[ARM][Block placement] Check the predecessor exists before processing it

[samtebbs]

Summary

Not all machine loops will have a predecessor. so the pass needs to
check it before continuing.

[dmgreen]

Thanks. LGTM

llvm/lib/Target/ARM/ARMBlockPlacement.cpp

bool ARMBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
 
   // Find loops with a backwards branching WLS.
   // This requires looping over the loops in the function, checking each
   // preheader for a WLS and if its target is before the preheader. If moving
   // the target block wouldn't produce another backwards WLS or a new forwards
   // LE branch then move the target block after the preheader.
   for (auto *ML : *MLI) {
     MachineBasicBlock *Preheader = ML->getLoopPredecessor();
+    if (!Preheader)
+      continue;
 
     for (auto &Terminator : Preheader->terminators()) {
       if (Terminator.getOpcode() != ARM::t2WhileLoopStart)
         continue;
       MachineBasicBlock *LoopExit = Terminator.getOperand(1).getMBB();
       // We don't want to move the function's entry block.
       if (!LoopExit->getPrevNode())
         continue;

llvm/test/CodeGen/Thumb2/block-placement.mir

--- |
   entry:
     unreachable
   }
 
   define void @backwards_branch_forwards_le(i32 %N, i32 %M, i32* nocapture %a, i32* nocapture %b, i32* nocapture %c) local_unnamed_addr #0 {
   entry:
     unreachable
   }
+
+  define void @no_preheader(i32 %N, i32 %M, i32* nocapture %a, i32* nocapture %b, i32* nocapture %c) local_unnamed_addr #0 {
+  entry:
+    unreachable
+  }
+
+  declare dso_local i32 @g(...) local_unnamed_addr #1
+
+  declare dso_local i32 @h(...) local_unnamed_addr #1
+
 ...
 ---
 name:            backwards_branch
 body:             |
   ; CHECK-LABEL: name: backwards_branch
   ; CHECK: bb.0:
   ; CHECK:   successors: %bb.2(0x80000000)
   ; CHECK:   tCMPi8 renamable $r0, 1, 14 /* CC::al */, $noreg, implicit-def $cpsr
bb.3:
     liveins: $lr, $r1, $r2
 
     renamable $r0 = tLDRi renamable $r2, 0, 14 /* CC::al */, $noreg
     tSTRi killed renamable $r0, renamable $r1, 0, 14 /* CC::al */, $noreg
     renamable $lr = t2LoopEndDec killed renamable $lr, %bb.3, implicit-def dead $cpsr
     t2B %bb.1, 14 /* CC::al */, $noreg
 
