[llvm] [CodeGen] Check physical def kill flag in MachineInstr::isDead (PR #168684)

[Luke Lau via llvm-commits]

https://github.com/lukel97 created https://github.com/llvm/llvm-project/pull/168684

Currently when checking if a MachineInstr is dead we use LiveRegUnits to check if any physical defs are alive. However if the kill/dead flag is set on the def, then we don't need to rely on LiveRegUnits check.

This should save some cycles on the hot path and allows users of isDead which don't pass in LiveRegUnits to catch more dead instructions.




>From 74c8f1b3f523c13d24bfbce403daff906fba84a1 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Wed, 19 Nov 2025 16:26:48 +0800
Subject: [PATCH 1/2] Precommit tests

---
 llvm/test/CodeGen/RISCV/machine-sink.mir | 86 ++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 llvm/test/CodeGen/RISCV/machine-sink.mir

diff --git a/llvm/test/CodeGen/RISCV/machine-sink.mir b/llvm/test/CodeGen/RISCV/machine-sink.mir
new file mode 100644
index 0000000000000..5c7ba672f6ca7
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/machine-sink.mir
@@ -0,0 +1,86 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=riscv64 -run-pass machine-sink -sink-insts-to-avoid-spills -o - %s | FileCheck %s
+---
+name: no_delete_live_phys_def
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: no_delete_live_phys_def
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $x8
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:gpr = COPY $x8
+  ; CHECK-NEXT:   %y:gpr = COPY %x, implicit-def $x8
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %z:gpr = ADDI %y, 1
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  bb.0:
+    liveins: $x8
+    %x:gpr = COPY $x8
+    %y:gpr = COPY %x, implicit-def $x8
+    PseudoBR %bb.1
+  bb.1:
+    %z:gpr = ADDI %y, 1
+    PseudoBR %bb.1
+...
+---
+name: delete_dead_phys_def
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: delete_dead_phys_def
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $x8
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:gpr = COPY $x8
+  ; CHECK-NEXT:   %y:gpr = COPY %x, implicit-def dead $x8
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gpr = COPY %x, implicit-def dead $x8
+  ; CHECK-NEXT:   %z:gpr = ADDI [[COPY]], 1
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  bb.0:
+    liveins: $x8
+    %x:gpr = COPY $x8
+    %y:gpr = COPY %x, implicit-def dead $x8
+    PseudoBR %bb.1
+  bb.1:
+    %z:gpr = ADDI %y, 1
+    PseudoBR %bb.1
+...
+---
+name: reserved_phys_def
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: reserved_phys_def
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $x8
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:gpr = COPY $x8
+  ; CHECK-NEXT:   %y:gpr = COPY %x, implicit-def dead $x2
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gpr = COPY %x, implicit-def dead $x2
+  ; CHECK-NEXT:   %z:gpr = ADDI [[COPY]], 1
+  ; CHECK-NEXT:   PseudoBR %bb.1
+  bb.0:
+    liveins: $x8
+    %x:gpr = COPY $x8
+    %y:gpr = COPY %x, implicit-def dead $x2
+    PseudoBR %bb.1
+
+  bb.1:
+    %z:gpr = ADDI %y, 1
+    PseudoBR %bb.1
+...

>From 215423a567a91967a057e2c08ff6884a99c0f2b7 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Wed, 19 Nov 2025 16:27:11 +0800
Subject: [PATCH 2/2] [CodeGen] Check physical def kill flag in
 MachineInstr::isDead

Currently when checking if a MachineInstr is dead we use LiveRegUnits to check if any physical defs are alive. However if the kill/dead flag is set on the def, then we don't need to rely on LiveRegUnits check.

This should save some cycles on the hot path and allows users of isDead which don't pass in LiveRegUnits to catch more dead instructions.
---
 llvm/lib/CodeGen/MachineInstr.cpp        | 3 ++-
 llvm/test/CodeGen/RISCV/machine-sink.mir | 1 -
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index eb46124d9eb5f..d6790714b9660 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -1398,7 +1398,8 @@ bool MachineInstr::isDead(const MachineRegisterInfo &MRI,
     Register Reg = MO.getReg();
     if (Reg.isPhysical()) {
       // Don't delete live physreg defs, or any reserved register defs.
-      if (!LivePhysRegs || !LivePhysRegs->available(Reg) || MRI.isReserved(Reg))
+      if ((!MO.isDead() && (!LivePhysRegs || !LivePhysRegs->available(Reg))) ||
+          MRI.isReserved(Reg))
         return false;
     } else {
       if (MO.isDead())
diff --git a/llvm/test/CodeGen/RISCV/machine-sink.mir b/llvm/test/CodeGen/RISCV/machine-sink.mir
index 5c7ba672f6ca7..213e6fd73e196 100644
--- a/llvm/test/CodeGen/RISCV/machine-sink.mir
+++ b/llvm/test/CodeGen/RISCV/machine-sink.mir
@@ -37,7 +37,6 @@ body: |
   ; CHECK-NEXT:   liveins: $x8
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   %x:gpr = COPY $x8
-  ; CHECK-NEXT:   %y:gpr = COPY %x, implicit-def dead $x8
   ; CHECK-NEXT:   PseudoBR %bb.1
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1: