[PATCH] D111201: [GlobalISel] Clear unreachable blocks' contents after selection.

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.
aemerson added reviewers: uabelho, arsenm, foad, paquette, qcolombet.
aemerson added a project: LLVM.
Herald added subscribers: hiraditya, rovka.
aemerson requested review of this revision.
Herald added a subscriber: wdng.

If these blocks are unreachable, then we can discard all of the instructions. However, keep the block around because it may have an address taken or the block may have a stale reference from a PHI somewhere. Instead of finding those PHIs and fixing them up, just leave the block empty.

This was motivated by https://reviews.llvm.org/D110603 which ran into this problem.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D111201

Files:
  llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/select-unreachable-blocks.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/select-unreachable-blocks.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/select-unreachable-blocks.mir
@@ -0,0 +1,61 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64-unknown-unknown -o - -verify-machineinstrs -run-pass=instruction-select %s | FileCheck %s
+---
+name:            test_unreachable_delete
+alignment:       4
+exposesReturnsTwice: false
+legalized:       true
+regBankSelected: true
+selected:        false
+tracksRegLiveness: true
+liveins:
+body:             |
+  ; CHECK-LABEL: name: test_unreachable_delete
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT:   liveins: $w0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gpr32sp = COPY $w0
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:gpr32 = COPY $wzr
+  ; CHECK-NEXT:   [[MOVi32imm:%[0-9]+]]:gpr32 = MOVi32imm 1
+  ; CHECK-NEXT:   B %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[ADDWri:%[0-9]+]]:gpr32sp = ADDWri [[COPY]], 2, 0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   [[PHI:%[0-9]+]]:gpr32 = PHI [[ADDWri]], %bb.2
+  ; CHECK-NEXT:   $w0 = COPY [[PHI]]
+  ; CHECK-NEXT:   RET_ReallyLR implicit $w0
+  bb.1:
+    successors: %bb.3
+    liveins: $w0
+
+    %0:gpr(s32) = COPY $w0
+    %1:gpr(s32) = G_CONSTANT i32 0
+    %3:gpr(s32) = G_CONSTANT i32 1
+    %5:gpr(s32) = G_CONSTANT i32 2
+    G_BR %bb.3
+
+  bb.5:
+    ; This block is unreachable.
+    %unreachable_inst:gpr(s32) = G_XOR %1, %3
+    $w0 = COPY %unreachable_inst(s32)
+
+  bb.3:
+    successors: %bb.4(0x80000000)
+
+    %6:gpr(s32) = G_ADD %0, %5
+
+  bb.4:
+    %7:gpr(s32) = G_PHI %6(s32), %bb.3
+    $w0 = COPY %7(s32)
+    RET_ReallyLR implicit $w0
+
+...
Index: llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
+++ llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
@@ -130,9 +130,12 @@
   // Until then, keep track of the number of blocks to assert that we don't.
   const size_t NumBlocks = MF.size();
 #endif
+  // Keep track of selected blocks, so we can delete unreachable ones later.
+  DenseSet<MachineBasicBlock *> SelectedBlocks;
 
   for (MachineBasicBlock *MBB : post_order(&MF)) {
     ISel->CurMBB = MBB;
+    SelectedBlocks.insert(MBB);
     if (MBB->empty())
       continue;
 
@@ -205,6 +208,15 @@
     if (MBB.empty())
       continue;
 
+    if (!SelectedBlocks.contains(&MBB)) {
+      // This is an unreachable block and therefore hasn't been selected, since
+      // the main selection loop above uses a postorder block traversal.
+      // We delete all the instructions in this block since it's unreachable.
+      MBB.clear();
+      // Don't delete the block in case the block has it's address taken or is
+      // still being referenced by a phi somewhere.
+      continue;
+    }
     // Try to find redundant copies b/w vregs of the same register class.
     bool ReachedBegin = false;
     for (auto MII = std::prev(MBB.end()), Begin = MBB.begin(); !ReachedBegin;) {


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