[globalisel][combine] When placing truncates, handle the case when the BB is empty

GlobalISel uses MIR with implicit fallthrough on each basic block. As a result,
getFirstNonPhi() can return end().

llvm-svn: 343829

lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -110,7 +110,8 @@ PreferredTuple ChoosePreferredUse(PreferredTuple &CurrentUse,
 /// want to try harder to find a dominating block.
 static void InsertInsnsWithoutSideEffectsBeforeUse(
     MachineIRBuilder &Builder, MachineInstr &DefMI, MachineOperand &UseMO,
-    std::function<void(MachineBasicBlock::iterator)> Inserter) {
+    std::function<void(MachineBasicBlock *, MachineBasicBlock::iterator)>
+        Inserter) {
   MachineInstr &UseMI = *UseMO.getParent();
 
   MachineBasicBlock *InsertBB = UseMI.getParent();
@@ -125,16 +126,26 @@ static void InsertInsnsWithoutSideEffectsBeforeUse(
   // the def instead of at the start of the block.
   if (InsertBB == DefMI.getParent()) {
     MachineBasicBlock::iterator InsertPt = &DefMI;
-    Inserter(std::next(InsertPt));
+    Inserter(InsertBB, std::next(InsertPt));
     return;
   }
 
   // Otherwise we want the start of the BB
-  Inserter(InsertBB->getFirstNonPHI());
+  Inserter(InsertBB, InsertBB->getFirstNonPHI());
 }
 } // end anonymous namespace
 
 bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
+  struct InsertionPoint {
+    MachineOperand *UseMO;
+    MachineBasicBlock *InsertIntoBB;
+    MachineBasicBlock::iterator InsertBefore;
+    InsertionPoint(MachineOperand *UseMO, MachineBasicBlock *InsertIntoBB,
+                   MachineBasicBlock::iterator InsertBefore)
+        : UseMO(UseMO), InsertIntoBB(InsertIntoBB), InsertBefore(InsertBefore) {
+    }
+  };
+
   // We match the loads and follow the uses to the extend instead of matching
   // the extends and following the def to the load. This is because the load
   // must remain in the same position for correctness (unless we also add code
@@ -193,7 +204,7 @@ bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
 
   // Rewrite all the uses to fix up the types.
   SmallVector<MachineInstr *, 1> ScheduleForErase;
-  SmallVector<std::pair<MachineOperand *, MachineInstr *>, 4> ScheduleForInsert;
+  SmallVector<InsertionPoint, 4> ScheduleForInsert;
   for (auto &UseMO : MRI.use_operands(LoadValue.getReg())) {
     MachineInstr *UseMI = UseMO.getParent();
 
@@ -243,8 +254,9 @@ bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
           //    ... = ... %3(s64)
           InsertInsnsWithoutSideEffectsBeforeUse(
               Builder, MI, UseMO,
-              [&](MachineBasicBlock::iterator InsertBefore) {
-                ScheduleForInsert.emplace_back(&UseMO, &*InsertBefore);
+              [&](MachineBasicBlock *InsertIntoBB,
+                  MachineBasicBlock::iterator InsertBefore) {
+                ScheduleForInsert.emplace_back(&UseMO, InsertIntoBB, InsertBefore);
               });
         }
         continue;
@@ -259,27 +271,29 @@ bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
     // The use isn't an extend. Truncate back to the type we originally loaded.
     // This is free on many targets.
     InsertInsnsWithoutSideEffectsBeforeUse(
-        Builder, MI, UseMO, [&](MachineBasicBlock::iterator InsertBefore) {
-          ScheduleForInsert.emplace_back(&UseMO, &*InsertBefore);
+        Builder, MI, UseMO,
+        [&](MachineBasicBlock *InsertIntoBB,
+            MachineBasicBlock::iterator InsertBefore) {
+          ScheduleForInsert.emplace_back(&UseMO, InsertIntoBB, InsertBefore);
         });
   }
 
   DenseMap<MachineBasicBlock *, MachineInstr *> EmittedInsns;
   for (auto &InsertionInfo : ScheduleForInsert) {
-    MachineOperand *UseMO = InsertionInfo.first;
-    MachineInstr *InsertBefore = InsertionInfo.second;
+    MachineOperand *UseMO = InsertionInfo.UseMO;
+    MachineBasicBlock *InsertIntoBB = InsertionInfo.InsertIntoBB;
+    MachineBasicBlock::iterator InsertBefore = InsertionInfo.InsertBefore;
 
