[mips] Deal with empty blocks in the mips hazard scheduler

This patch teaches the hazard scheduler how to handle empty blocks
when search for the next real instruction when dealing with forbidden
slots.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D31293

llvm-svn: 299427

2 files changed, 106 insertions, 11 deletions

diff --git a/llvm/lib/Target/Mips/MipsHazardSchedule.cpp b/llvm/lib/Target/Mips/MipsHazardSchedule.cpp
index 87446e38aca..f6fcf6ec938 100644
--- a/llvm/lib/Target/Mips/MipsHazardSchedule.cpp
+++ b/llvm/lib/Target/Mips/MipsHazardSchedule.cpp
@@ -103,23 +103,24 @@ static Iter getNextMachineInstrInBB(Iter Position) {
 
 // Find the next real instruction from the current position, looking through
 // basic block boundaries.
-static Iter getNextMachineInstr(Iter Position, MachineBasicBlock *Parent) {
+static std::pair<Iter, bool> getNextMachineInstr(Iter Position, MachineBasicBlock * Parent) {
   if (Position == Parent->end()) {
-    MachineBasicBlock *Succ = Parent->getNextNode();
-    if (Succ != nullptr && Parent->isSuccessor(Succ)) {
-      Position = Succ->begin();
-      Parent = Succ;
-    } else {
-      llvm_unreachable(
-          "Should have identified the end of the function earlier!");
-    }
+    do {
+      MachineBasicBlock *Succ = Parent->getNextNode();
+      if (Succ != nullptr && Parent->isSuccessor(Succ)) {
+        Position = Succ->begin();
+        Parent = Succ;
+      } else {
+        return std::make_pair(Position, true);
+      }
+    } while (Parent->empty());
   }
 
   Iter Instr = getNextMachineInstrInBB(Position);
   if (Instr == Parent->end()) {
     return getNextMachineInstr(Instr, Parent);
   }
-  return Instr;
+  return std::make_pair(Instr, false);
 }
 
 bool MipsHazardSchedule::runOnMachineFunction(MachineFunction &MF) {
@@ -145,7 +146,9 @@ bool MipsHazardSchedule::runOnMachineFunction(MachineFunction &MF) {
       bool LastInstInFunction =
           std::next(I) == FI->end() && std::next(FI) == MF.end();
       if (!LastInstInFunction) {
-        Inst = getNextMachineInstr(std::next(I), &*FI);
+        std::pair<Iter, bool> Res = getNextMachineInstr(std::next(I), &*FI);
+        LastInstInFunction |= Res.second;
+        Inst = Res.first;
       }
 
       if (LastInstInFunction || !TII->SafeInForbiddenSlot(*Inst)) {
diff --git a/llvm/test/CodeGen/Mips/compactbranches/empty-block.mir b/llvm/test/CodeGen/Mips/compactbranches/empty-block.mir
new file mode 100644
index 00000000000..7831e51e315
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/compactbranches/empty-block.mir
@@ -0,0 +1,92 @@
+# RUN: llc -march=mipsel -mcpu=mips32r6 -start-after=block-placement %s -o - | FileCheck %s
+
+# Check that empty blocks in the cfg don't cause the mips hazard scheduler to
+# crash and that the nop is inserted correctly.
+
+# CHECK:  blezc
+# CHECK:  nop
+# CHECK: # BB#1:
+# CHECK:  .insn
+# CHECK: # BB#2:
+# CHECK:  .insn
+# CHECK: # BB#3:
+# CHECK:  jal
+
+--- |
+  ; ModuleID = '<stdin>'
+  source_filename = "<stdin>"
+  target datalayout = "e-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64"
+
+  declare i32 @k()
+
+  declare void @f(i32)
+
+  define void @l5() {
+  entry:
+    %call = tail call i32 @k()
+    %cmp = icmp sgt i32 %call, 0
+    br i1 %cmp, label %if.then, label %if.end
+
+  if.then:                                          ; preds = %entry
+    tail call void @f(i32 signext 2)
+    br label %if.end
+
+  if.end:                                           ; preds = %if.then, %entry
+    ret void
+  }
+
+---
+name:            l5
+alignment:       2
+exposesReturnsTwice: false
+noVRegs:         true
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       24
+  offsetAdjustment: 0
+  maxAlignment:    4
+  adjustsStack:    true
+  hasCalls:        true
+  maxCallFrameSize: 16
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+stack:
+  - { id: 0, type: spill-slot, offset: -4, size: 4, alignment: 4, callee-saved-register: '%ra' }
+body:             |
+  bb.0.entry:
+    successors: %bb.1.if.then(0x50000000), %bb.4.if.end(0x30000000)
+    liveins: %ra
+
+    %sp = ADDiu %sp, -24
+    CFI_INSTRUCTION def_cfa_offset 24
+    SW killed %ra, %sp, 20 :: (store 4 into %stack.0)
+    CFI_INSTRUCTION offset %ra_64, -4
+    JAL @k, csr_o32_fp64, implicit-def dead %ra, implicit-def %sp, implicit-def %v0
+    BLEZ %v0, %bb.4.if.end, implicit-def %at
+
+  bb.1.if.then:
+    successors: %bb.2.if.then(0x80000000)
+
+  bb.2.if.then:
+    successors: %bb.3.if.then(0x80000000)
+
+  bb.3.if.then:
+    successors: %bb.4.if.end(0x80000000)
+
+    %a0 = ADDiu %zero, 2
+    JAL @f, csr_o32_fp64, implicit-def dead %ra, implicit killed %a0, implicit-def %sp
+
+  bb.4.if.end:
+    %ra = LW %sp, 20 :: (load 4 from %stack.0)
+    %sp = ADDiu %sp, 24
+    PseudoReturn undef %ra
+
+...