[llvm-branch-commits] [llvm] [CodeGen] Enhance createFrom for sub-reg aware cloning (PR #174999)

[via llvm-branch-commits]

https://github.com/easyonaadit updated https://github.com/llvm/llvm-project/pull/174999

>From 4cbed3bef07f5077dada9f816d1fcece3e2a4601 Mon Sep 17 00:00:00 2001
From: Christudasan Devadasan <Christudasan.Devadasan at amd.com>
Date: Wed, 7 Jan 2026 12:42:45 +0000
Subject: [PATCH 1/2] [CodeGen] Enhance createFrom for sub-reg aware cloning

Instead of just cloning the virtual register, this
function now creates a new virtual register derived
from a subregister class of the original value.
---
 llvm/include/llvm/CodeGen/LiveRangeEdit.h |  7 +++++--
 llvm/lib/CodeGen/LiveRangeEdit.cpp        | 10 +++++++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/LiveRangeEdit.h b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
index 00e1b6623efb4..c0c16f12eac49 100644
--- a/llvm/include/llvm/CodeGen/LiveRangeEdit.h
+++ b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
@@ -155,8 +155,11 @@ class LLVM_ABI LiveRangeEdit : private MachineRegisterInfo::Delegate {
 
   ArrayRef<Register> regs() const { return ArrayRef(NewRegs).slice(FirstNew); }
 
-  /// createFrom - Create a new virtual register based on OldReg.
-  Register createFrom(Register OldReg);
+  /// createFrom - Create a new virtual register based on OldReg. If \p RC is
+  /// non-null, constrain the register class of the new vreg. Partial reloads
+  /// of tuple regclasses will feed-in a subregclass derived from the regclass
+  /// of OldReg.
+  Register createFrom(Register OldReg, const TargetRegisterClass *RC = nullptr);
 
   /// create - Create a new register with the same class and original slot as
   /// parent.
diff --git a/llvm/lib/CodeGen/LiveRangeEdit.cpp b/llvm/lib/CodeGen/LiveRangeEdit.cpp
index 1d6b835f16468..9e0c6c8558d2c 100644
--- a/llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ b/llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -52,8 +52,16 @@ LiveInterval &LiveRangeEdit::createEmptyIntervalFrom(Register OldReg,
   return LI;
 }
 
-Register LiveRangeEdit::createFrom(Register OldReg) {
+Register LiveRangeEdit::createFrom(Register OldReg,
+                                   const TargetRegisterClass *RC) {
   Register VReg = MRI.cloneVirtualRegister(OldReg);
+
+  // If RC is given, set it now. This is needed for cloning a new VReg with a
+  // smaller RC from its original virtual register, mainly used for subreg
+  // reload. The cloning is done first to ensure the vreg flags are preserved.
+  if (RC)
+    MRI.setRegClass(VReg, RC);
+
   if (VRM) {
     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
   }

>From 8140fc2b4ad98e17a40c9dbb3729793989d00625 Mon Sep 17 00:00:00 2001
From: Aaditya <Aaditya.AlokDeshpande at amd.com>
Date: Wed, 30 Sep 2026 16:56:40 +0530
Subject: [PATCH 2/2] pass in subregidx rather than a regclass

---
 llvm/include/llvm/CodeGen/LiveRangeEdit.h | 10 +++++-----
 llvm/lib/CodeGen/LiveRangeEdit.cpp        | 19 ++++++++++++-------
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/LiveRangeEdit.h b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
index c0c16f12eac49..309dd57993d1e 100644
--- a/llvm/include/llvm/CodeGen/LiveRangeEdit.h
+++ b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
@@ -155,11 +155,11 @@ class LLVM_ABI LiveRangeEdit : private MachineRegisterInfo::Delegate {
 
   ArrayRef<Register> regs() const { return ArrayRef(NewRegs).slice(FirstNew); }
 
-  /// createFrom - Create a new virtual register based on OldReg. If \p RC is
-  /// non-null, constrain the register class of the new vreg. Partial reloads
-  /// of tuple regclasses will feed-in a subregclass derived from the regclass
-  /// of OldReg.
-  Register createFrom(Register OldReg, const TargetRegisterClass *RC = nullptr);
+  /// createFrom - Create a new virtual register based on OldReg. If \p
+  /// SubRegIdx is non-null, constrain the register class of the new vreg.
+  /// Partial reloads of tuple regclasses will feed-in a subregidx derived from
+  /// OldReg.
+  Register createFrom(Register OldReg, const unsigned SubRegIdx = {});
 
   /// create - Create a new register with the same class and original slot as
   /// parent.
diff --git a/llvm/lib/CodeGen/LiveRangeEdit.cpp b/llvm/lib/CodeGen/LiveRangeEdit.cpp
index 9e0c6c8558d2c..682d4926093f6 100644
--- a/llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ b/llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -52,15 +52,20 @@ LiveInterval &LiveRangeEdit::createEmptyIntervalFrom(Register OldReg,
   return LI;
 }
 
-Register LiveRangeEdit::createFrom(Register OldReg,
-                                   const TargetRegisterClass *RC) {
+Register LiveRangeEdit::createFrom(Register OldReg, const unsigned SubRegIdx) {
   Register VReg = MRI.cloneVirtualRegister(OldReg);
 
-  // If RC is given, set it now. This is needed for cloning a new VReg with a
-  // smaller RC from its original virtual register, mainly used for subreg
-  // reload. The cloning is done first to ensure the vreg flags are preserved.
-  if (RC)
-    MRI.setRegClass(VReg, RC);
+  // If SubRegIdx is given, set it now. This is needed for cloning a new VReg
+  // with a smaller RC from its original virtual register, mainly used for
+  // subreg reload. The cloning is done first to ensure the vreg flags are
+  // preserved.
+  if (SubRegIdx) {
+    const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
+    const TargetRegisterClass *RC =
+        TRI.getSubRegisterClass(MRI.getRegClass(OldReg), SubRegIdx);
+    if (RC)
+      MRI.setRegClass(VReg, RC);
+  }
 
   if (VRM) {
     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));