[GlobalISel][DebugInfo] Salvage trivially dead instructions

[dzhidzhoev]

Summary

Use salvageDebugInfo for instructions erased as trivially dead in
GlobalISel.

It would be helpful to implement support of G_PTR_ADD and G_FRAME_INDEX
in salvageDebugInfo in future in order to preserve more variable
location.

Statistics of llvm object files built with vs without this commit on -O2 optimization level (CMAKE_BUILD_TYPE=RelWithDebInfo, -fglobal-isel) on Aarch64
(macOS):

Number of variables covered by location statistics:

• test-suite GISel - 1074837
• test-suite Patch - 1109099 (+3.18%)
• llvm GISel - 11914074
• llvm Patch - 12356266 (+3.71%)

Number of variables with 100% coverage of parent scope:

• test-suite GISel - 361477
• test-suite Patch - 393043 (+8.73%)
• llvm GISel - 4575700
• llvm GISel+patch - 4858319 (+6.17%)

[arsenm]

LGTM

llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp

#endif
         --MII;
 
       LLVM_DEBUG(dbgs() << "Selecting: \n  " << MI);
 
       // We could have folded this instruction away already, making it dead.
       // If so, erase it.
       if (isTriviallyDead(MI, MRI)) {
         LLVM_DEBUG(dbgs() << "Is dead; erasing.\n");
+        salvageDebugInfo(MRI, MI);
         MI.eraseFromParent();
         continue;
       }
 
       // Eliminate hints.
       if (isPreISelGenericOptimizationHint(MI.getOpcode())) {
         Register DstReg = MI.getOperand(0).getReg();
         Register SrcReg = MI.getOperand(1).getReg();

llvm/lib/CodeGen/GlobalISel/Legalizer.cpp

do {
     LLVM_DEBUG(dbgs() << "=== New Iteration ===\n");
     assert(RetryList.empty() && "Expected no instructions in RetryList");
     unsigned NumArtifacts = ArtifactList.size();
     while (!InstList.empty()) {
       MachineInstr &MI = *InstList.pop_back_val();
       assert(isPreISelGenericOpcode(MI.getOpcode()) &&
              "Expecting generic opcode");
       if (isTriviallyDead(MI, MRI)) {
+        salvageDebugInfo(MRI, MI);
         eraseInstr(MI, MRI, &LocObserver);
         continue;
       }
 
       // Do the legalization for this instruction.
       auto Res = Helper.legalizeInstrStep(MI, LocObserver);
       // Error out if we couldn't legalize this instruction. We may want to
       // fall back to DAG ISel instead in the future.
if (!RetryList.empty()) {
       }
     }
     LocObserver.checkpoint();
     while (!ArtifactList.empty()) {
       MachineInstr &MI = *ArtifactList.pop_back_val();
       assert(isPreISelGenericOpcode(MI.getOpcode()) &&
              "Expecting generic opcode");
       if (isTriviallyDead(MI, MRI)) {
+        salvageDebugInfo(MRI, MI);
         eraseInstr(MI, MRI, &LocObserver);
         continue;
       }
       SmallVector<MachineInstr *, 4> DeadInstructions;
       LLVM_DEBUG(dbgs() << "Trying to combine: " << MI);
       if (ArtCombiner.tryCombineInstruction(MI, DeadInstructions,
                                             WrapperObserver)) {
         WorkListObserver.printNewInstrs();

llvm/test/CodeGen/AArch64/GlobalISel/select-dbg-value.mir

 legalized:       true
 regBankSelected: true
 body: |
   bb.0:
     liveins: $w0
     ; CHECK-LABEL: name: test_dbg_value_dead
     ; CHECK: liveins: $w0
     ; CHECK-NEXT: {{  $}}
-    ; CHECK-NEXT: DBG_VALUE %0:gpr32, $noreg, !7, !DIExpression(), debug-location !9
+    ; CHECK-NEXT: DBG_VALUE %1:gpr64, $noreg, !7, !DIExpression(), debug-location !9
     %0:gpr(s32) = COPY $w0
-    DBG_VALUE %0(s32), $noreg, !7, !DIExpression(), debug-location !9
+    %1:gpr(s64) = G_ZEXT %0:gpr(s32)
+    DBG_VALUE %1(s64), $noreg, !7, !DIExpression(), debug-location !9
 ...