[ImplicitNullChecks] Work with implicit defs.

Summary:
This change generalizes the implicit null checks pass to work with
instructions that don't have any explicit register defs.  This lets us
use X86's `cmp` against memory as faulting load instructions.

Reviewers: reames, JosephTremoulet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11286

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242703 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ImplicitNullChecks.cpp

@@ -238,7 +238,7 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
     unsigned BaseReg, Offset;
     if (TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI))
       if (MI->mayLoad() && !MI->isPredicable() && BaseReg == PointerReg &&
-          Offset < PageSize && MI->getDesc().getNumDefs() == 1 &&
+          Offset < PageSize && MI->getDesc().getNumDefs() <= 1 &&
           IsSafeToHoist(MI)) {
         NullCheckList.emplace_back(MI, MBP.ConditionDef, &MBB, NotNullSucc,
                                    NullSucc);
@@ -281,14 +281,19 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
 MachineInstr *ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
                                                      MachineBasicBlock *MBB,
                                                      MCSymbol *HandlerLabel) {
+  const unsigned NoRegister = 0; // Guaranteed to be the NoRegister value for
+                                 // all targets.
+
   DebugLoc DL;
   unsigned NumDefs = LoadMI->getDesc().getNumDefs();
-  assert(NumDefs == 1 && "other cases unhandled!");
+  assert(NumDefs <= 1 && "other cases unhandled!");
-  (void)NumDefs;
 
-  unsigned DefReg = LoadMI->defs().begin()->getReg();
+  unsigned DefReg = NoRegister;
-  assert(std::distance(LoadMI->defs().begin(), LoadMI->defs().end()) == 1 &&
+  if (NumDefs != 0) {
-         "expected exactly one def!");
+    DefReg = LoadMI->defs().begin()->getReg();
+    assert(std::distance(LoadMI->defs().begin(), LoadMI->defs().end()) == 1 &&
+           "expected exactly one def!");
+  }
 
   auto MIB = BuildMI(MBB, DL, TII->get(TargetOpcode::FAULTING_LOAD_OP), DefReg)
                  .addSym(HandlerLabel)

lib/Target/X86/X86MCInstLower.cpp

@@ -875,7 +875,10 @@ void X86AsmPrinter::LowerFAULTING_LOAD_OP(const MachineInstr &MI,
 
   MCInst LoadMI;
   LoadMI.setOpcode(LoadOpcode);
-  LoadMI.addOperand(MCOperand::createReg(LoadDefRegister));
+
+  if (LoadDefRegister != X86::NoRegister)
+    LoadMI.addOperand(MCOperand::createReg(LoadDefRegister));
+
   for (auto I = MI.operands_begin() + LoadOperandsBeginIdx,
             E = MI.operands_end();
        I != E; ++I)

test/CodeGen/X86/implicit-null-check.ll

@@ -101,6 +101,40 @@ define i32 @imp_null_check_hoist_over_unrelated_load(i32* %x, i32* %y, i32* %z)
   ret i32 %t1
 }
 
+define i32 @imp_null_check_via_mem_comparision(i32* %x, i32 %val) {
+; CHECK-LABEL: _imp_null_check_via_mem_comparision
+; CHECK: Ltmp9:
+; CHECK: cmpl   %esi, 4(%rdi)
+; CHECK: jge    LBB4_2
+; CHECK: movl   $100, %eax
+; CHECK: retq
+; CHECK: Ltmp8:
+; CHECK: movl   $42, %eax
+; CHECK: retq
+; CHECK: LBB4_2:
+; CHECK: movl   $200, %eax
+; CHECK: retq
+
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %x.loc = getelementptr i32, i32* %x, i32 1
+  %t = load i32, i32* %x.loc
+  %m = icmp slt i32 %t, %val
+  br i1 %m, label %ret_100, label %ret_200
+
+ ret_100:
+  ret i32 100
+
+ ret_200:
+  ret i32 200
+}
+
 !0 = !{}
 
 ; CHECK-LABEL: __LLVM_FaultMaps:
@@ -113,7 +147,7 @@ define i32 @imp_null_check_hoist_over_unrelated_load(i32* %x, i32* %y, i32* %z)
 ; CHECK-NEXT: .short 0
 
 ; # functions:
-; CHECK-NEXT: .long 4
+; CHECK-NEXT: .long 5
 
 ; FunctionAddr:
 ; CHECK-NEXT: .quad _imp_null_check_add_result
@@ -167,9 +201,22 @@ define i32 @imp_null_check_hoist_over_unrelated_load(i32* %x, i32* %y, i32* %z)
 ; Fault[0].HandlerOffset:
 ; CHECK-NEXT: .long Ltmp0-_imp_null_check_load
 
+; FunctionAddr:
+; CHECK-NEXT: .quad     _imp_null_check_via_mem_comparision
+; NumFaultingPCs
+; CHECK-NEXT: .long   1
+; Reserved:
+; CHECK-NEXT: .long   0
+; Fault[0].Type:
+; CHECK-NEXT: .long   1
+; Fault[0].FaultOffset:
+; CHECK-NEXT: .long   Ltmp9-_imp_null_check_via_mem_comparision
+; Fault[0].HandlerOffset:
+; CHECK-NEXT: .long   Ltmp8-_imp_null_check_via_mem_comparision
+
 ; OBJDUMP: FaultMap table:
 ; OBJDUMP-NEXT: Version: 0x1
-; OBJDUMP-NEXT: NumFunctions: 4
+; OBJDUMP-NEXT: NumFunctions: 5
 ; OBJDUMP-NEXT: FunctionAddress: 0x000000, NumFaultingPCs: 1
 ; OBJDUMP-NEXT: Fault kind: FaultingLoad, faulting PC offset: 0, handling PC offset: 5
 ; OBJDUMP-NEXT: FunctionAddress: 0x000000, NumFaultingPCs: 1