[llvm] 0312369 - [GlobalISel] Defer RegBankSelect operand mapper creation (#196985)

[via llvm-commits]

Author: Cullen Rhodes
Date: 2026-05-12T07:02:33+01:00
New Revision: 03123695573f476775b43a4dff4ea712f5b78909

URL: https://github.com/llvm/llvm-project/commit/03123695573f476775b43a4dff4ea712f5b78909
DIFF: https://github.com/llvm/llvm-project/commit/03123695573f476775b43a4dff4ea712f5b78909.diff

LOG: [GlobalISel] Defer RegBankSelect operand mapper creation (#196985)

RegBankSelect::applyMapping constructs an OperandsMapper before applying
repairs. Default mappings that only need Reassign repairs only update
the register bank and do not create replacement operands, so the generic
applyDefaultMapping path has no rewriting work to do in that case.

Defer OperandsMapper creation until an Insert repair actually needs new
virtual registers. If no mapper was needed for a default mapping, return
after applying the repairs.

CTMark geomean -0.23% improvement on aarch64-O0-g.

https://llvm-compile-time-tracker.com/compare.php?from=ed50ea52004259af958bb3e5636268342c49ee62&to=1a4730426e14969626cad43c6b06e93bde707bd1&stat=instructions%3Au

Assisted-by: Codex

Added: 
    

Modified: 
    llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp b/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
index 4dfd5179a4e56..bc6b5df99d2e7 100644
--- a/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
@@ -45,6 +45,7 @@
 #include <cstdint>
 #include <limits>
 #include <memory>
+#include <optional>
 #include <utility>
 
 #define DEBUG_TYPE "regbankselect"
@@ -595,7 +596,7 @@ bool RegBankSelect::applyMapping(
     MachineInstr &MI, const RegisterBankInfo::InstructionMapping &InstrMapping,
     SmallVectorImpl<RegBankSelect::RepairingPlacement> &RepairPts) {
   // OpdMapper will hold all the information needed for the rewriting.
-  RegisterBankInfo::OperandsMapper OpdMapper(MI, InstrMapping, *MRI);
+  std::optional<RegisterBankInfo::OperandsMapper> OpdMapper;
 
   // First, place the repairing code.
   for (RepairingPlacement &RepairPt : RepairPts) {
@@ -620,8 +621,10 @@ bool RegBankSelect::applyMapping(
       // Don't insert additional instruction for debug instruction.
       if (MI.isDebugInstr())
         break;
-      OpdMapper.createVRegs(OpIdx);
-      if (!repairReg(MO, ValMapping, RepairPt, OpdMapper.getVRegs(OpIdx)))
+      if (!OpdMapper)
+        OpdMapper.emplace(MI, InstrMapping, *MRI);
+      OpdMapper->createVRegs(OpIdx);
+      if (!repairReg(MO, ValMapping, RepairPt, OpdMapper->getVRegs(OpIdx)))
         return false;
       break;
     default:
@@ -629,9 +632,16 @@ bool RegBankSelect::applyMapping(
     }
   }
 
+  // Default mappings only need rewriting when repairs create new operands.
+  if (!OpdMapper && InstrMapping.getID() == RegisterBankInfo::DefaultMappingID)
+    return true;
+
+  if (!OpdMapper)
+    OpdMapper.emplace(MI, InstrMapping, *MRI);
   // Second, rewrite the instruction.
-  LLVM_DEBUG(dbgs() << "Actual mapping of the operands: " << OpdMapper << '\n');
-  RBI->applyMapping(MIRBuilder, OpdMapper);
+  LLVM_DEBUG(dbgs() << "Actual mapping of the operands: " << *OpdMapper
+                    << '\n');
+  RBI->applyMapping(MIRBuilder, *OpdMapper);
 
   return true;
 }