[X86] Model FAULTING_LOAD_OP as a terminator and branch.

This operation may branch to the handler block and we do not want it
to happen anywhere within the basic block.
Moreover, by marking it "terminator and branch" the machine verifier
does not wrongly assume (because of AnalyzeBranch not knowing better)
the branch is analyzable. Indeed, the target was seeing only the
unconditional branch and not the faulting load op and thought it was
a simple unconditional block.
The machine verifier was complaining because of that and moreover,
other optimizations could have done wrong transformation!

In the process, simplify the representation of the handler block in
the faulting load op. Now, we directly reference the handler block
instead of using a label. This has the benefits of:
1. MC knows how to issue a label for a BB, so leave that to it.
2. Accessing the target BB from its label is painful, whereas it is
   direct from a MBB operand.

Note: The 2 bytes offset in implicit-null-check.ll comes from the
fact the unconditional jumps are not removed anymore, as the whole
terminator sequence is not analyzable anymore.

Will fix it in a subsequence commit.

llvm-svn: 268327

1 files changed, 8 insertions, 13 deletions

diff --git a/llvm/lib/CodeGen/ImplicitNullChecks.cpp b/llvm/lib/CodeGen/ImplicitNullChecks.cpp
index c923ddd9471..dfd1be8c8ea 100644
--- a/llvm/lib/CodeGen/ImplicitNullChecks.cpp
+++ b/llvm/lib/CodeGen/ImplicitNullChecks.cpp
@@ -95,7 +95,7 @@ class ImplicitNullChecks : public MachineFunctionPass {
   bool analyzeBlockForNullChecks(MachineBasicBlock &MBB,
                                  SmallVectorImpl<NullCheck> &NullCheckList);
   MachineInstr *insertFaultingLoad(MachineInstr *LoadMI, MachineBasicBlock *MBB,
-                                   MCSymbol *HandlerLabel);
+                                   MachineBasicBlock *HandlerMBB);
   void rewriteNullChecks(ArrayRef<NullCheck> NullCheckList);
 
 public:
@@ -349,11 +349,12 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
 
 /// Wrap a machine load instruction, LoadMI, into a FAULTING_LOAD_OP machine
 /// instruction.  The FAULTING_LOAD_OP instruction does the same load as LoadMI
-/// (defining the same register), and branches to HandlerLabel if the load
+/// (defining the same register), and branches to HandlerMBB if the load
 /// faults.  The FAULTING_LOAD_OP instruction is inserted at the end of MBB.
-MachineInstr *ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
-                                                     MachineBasicBlock *MBB,
-                                                     MCSymbol *HandlerLabel) {
+MachineInstr *
+ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
+                                       MachineBasicBlock *MBB,
+                                       MachineBasicBlock *HandlerMBB) {
   const unsigned NoRegister = 0; // Guaranteed to be the NoRegister value for
                                  // all targets.
 
@@ -369,7 +370,7 @@ MachineInstr *ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
   }
 
   auto MIB = BuildMI(MBB, DL, TII->get(TargetOpcode::FAULTING_LOAD_OP), DefReg)
-                 .addSym(HandlerLabel)
+                 .addMBB(HandlerMBB)
                  .addImm(LoadMI->getOpcode());
 
   for (auto &MO : LoadMI->uses())
@@ -386,8 +387,6 @@ void ImplicitNullChecks::rewriteNullChecks(
   DebugLoc DL;
 
   for (auto &NC : NullCheckList) {
-    MCSymbol *HandlerLabel = MMI->getContext().createTempSymbol();
-
     // Remove the conditional branch dependent on the null check.
     unsigned BranchesRemoved = TII->RemoveBranch(*NC.CheckBlock);
     (void)BranchesRemoved;
@@ -398,7 +397,7 @@ void ImplicitNullChecks::rewriteNullChecks(
     // touch the successors list for any basic block since we haven't changed
     // control flow, we've just made it implicit.
     MachineInstr *FaultingLoad =
-        insertFaultingLoad(NC.MemOperation, NC.CheckBlock, HandlerLabel);
+        insertFaultingLoad(NC.MemOperation, NC.CheckBlock, NC.NullSucc);
     // Now the value of the MemOperation, if any, is live-in of block
     // of MemOperation.
     unsigned Reg = FaultingLoad->getOperand(0).getReg();
@@ -414,10 +413,6 @@ void ImplicitNullChecks::rewriteNullChecks(
     TII->InsertBranch(*NC.CheckBlock, NC.NotNullSucc, nullptr, /*Cond=*/None,
                       DL);
 
-    // Emit the HandlerLabel as an EH_LABEL.
-    BuildMI(*NC.NullSucc, NC.NullSucc->begin(), DL,
-            TII->get(TargetOpcode::EH_LABEL)).addSym(HandlerLabel);
-
     NumImplicitNullChecks++;
   }
 }