[RISCV] Add bounds check before use on returned iterator.

[anmolparalkar-nxp]

Summary

Check iterator validity before use; fixes crash seen in the RISC-V
Zcmp Push/Pop optimization when compiling internal benchmark.

Includes test case (pre fix reproducer derived using bugpoint).

[wangpc]

LGTM.

[asb]

LGTM.

llvm/lib/Target/RISCV/RISCVPushPopOptimizer.cpp

bool RISCVPushPopOpt::runOnMachineFunction(MachineFunction &Fn) {
   // per function and then clear the register units each time we determine
   // correct return value for the POP.
   ModifiedRegUnits.init(*TRI);
   UsedRegUnits.init(*TRI);
   bool Modified = false;
   for (auto &MBB : Fn) {
     MachineBasicBlock::iterator MBBI = containsPop(MBB);
     MachineBasicBlock::iterator NextI = next_nodbg(MBBI, MBB.end());
-    if (MBBI != MBB.end() && NextI->getOpcode() == RISCV::PseudoRET)
+    if (MBBI != MBB.end() && NextI != MBB.end() &&
+        NextI->getOpcode() == RISCV::PseudoRET)
       Modified |= usePopRet(MBBI, NextI, adjustRetVal(MBBI));
   }
   return Modified;
 }
 
 /// createRISCVPushPopOptimizationPass - returns an instance of the
 /// Push/Pop optimization pass.
 FunctionPass *llvm::createRISCVPushPopOptimizationPass() {
   return new RISCVPushPopOpt();
 }

llvm/test/CodeGen/RISCV/push-pop-opt-crash.ll

+; RUN: llc -mattr=+zcmp -verify-machineinstrs  \
+; RUN: -mtriple=riscv32 -target-abi ilp32 < %s \
+; RUN: | FileCheck %s -check-prefixes=RV32IZCMP
+; RUN: llc -mattr=+zcmp -verify-machineinstrs  \
+; RUN: -mtriple=riscv64 -target-abi ilp64 < %s \
+; RUN: | FileCheck %s -check-prefixes=RV64IZCMP
+
+; This source code exposed a crash in the RISC-V Zcmp Push/Pop optimization
+; pass. The root cause was: Not doing a bounds check before using a returned
+; iterator.
+
+declare dso_local void @f1() local_unnamed_addr
+declare dso_local void @f2() local_unnamed_addr
+define  dso_local void @f0() local_unnamed_addr {
+; RV32IZCMP-LABEL: f0:
+; RV32IZCMP:  .cfi_startproc
+; RV32IZCMP-NEXT: # %bb.0:                                # %entry
+; RV32IZCMP-NEXT:   bnez  zero, .LBB0_2
+; RV32IZCMP-NEXT: # %bb.1:                                # %if.T
+; RV32IZCMP-NEXT:   cm.push {ra}, -16
+; RV32IZCMP-NEXT:   .cfi_def_cfa_offset 16
+; RV32IZCMP-NEXT:   .cfi_offset ra, -4
+; RV32IZCMP-NEXT:   call  f1
+; RV32IZCMP-NEXT:   cm.pop  {ra}, 16
+; RV32IZCMP-NEXT: .LBB0_2:                                # %if.F
+; RV32IZCMP-NEXT:   tail  f2
+; RV32IZCMP-NEXT: .Lfunc_end0:
+
+; RV64IZCMP-LABEL: f0:
+; RV64IZCMP:  .cfi_startproc
+; RV64IZCMP-NEXT: # %bb.0:                                # %entry
+; RV64IZCMP-NEXT:   bnez  zero, .LBB0_2
+; RV64IZCMP-NEXT: # %bb.1:                                # %if.T
+; RV64IZCMP-NEXT:   cm.push {ra}, -16
+; RV64IZCMP-NEXT:   .cfi_def_cfa_offset 16
+; RV64IZCMP-NEXT:   .cfi_offset ra, -8
+; RV64IZCMP-NEXT:   call  f1
+; RV64IZCMP-NEXT:   cm.pop  {ra}, 16
+; RV64IZCMP-NEXT: .LBB0_2:                                # %if.F
+; RV64IZCMP-NEXT:   tail  f2
+; RV64IZCMP-NEXT: .Lfunc_end0:
+entry:
+  br i1 poison, label %if.T, label %if.F
+
+if.T:
+  tail call void @f1()
+  br label %if.F
+
+if.F:
+  tail call void @f2()
+  ret void
+}