-    MachineInstr *PreviouslyEmitted =
-        EmittedInsns.lookup(InsertBefore->getParent());
+    MachineInstr *PreviouslyEmitted = EmittedInsns.lookup(InsertIntoBB);
     if (PreviouslyEmitted) {
       UseMO->setReg(PreviouslyEmitted->getOperand(0).getReg());
       continue;
     }
 
-    Builder.setInsertPt(*InsertBefore->getParent(), InsertBefore);
+    Builder.setInsertPt(*InsertIntoBB, InsertBefore);
     unsigned NewDstReg = MRI.cloneVirtualRegister(MI.getOperand(0).getReg());
     MachineInstr *NewMI = Builder.buildTrunc(NewDstReg, ChosenDstReg);
-    EmittedInsns[InsertBefore->getParent()] = NewMI;
+    EmittedInsns[InsertIntoBB] = NewMI;
     UseMO->setReg(NewDstReg);
     Observer.createdInstr(*NewMI);
   }

test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-extending-loads-cornercases.mir

@@ -35,6 +35,19 @@
     ret void
   }
 
+  define void @sink_to_phi_emptyblock(i8* %addr) {
+  entry:
+    br i1 0, label %if, label %else
+  if:
+    br label %exit
+  else:
+    br label %else2
+  else2:
+    br label %exit
+  exit:
+    ret void
+  }
+
   define void @use_doesnt_def_anything(i8* %addr) {
   entry:
     ret void
@@ -44,7 +57,6 @@
   entry:
     ret void
   }
-
 ...
 
 ---
@@ -178,6 +190,52 @@ body: |
     $w1 = COPY %9
 ...
 
+---
+name:            sink_to_phi_emptyblock
+# CHECK-LABEL: name: sink_to_phi_emptyblock
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0, $w1
+    successors: %bb.1(0x40000000), %bb.2(0x40000000); %bb.1(50.00%), %bb.2(50.00%)
+    ; CHECK: [[T0:%[0-9]+]]:_(s32) = G_SEXTLOAD
+    %0:_(p0) = COPY $x0
+    %1:_(s32) = COPY $w1
+    %2:_(s8) = G_LOAD %0 :: (load 1 from %ir.addr)
+    %3:_(s32) = G_SEXT %2
+    %4:_(s32) = G_CONSTANT i32 1
+    %5:_(s1) = G_ICMP intpred(ne), %1:_(s32), %4:_
+    G_BRCOND %5:_(s1), %bb.1
+    G_BR %bb.2.else
+  bb.1.if:
+  ; CHECK: bb.1.if:
+    successors: %bb.4(0x80000000)
+    %10:_(s8) = G_CONSTANT i8 1
+    ; CHECK: [[T1:%[0-9]+]]:_(s8) = G_TRUNC [[T0]](s32)
+    %6:_(s8) = G_ADD %2, %10
+    ; CHECK: [[T2:%[0-9]+]]:_(s8) = G_ADD [[T1]], {{.*}}
+    G_BR %bb.4.exit
+  bb.2.else:
+  ; CHECK: bb.2.else:
+    successors: %bb.3(0x80000000)
+    G_BR %bb.3.else2
+  bb.3.else2:
+  ; CHECK: bb.3.else2:
+    successors: %bb.4(0x80000000)
+    ; CHECK: [[T4:%[0-9]+]]:_(s8) = G_TRUNC [[T0]](s32)
+    ; Fallthrough
+  bb.4.exit:
+  ; CHECK: bb.4.exit:
+    %8:_(s8) = G_PHI %6:_(s8), %bb.1, %2:_(s8), %bb.3
+    ; CHECK: [[T5:%[0-9]+]]:_(s8) = G_PHI [[T2]](s8), %bb.1, [[T4]](s8)
+    %9:_(s32) = G_ZEXT %8
+    ; CHECK: [[T6:%[0-9]+]]:_(s32) = G_ZEXT [[T5]](s8)
+    ; CHECK: $w0 = COPY [[T0]](s32)
+    ; CHECK: $w1 = COPY [[T6]](s32)
+    $w0 = COPY %3
+    $w1 = COPY %9
+...
+
 ---
 name:            use_doesnt_def_anything
 # CHECK-LABEL: name: use_doesnt_def_anything