cond.cleanup:
 # CHECK: bb.3.for.header:

test/CodeGen/Thumb2/LowOverheadLoops/revert-non-header.mir

@@ -3,7 +3,7 @@
 # CHECK: bb.5.for.inc16:
 # CHECK:   $lr = t2SUBri killed renamable $lr, 1, 14
 # CHECK:   t2CMPri $lr, 0, 14
-# CHECK:   t2Bcc %bb.6, 1
+# CHECK:   tBcc %bb.6, 1
 # CHECK:   tB %bb.2
 # CHECK: bb.6.for.cond4.preheader:
 

test/CodeGen/Thumb2/LowOverheadLoops/revert-non-loop.mir

@@ -5,11 +5,11 @@
 # CHECK:   tBcc %bb.2, 3
 # CHECK: bb.1.not.preheader:
 # CHECK:   t2CMPri renamable $lr, 0, 14
-# CHECK:   t2Bcc %bb.4, 0
+# CHECK:   tBcc %bb.4, 0
 # CHECK:   tB %bb.2
 # CHECK: bb.3.while.body:
 # CHECK:   t2CMPri $lr, 0, 14
-# CHECK:   t2Bcc %bb.3, 1
+# CHECK:   tBcc %bb.3, 1
 # CHECK:   tB %bb.4
 # CHECK: bb.4.while.end: