[AMDGPU] Fix end() iterator dereference in SILowerSGPRSpills (#88828)

Author: bcahoon

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

@@ -278,7 +278,8 @@ void SILowerSGPRSpills::extendWWMVirtRegLiveness(MachineFunction &MF,
   for (auto Reg : MFI->getSGPRSpillVGPRs()) {
     for (MachineBasicBlock *SaveBlock : SaveBlocks) {
       MachineBasicBlock::iterator InsertBefore = SaveBlock->begin();
-      auto MIB = BuildMI(*SaveBlock, *InsertBefore, InsertBefore->getDebugLoc(),
+      DebugLoc DL = SaveBlock->findDebugLoc(InsertBefore);
+      auto MIB = BuildMI(*SaveBlock, InsertBefore, DL,
                          TII->get(AMDGPU::IMPLICIT_DEF), Reg);
       MFI->setFlag(Reg, AMDGPU::VirtRegFlag::WWM_REG);
       // Set SGPR_SPILL asm printer flag
@@ -294,10 +295,10 @@ void SILowerSGPRSpills::extendWWMVirtRegLiveness(MachineFunction &MF,
   for (MachineBasicBlock *RestoreBlock : RestoreBlocks) {
     MachineBasicBlock::iterator InsertBefore =
         RestoreBlock->getFirstTerminator();
+    DebugLoc DL = RestoreBlock->findDebugLoc(InsertBefore);
     for (auto Reg : MFI->getSGPRSpillVGPRs()) {
-      auto MIB =
-          BuildMI(*RestoreBlock, *InsertBefore, InsertBefore->getDebugLoc(),
-                  TII->get(TargetOpcode::KILL));
+      auto MIB = BuildMI(*RestoreBlock, InsertBefore, DL,
+                         TII->get(TargetOpcode::KILL));
       MIB.addReg(Reg);
       if (LIS)
         LIS->InsertMachineInstrInMaps(*MIB);

llvm/test/CodeGen/AMDGPU/sgpr-spills-empty-prolog-block.mir

@@ -0,0 +1,33 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 4
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -verify-machineinstrs -run-pass=si-lower-sgpr-spills -o - %s | FileCheck %s
+
+# Test that the si-lower-sgpr-spills pass does not defeference a
+# machine instruction iterator that is equal to end().
+
+---
+name:            empty_prolog_block
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: 0, size: 32, alignment: 4,
+      stack-id: sgpr-spill, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+machineFunctionInfo:
+  hasSpilledSGPRs: true
+body:             |
+  ; CHECK-LABEL: name: empty_prolog_block
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $sgpr0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:vgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   $sgpr0 = SI_RESTORE_S32_FROM_VGPR [[DEF]], 0
+  ; CHECK-NEXT:   KILL [[DEF]]
+  ; CHECK-NEXT:   S_ENDPGM 0
+  bb.0:
+    liveins: $sgpr0
+  bb.1:
+    renamable $sgpr0 = SI_SPILL_S32_RESTORE %stack.0, implicit $exec, implicit $sgpr32
+    S_ENDPGM 0
+
+...