 ...
+---
+name:            no_preheader
+body:             |
+  ; CHECK-LABEL: name: no_preheader
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.2(0x30000000), %bb.1(0x50000000)
+  ; CHECK:   frame-setup tPUSH 14 /* CC::al */, $noreg, killed $r4, killed $r5, $r7, killed $lr, implicit-def $sp, implicit $sp
+  ; CHECK:   frame-setup CFI_INSTRUCTION def_cfa_offset 16
+  ; CHECK:   frame-setup CFI_INSTRUCTION offset $lr, -4
+  ; CHECK:   frame-setup CFI_INSTRUCTION offset $r7, -8
+  ; CHECK:   frame-setup CFI_INSTRUCTION offset $r5, -12
+  ; CHECK:   frame-setup CFI_INSTRUCTION offset $r4, -16
+  ; CHECK:   $r7 = frame-setup tADDrSPi $sp, 2, 14 /* CC::al */, $noreg
+  ; CHECK:   frame-setup CFI_INSTRUCTION def_cfa $r7, 8
+  ; CHECK:   $r4 = tMOVr killed $r0, 14 /* CC::al */, $noreg
+  ; CHECK:   tBL 14 /* CC::al */, $noreg, @g, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def $r0
+  ; CHECK:   tCMPi8 killed renamable $r0, 0, 14 /* CC::al */, $noreg, implicit-def $cpsr
+  ; CHECK:   t2Bcc %bb.2, 0 /* CC::eq */, killed $cpsr
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.4(0x80000000)
+  ; CHECK:   renamable $r0, dead $cpsr = tMOVi8 4, 14 /* CC::al */, $noreg
+  ; CHECK:   renamable $r5 = t2LDRSHi12 killed renamable $r0, 0, 14 /* CC::al */, $noreg
+  ; CHECK:   t2B %bb.4, 14 /* CC::al */, $noreg
+  ; CHECK: bb.2:
+  ; CHECK:   successors: %bb.4(0x80000000)
+  ; CHECK:   renamable $r5, dead $cpsr = tMOVi8 0, 14 /* CC::al */, $noreg
+  ; CHECK:   t2B %bb.4, 14 /* CC::al */, $noreg
+  ; CHECK: bb.3:
+  ; CHECK:   successors: %bb.4(0x80000000)
+  ; CHECK:   $r0 = tMOVr $r5, 14 /* CC::al */, $noreg
+  ; CHECK:   tBL 14 /* CC::al */, $noreg, @h, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit killed $r0, implicit-def $sp, implicit-def dead $r0
+  ; CHECK: bb.4:
+  ; CHECK:   successors: %bb.5(0x04000000), %bb.3(0x7c000000)
+  ; CHECK:   renamable $r0 = tLDRi renamable $r4, 0, 14 /* CC::al */, $noreg
+  ; CHECK:   tCMPi8 killed renamable $r0, 0, 14 /* CC::al */, $noreg, implicit-def $cpsr
+  ; CHECK:   t2Bcc %bb.3, 1 /* CC::ne */, killed $cpsr
+  ; CHECK: bb.5:
+  ; CHECK:   frame-destroy tPOP_RET 14 /* CC::al */, $noreg, def $r4, def $r5, def $r7, def $pc
+  bb.0:
+    successors: %bb.1(0x30000000), %bb.2(0x50000000)
+    liveins: $r0, $r4, $r5, $lr
+
+    frame-setup tPUSH 14 /* CC::al */, $noreg, killed $r4, killed $r5, $r7, killed $lr, implicit-def $sp, implicit $sp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 16
+    frame-setup CFI_INSTRUCTION offset $lr, -4
+    frame-setup CFI_INSTRUCTION offset $r7, -8
+    frame-setup CFI_INSTRUCTION offset $r5, -12
+    frame-setup CFI_INSTRUCTION offset $r4, -16
+    $r7 = frame-setup tADDrSPi $sp, 2, 14 /* CC::al */, $noreg
+    frame-setup CFI_INSTRUCTION def_cfa $r7, 8
+    $r4 = tMOVr killed $r0, 14 /* CC::al */, $noreg
+    tBL 14 /* CC::al */, $noreg, @g, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def $r0
+    tCMPi8 killed renamable $r0, 0, 14 /* CC::al */, $noreg, implicit-def $cpsr
+    t2Bcc %bb.1, 0 /* CC::eq */, killed $cpsr
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+    liveins: $r4
+
+    renamable $r0, dead $cpsr = tMOVi8 4, 14 /* CC::al */, $noreg
+    renamable $r5 = t2LDRSHi12 killed renamable $r0, 0, 14 /* CC::al */, $noreg
+    t2B %bb.3, 14 /* CC::al */, $noreg
+
+  bb.1:
+    successors: %bb.3(0x80000000)
+    liveins: $r4
+
+    renamable $r5, dead $cpsr = tMOVi8 0, 14 /* CC::al */, $noreg
+    t2B %bb.3, 14 /* CC::al */, $noreg
+
+  bb.4:
+    successors: %bb.3(0x80000000)
+    liveins: $r4, $r5
+
+    $r0 = tMOVr $r5, 14 /* CC::al */, $noreg
+    tBL 14 /* CC::al */, $noreg, @h, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit killed $r0, implicit-def $sp, implicit-def dead $r0
+
+  bb.3:
+    successors: %bb.5(0x04000000), %bb.4(0x7c000000)
+    liveins: $r4, $r5
+
+    renamable $r0 = tLDRi renamable $r4, 0, 14 /* CC::al */, $noreg
+    tCMPi8 killed renamable $r0, 0, 14 /* CC::al */, $noreg, implicit-def $cpsr
+    t2Bcc %bb.4, 1 /* CC::ne */, killed $cpsr
+
+  bb.5:
+    frame-destroy tPOP_RET 14 /* CC::al */, $noreg, def $r4, def $r5, def $r7, def $pc
+...