[llvm] [LiveVariables] Mark use as implicit-def if def is a subregister (PR #119446)

[via llvm-commits]

https://github.com/jofrn created https://github.com/llvm/llvm-project/pull/119446

LiveVariables will mark instructions with their implicit subregister uses. However, it will miss marking the subregister as an implicit-def if its own definition is a subregister of it, i.e. `$r3 = OP val, implicit-def $r0_r1_r2_r3, ..., implicit $r2_r3`, which defines $sr3 on the same line it is used.

This change ensures such uses are marked as implicit-def, i.e. `$r3 = OP val, implicit-def $r0_r1_r2_r3, ..., implicit-def $r2_r3`.

>From 59393473a837dc822efc369cf775adfe5561f091 Mon Sep 17 00:00:00 2001
From: jofrn <jofernau at amd.com>
Date: Tue, 10 Dec 2024 11:53:21 -0800
Subject: [PATCH] [LiveVariables] Mark use as implicit-def if def is a
 subregister

LiveVariables will mark instructions with their implicit subregister
uses. However, it will miss marking the subregister as an implicit-def
if its own definition is a subregister of it, i.e.
`$r3 = OP val, implicit-def $r0_r1_r2_r3, ..., implicit $r2_r3`,
which defines $sr3 on the same line it is used.

This change ensures such uses are marked as implicit-def, i.e.
`$r3 = OP val, implicit-def $r0_r1_r2_r3, ..., implicit-def $r2_r3`.
---
 llvm/lib/CodeGen/LiveVariables.cpp            | 13 +++++-
 .../CodeGen/AMDGPU/implicitdef-subreg.mir     | 44 +++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/AMDGPU/implicitdef-subreg.mir

diff --git a/llvm/lib/CodeGen/LiveVariables.cpp b/llvm/lib/CodeGen/LiveVariables.cpp
index f17d60dc22dda9..e6753563a24c28 100644
--- a/llvm/lib/CodeGen/LiveVariables.cpp
+++ b/llvm/lib/CodeGen/LiveVariables.cpp
@@ -277,10 +277,21 @@ void LiveVariables::HandlePhysRegUse(Register Reg, MachineInstr &MI) {
           continue;
         if (PartDefRegs.count(SubReg))
           continue;
+
+        // Check if SubReg is defined at LastPartialDef.
+        bool IsDefinedHere = false;
+        for (int i = 0; i < LastPartialDef->getNumOperands(); ++i) {
+          auto MO = LastPartialDef->getOperand(i);
+          if (!MO.isDef()) continue;
+          if (TRI->isSubRegister(SubReg, MO.getReg())) {
+            IsDefinedHere = true;
+            break;
+          }
+        }
         // This part of Reg was defined before the last partial def. It's killed
         // here.
         LastPartialDef->addOperand(MachineOperand::CreateReg(SubReg,
-                                                             false/*IsDef*/,
+                                                             IsDefinedHere/*IsDef*/,
                                                              true/*IsImp*/));
         PhysRegDef[SubReg] = LastPartialDef;
         for (MCPhysReg SS : TRI->subregs(SubReg))
diff --git a/llvm/test/CodeGen/AMDGPU/implicitdef-subreg.mir b/llvm/test/CodeGen/AMDGPU/implicitdef-subreg.mir
new file mode 100644
index 00000000000000..9b11bb482a028e
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/implicitdef-subreg.mir
@@ -0,0 +1,44 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=amdgcn -mcpu=gfx1010 -O1 %s --run-pass=livevars -implicitdef -o - | FileCheck %s
+---
+name:            sgpr_copy
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: sgpr_copy
+    ; CHECK: %sval:sreg_32 = S_MOV_B32 0
+    ; CHECK-NEXT: $sgpr0 = COPY %sval
+    ; CHECK-NEXT: $sgpr1 = COPY %sval
+    ; CHECK-NEXT: $sgpr2 = COPY %sval
+    ; CHECK-NEXT: $sgpr3 = COPY killed %sval, implicit-def $sgpr0_sgpr1_sgpr2_sgpr3, implicit $sgpr0, implicit $sgpr1, implicit $sgpr2, implicit $sgpr0_sgpr1, implicit $sgpr0_sgpr1_sgpr2, implicit-def $sgpr2_sgpr3
+    ; CHECK-NEXT: dead $sgpr30_sgpr31 = COPY killed $sgpr0_sgpr1_sgpr2_sgpr3
+    %sval:sreg_32 = S_MOV_B32 0
+
+    $sgpr0 = COPY %sval
+    $sgpr1 = COPY %sval
+    $sgpr2 = COPY %sval
+    $sgpr3 = COPY %sval
+    $sgpr30_sgpr31 = COPY $sgpr0_sgpr1_sgpr2_sgpr3
+
+...
+---
+name:            vgpr_copy
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: vgpr_copy
+    ; CHECK: %sval:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    ; CHECK-NEXT: $vgpr0 = COPY %sval
+    ; CHECK-NEXT: $vgpr1 = COPY %sval
+    ; CHECK-NEXT: $vgpr2 = COPY %sval
+    ; CHECK-NEXT: $vgpr3 = COPY killed %sval, implicit-def $vgpr0_vgpr1_vgpr2_vgpr3, implicit $vgpr0, implicit $vgpr1, implicit $vgpr2, implicit $vgpr0_vgpr1, implicit $vgpr0_vgpr1_vgpr2, implicit-def $vgpr1_vgpr2_vgpr3
+    ; CHECK-NEXT: dead [[COPY:%[0-9]+]]:vgpr_32 = COPY killed $vgpr0_vgpr1_vgpr2_vgpr3
+    %sval:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+
+    $vgpr0 = COPY %sval
+    $vgpr1 = COPY %sval
+    $vgpr2 = COPY %sval
+    $vgpr3 = COPY %sval
+    %0:vgpr_32 = COPY $vgpr0_vgpr1_vgpr2_vgpr3
+
+...