[RISCV][GISel] Sync MaxIterations/ObserverLvl/EnableFullDCE for PreLegalizer combiners with AArch64.

Author: topperc

llvm/lib/Target/RISCV/GISel/RISCVO0PreLegalizerCombiner.cpp

@@ -131,15 +131,19 @@ bool RISCVO0PreLegalizerCombiner::runOnMachineFunction(MachineFunction &MF) {
   CombinerInfo CInfo(/*AllowIllegalOps*/ true, /*ShouldLegalizeIllegal*/ false,
                      /*LegalizerInfo*/ nullptr, /*EnableOpt*/ false,
                      F.hasOptSize(), F.hasMinSize());
+  // Disable fixed-point iteration in the Combiner. This improves compile-time
+  // at the cost of possibly missing optimizations. See PR#94291 for details.
+  CInfo.MaxIterations = 1;
+
   RISCVO0PreLegalizerCombinerImpl Impl(MF, CInfo, &TPC, *KB,
                                        /*CSEInfo*/ nullptr, RuleConfig, ST);
   return Impl.combineMachineInstrs();
 }
 
 char RISCVO0PreLegalizerCombiner::ID = 0;
 INITIALIZE_PASS_BEGIN(RISCVO0PreLegalizerCombiner, DEBUG_TYPE,
-                      "Combine RISC-V machine instrs before legalization", false,
-                      false)
+                      "Combine RISC-V machine instrs before legalization",
+                      false, false)
 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 INITIALIZE_PASS_DEPENDENCY(GISelKnownBitsAnalysis)
 INITIALIZE_PASS_DEPENDENCY(GISelCSEAnalysisWrapperPass)

llvm/lib/Target/RISCV/GISel/RISCVPreLegalizerCombiner.cpp

@@ -146,6 +146,12 @@ bool RISCVPreLegalizerCombiner::runOnMachineFunction(MachineFunction &MF) {
   CombinerInfo CInfo(/*AllowIllegalOps*/ true, /*ShouldLegalizeIllegal*/ false,
                      /*LegalizerInfo*/ nullptr, EnableOpt, F.hasOptSize(),
                      F.hasMinSize());
+  // Disable fixed-point iteration to reduce compile-time
+  CInfo.MaxIterations = 1;
+  CInfo.ObserverLvl = CombinerInfo::ObserverLevel::SinglePass;
+  // This is the first Combiner, so the input IR might contain dead
+  // instructions.
+  CInfo.EnableFullDCE = true;
   RISCVPreLegalizerCombinerImpl Impl(MF, CInfo, &TPC, *KB, CSEInfo, RuleConfig,
                                      ST, MDT, LI);
   return Impl.combineMachineInstrs();