[llvm] dfe05b8 - [GlobalISel] Provide a fast path for ResetMachineFunctionPass when the function is empty (#177341)

[via llvm-commits]

Author: Ryan Cowan
Date: 2026-01-29T16:29:49Z
New Revision: dfe05b84b36588d6d576b668933377f0d464aca7

URL: https://github.com/llvm/llvm-project/commit/dfe05b84b36588d6d576b668933377f0d464aca7
DIFF: https://github.com/llvm/llvm-project/commit/dfe05b84b36588d6d576b668933377f0d464aca7.diff

LOG: [GlobalISel] Provide a fast path for ResetMachineFunctionPass when the function is empty (#177341)

As part of https://github.com/llvm/llvm-project/pull/174746 I
encountered a compile time regression due to ResetMachineFunction
performing full resets on empty functions. In normal operation, this
behaviour is not likely to impact compile time, as the pass is only
inserted when using GlobalISel.

https://github.com/llvm/llvm-project/pull/174746 includes GlobalISel
passes in the SDAG pipeline (only on AArch64) and skips them if a given
function is not optnone surfacing this. By checking if the
MachineFunction is empty we can perform a more lightweight reset that
just sets the required flags, reducing the impact of this change.

Added: 
    

Modified: 
    llvm/include/llvm/CodeGen/MachineFunction.h
    llvm/lib/CodeGen/ResetMachineFunctionPass.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/CodeGen/MachineFunction.h b/llvm/include/llvm/CodeGen/MachineFunction.h
index c22a0101ef478..edb8963ce42b6 100644
--- a/llvm/include/llvm/CodeGen/MachineFunction.h
+++ b/llvm/include/llvm/CodeGen/MachineFunction.h
@@ -239,6 +239,14 @@ class MachineFunctionProperties {
     return *this;
   }
 
+  /// Reset all properties and re-establish baseline invariants.
+  MachineFunctionProperties &resetToInitial() {
+    reset();
+    setIsSSA();
+    setTracksLiveness();
+    return *this;
+  }
+
   MachineFunctionProperties &set(const MachineFunctionProperties &MFP) {
     Properties |= MFP.Properties;
     return *this;

diff  --git a/llvm/lib/CodeGen/ResetMachineFunctionPass.cpp b/llvm/lib/CodeGen/ResetMachineFunctionPass.cpp
index 8739643e25adf..14c3cc2ebe354 100644
--- a/llvm/lib/CodeGen/ResetMachineFunctionPass.cpp
+++ b/llvm/lib/CodeGen/ResetMachineFunctionPass.cpp
@@ -60,26 +60,36 @@ namespace {
       llvm::scope_exit ClearVRegTypesOnReturn(
           [&MF]() { MF.getRegInfo().clearVirtRegTypes(); });
 
-      if (MF.getProperties().hasFailedISel()) {
-        if (AbortOnFailedISel)
-          report_fatal_error("Instruction selection failed");
-        LLVM_DEBUG(dbgs() << "Resetting: " << MF.getName() << '\n');
-        ++NumFunctionsReset;
+      if (!MF.getProperties().hasFailedISel())
+        return false;
+
+      if (AbortOnFailedISel)
+        report_fatal_error("Instruction selection failed");
+
+      LLVM_DEBUG(dbgs() << "Resetting: " << MF.getName() << '\n');
+      ++NumFunctionsReset;
+
+      if (MF.empty()) {
+        // Nothing was materialized in the MachineFunction, so avoid the cost of
+        // tearing down and rebuilding all of the per-function state. Just clear
+        // the FailedISel bit so the SelectionDAG pipeline can proceed.
+        auto &Props = MF.getProperties();
+        Props.resetToInitial();
+      } else {
         MF.reset();
         MF.initTargetMachineFunctionInfo(MF.getSubtarget());
 
         const TargetMachine &TM = MF.getTarget();
         // MRI callback for target specific initializations.
         TM.registerMachineRegisterInfoCallback(MF);
+      }
 
-        if (EmitFallbackDiag) {
-          const Function &F = MF.getFunction();
-          DiagnosticInfoISelFallback DiagFallback(F);
-          F.getContext().diagnose(DiagFallback);
-        }
-        return true;
+      if (EmitFallbackDiag) {
+        const Function &F = MF.getFunction();
+        DiagnosticInfoISelFallback DiagFallback(F);
+        F.getContext().diagnose(DiagFallback);
       }
-      return false;
+      return true;
     }
 
   };