[llvm] [CodeGen] Untangle RegisterCoalescer from LRE's ScannedRemattable flag [nfc[ (PR #159839)

[Philip Reames via llvm-commits]

https://github.com/preames created https://github.com/llvm/llvm-project/pull/159839

LiveRangeEdit's rematerialization checking logic is used in two quite different ways.  For SplitKit and InlineSpiller, we're analyzing all defs associated with a live interval, doing that analysis up front, and then using the result a bit later.  The RegisterCoalescer, we're analysing exactly one ValNo at a time, and using the legality result immediately.  LRE had a checkRematerializable which existed basically to adapt the later into the former usage model.

Instead, this change bypasses the ScannedRemat and Remattable structures, and directly queries the underlying routines.  This is easy to read, and makes it more clear as to which uses actually need the defered analysis.  (A following change may try to unwind that too, but it's not strictly NFC.)

>From 84d5dbd99b4e76de813a1ccbf4feb740a5771c07 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Fri, 19 Sep 2025 12:24:22 -0700
Subject: [PATCH] [CodeGen] Untangle RegisterCoalescer from LRE's
 ScannedRemattable flag [nfc]

LiveRangeEdit's rematerialization checking logic is used in two quite
different ways.  For SplitKit and InlineSpiller, we're analyzing all
defs associated with a live interval, doing that analysis up front,
and then using the result a bit later.  The RegisterCoalescer, we're
analysing exactly one ValNo at a time, and using the legality result
immediately.  LRE had a checkRematerializable which existed basically
to adapt the later into the former usage model.

Instead, this change bypasses the ScannedRemat and Remattable
structures, and directly queries the underlying routines.  This is
easy to read, and makes it more clear as to which uses actually need
the defered analysis.  (A following change may try to unwind that
too, but it's not strictly NFC.)
---
 llvm/include/llvm/CodeGen/LiveRangeEdit.h |  8 ++------
 llvm/lib/CodeGen/LiveRangeEdit.cpp        | 13 ++-----------
 llvm/lib/CodeGen/RegisterCoalescer.cpp    | 13 +++++++------
 3 files changed, 11 insertions(+), 23 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/LiveRangeEdit.h b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
index 3d5df194c71c9..db1785de255f0 100644
--- a/llvm/include/llvm/CodeGen/LiveRangeEdit.h
+++ b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
@@ -176,14 +176,10 @@ class LiveRangeEdit : private MachineRegisterInfo::Delegate {
   Register create() { return createFrom(getReg()); }
 
   /// anyRematerializable - Return true if any parent values may be
-  /// rematerializable.
-  /// This function must be called before any rematerialization is attempted.
+  /// rematerializable.  This function must be called before
+  /// canRematerializeAt is called..
   bool anyRematerializable();
 
-  /// checkRematerializable - Manually add VNI to the list of rematerializable
-  /// values if DefMI may be rematerializable.
-  bool checkRematerializable(VNInfo *VNI, const MachineInstr *DefMI);
-
   /// Remat - Information needed to rematerialize at a specific location.
   struct Remat {
     const VNInfo *const ParentVNI;  // parent_'s value at the remat location.
diff --git a/llvm/lib/CodeGen/LiveRangeEdit.cpp b/llvm/lib/CodeGen/LiveRangeEdit.cpp
index 5514e4eb6cf3e..33e980a5993d3 100644
--- a/llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ b/llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -68,16 +68,6 @@ Register LiveRangeEdit::createFrom(Register OldReg) {
   return VReg;
 }
 
-bool LiveRangeEdit::checkRematerializable(VNInfo *VNI,
-                                          const MachineInstr *DefMI) {
-  assert(DefMI && "Missing instruction");
-  ScannedRemattable = true;
-  if (!TII.isTriviallyReMaterializable(*DefMI))
-    return false;
-  Remattable.insert(VNI);
-  return true;
-}
-
 void LiveRangeEdit::scanRemattable() {
   for (VNInfo *VNI : getParent().valnos) {
     if (VNI->isUnused())
@@ -90,7 +80,8 @@ void LiveRangeEdit::scanRemattable() {
     MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
     if (!DefMI)
       continue;
-    checkRematerializable(OrigVNI, DefMI);
+    if (TII.isTriviallyReMaterializable(*DefMI))
+      Remattable.insert(OrigVNI);
   }
   ScannedRemattable = true;
 }
diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index b8486f6560c5f..db00f54daeb62 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1325,9 +1325,7 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   if (!TII->isAsCheapAsAMove(*DefMI))
     return false;
 
-  SmallVector<Register, 8> NewRegs;
-  LiveRangeEdit Edit(&SrcInt, NewRegs, *MF, *LIS, nullptr, this);
-  if (!Edit.checkRematerializable(ValNo, DefMI))
+  if (!TII->isTriviallyReMaterializable(*DefMI))
     return false;
 
   if (!definesFullReg(*DefMI, SrcReg))
@@ -1395,15 +1393,18 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     }
   }
 
-  LiveRangeEdit::Remat RM(ValNo);
-  RM.OrigMI = DefMI;
-  if (!Edit.canRematerializeAt(RM, ValNo, CopyIdx))
+  SmallVector<Register, 8> NewRegs;
+  LiveRangeEdit Edit(&SrcInt, NewRegs, *MF, *LIS, nullptr, this);
+  SlotIndex DefIdx = LIS->getInstructionIndex(*DefMI);
+  if (!Edit.allUsesAvailableAt(DefMI, DefIdx, CopyIdx))
     return false;
 
   DebugLoc DL = CopyMI->getDebugLoc();
   MachineBasicBlock *MBB = CopyMI->getParent();
   MachineBasicBlock::iterator MII =
       std::next(MachineBasicBlock::iterator(CopyMI));
+  LiveRangeEdit::Remat RM(ValNo);
+  RM.OrigMI = DefMI;
   Edit.rematerializeAt(*MBB, MII, DstReg, RM, *TRI, false, SrcIdx, CopyMI);
   MachineInstr &NewMI = *std::prev(MII);
   NewMI.setDebugLoc(DL);