RegisterCoalescer: Add undef flags in removePartialRedundancy

[arsenm]

If the copy being hoisted was undef, we have the same problems that eliminateUndefCopy needs to solve. We would effectively be introducing a new live out implicit_def. We need to add an undef flag to avoid artificially introducing a live through undef value. Previously, the verifier would fail due to the dead def inside the loop providing the live in value for the %1 use.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-regalloc

Author: Matt Arsenault (arsenm)

Changes

If the copy being hoisted was undef, we have the same problems that eliminateUndefCopy needs to solve. We would effectively be introducing a new live out implicit_def. We need to add an undef flag to avoid artificially introducing a live through undef value. Previously, the verifier would fail due to the dead def inside the loop providing the live in value for the %1 use.

---

Full diff: https://github.com/llvm/llvm-project/pull/75152.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/RegisterCoalescer.cpp (+13)
• (added) llvm/test/CodeGen/X86/coalescer-partial-redundancy-clear-dead-flag-undef-copy.mir (+47)

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index c067d87a9fd810..1b6b94c27fc614 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1214,6 +1214,19 @@ bool RegisterCoalescer::removePartialRedundancy(const CoalescerPair &CP,
   LIS->pruneValue(*static_cast<LiveRange *>(&IntB), CopyIdx.getRegSlot(),
                   &EndPoints);
   BValNo->markUnused();
+
+  if (CopyMI.getOperand(1).isUndef()) {
+    // We're introducing an undef phi def, and need to set undef on any users of
+    // the previously local def to avoid artifically extending the lifetime
+    // through the block.
+    for (MachineOperand &MO : MRI->use_nodbg_operands(IntB.reg())) {
+      const MachineInstr &MI = *MO.getParent();
+      SlotIndex UseIdx = LIS->getInstructionIndex(MI);
+      if (!IntB.liveAt(UseIdx))
+        MO.setIsUndef(true);
+    }
+  }
+
   // Extend IntB to the EndPoints of its original live interval.
   LIS->extendToIndices(IntB, EndPoints);
 
diff --git a/llvm/test/CodeGen/X86/coalescer-partial-redundancy-clear-dead-flag-undef-copy.mir b/llvm/test/CodeGen/X86/coalescer-partial-redundancy-clear-dead-flag-undef-copy.mir
new file mode 100644
index 00000000000000..5f33be0bc15559
--- /dev/null
+++ b/llvm/test/CodeGen/X86/coalescer-partial-redundancy-clear-dead-flag-undef-copy.mir
@@ -0,0 +1,47 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 4
+# RUN: llc -mtriple=x86_64-pc-linux-gnu -run-pass=register-coalescer -verify-coalescing -o - %s | FileCheck %s
+
+# Check for "Live range continues after dead def flag".
+
+# There are 2 copies of undef, but the registers also appear to be
+# live due to block live outs, and thus were not deleted as
+# eliminateUndefCopy only considered the live range, and not the undef
+# flag.
+#
+# removePartialRedundancy would move the COPY undef %0 in bb.1 to
+# bb.0.  The live range of %1 would then be extended to be live out of
+# %bb.1 for the backedge phi. This would then fail the verifier, since
+# the dead flag was no longer valid. This was fixed by directly
+# considering the undef flag to avoid considering this special case.
+
+---
+name: partial_redundancy_coalesce_undef_copy_live_out
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: partial_redundancy_coalesce_undef_copy_live_out
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $rdi
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gr32 = COPY $rdi
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   dead [[XOR32ri:%[0-9]+]]:gr32 = XOR32ri undef [[XOR32ri]], 1, implicit-def dead $eflags
+  ; CHECK-NEXT:   dead [[MOV32rr:%[0-9]+]]:gr32 = MOV32rr [[COPY]]
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gr32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  bb.0:
+    liveins: $rdi
+
+    %0:gr32 = COPY $rdi
+
+  bb.1:
+    %1:gr32 = COPY undef %0
+    dead %1:gr32 = XOR32ri %1, 1, implicit-def dead $eflags
+    dead %2:gr32 = MOV32rr killed %0
+    %0:gr32 = COPY killed undef %1
+    JMP_1 %bb.1
+
+...