Revert "[RISCV][GISEL] Introduce the RISCVPostLegalizerLowering pass (#108991)"

michaelmaitland · michaelmaitland · commit 9c5ad62e7440 · 2024-09-20T07:01:17.000-07:00
This reverts commit 6497283. Based on the discussions in #108991 that happened post merge, we have decided to remove this pass in favor of generating `RISCV::G_*` opcodes in the legalizer. We may reconsider moving that code elsewhere in the future so that we can do a better job during generic combines. We don't feel that doing it in instruciton selection is the right decision today. Firstly, it requires us to manually do regbankselect on the newly introduced instructions. Secondly, it is more difficult to test since the test output will contain whatever `RISCV::G_*` instructions select to (instead of `RISCV::G_*`). My personal opinion is that the legalizer pass can be split into an early legalizer and a late legalizer, both before regbankselect. The first legalizer would not introduce target specific generic opcodes and the generic combiner would run after it. The second legalizer would introduce the target specific generic opcodes. I think this approach is better than the lowerer because the legalizer guarantees that whatever we lower to is legal, and apparently because it is more performant at compared to the lowerer (although, I'm not sure how true this is).
diff --git a/llvm/lib/Target/RISCV/CMakeLists.txt b/llvm/lib/Target/RISCV/CMakeLists.txt
@@ -24,8 +24,6 @@ tablegen(LLVM RISCVGenPreLegalizeGICombiner.inc -gen-global-isel-combiner
               -combiners="RISCVPreLegalizerCombiner")
 tablegen(LLVM RISCVGenPostLegalizeGICombiner.inc -gen-global-isel-combiner
               -combiners="RISCVPostLegalizerCombiner")
-tablegen(LLVM RISCVGenPostLegalizeGILowering.inc -gen-global-isel-combiner
-              -combiners="RISCVPostLegalizerLowering")
 
 add_public_tablegen_target(RISCVCommonTableGen)
 
@@ -66,7 +64,6 @@ add_llvm_target(RISCVCodeGen
   GISel/RISCVInstructionSelector.cpp
   GISel/RISCVLegalizerInfo.cpp
   GISel/RISCVPostLegalizerCombiner.cpp
-  GISel/RISCVPostLegalizerLowering.cpp
   GISel/RISCVO0PreLegalizerCombiner.cpp
   GISel/RISCVPreLegalizerCombiner.cpp
   GISel/RISCVRegisterBankInfo.cpp
diff --git a/llvm/lib/Target/RISCV/GISel/RISCVPostLegalizerLowering.cpp b/llvm/lib/Target/RISCV/GISel/RISCVPostLegalizerLowering.cpp
diff --git a/llvm/lib/Target/RISCV/RISCV.h b/llvm/lib/Target/RISCV/RISCV.h
@@ -102,9 +102,6 @@ void initializeRISCVO0PreLegalizerCombinerPass(PassRegistry &);
 
 FunctionPass *createRISCVPreLegalizerCombiner();
 void initializeRISCVPreLegalizerCombinerPass(PassRegistry &);
-
-FunctionPass *createRISCVPostLegalizerLowering();
-void initializeRISCVPostLegalizerLoweringPass(PassRegistry &);
 } // namespace llvm
 
 #endif
diff --git a/llvm/lib/Target/RISCV/RISCVCombine.td b/llvm/lib/Target/RISCV/RISCVCombine.td
@@ -19,13 +19,6 @@ def RISCVO0PreLegalizerCombiner: GICombiner<
   "RISCVO0PreLegalizerCombinerImpl", [optnone_combines]> {
 }
 
-// Post-legalization combines which should happen at all optimization levels.
-// (E.g. ones that facilitate matching for the selector) For example, matching
-// pseudos.
-def RISCVPostLegalizerLowering
-    : GICombiner<"RISCVPostLegalizerLoweringImpl", []> {
-}
-
 // Post-legalization combines which are primarily optimizations.
 // TODO: Add more combines.
 def RISCVPostLegalizerCombiner
diff --git a/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp b/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp
@@ -111,7 +111,6 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeRISCVTarget() {
   initializeRISCVO0PreLegalizerCombinerPass(*PR);
   initializeRISCVPreLegalizerCombinerPass(*PR);
   initializeRISCVPostLegalizerCombinerPass(*PR);
-  initializeRISCVPostLegalizerLoweringPass(*PR);
   initializeKCFIPass(*PR);
   initializeRISCVDeadRegisterDefinitionsPass(*PR);
   initializeRISCVMakeCompressibleOptPass(*PR);
@@ -484,7 +483,6 @@ bool RISCVPassConfig::addLegalizeMachineIR() {
 void RISCVPassConfig::addPreRegBankSelect() {
   if (getOptLevel() != CodeGenOptLevel::None)
     addPass(createRISCVPostLegalizerCombiner());
-  addPass(createRISCVPostLegalizerLowering());
 }
 
 bool RISCVPassConfig::addRegBankSelect() {
diff --git a/llvm/test/CodeGen/RISCV/GlobalISel/gisel-commandline-option.ll b/llvm/test/CodeGen/RISCV/GlobalISel/gisel-commandline-option.ll
@@ -23,7 +23,6 @@
 ; ENABLED-NEXT:  Legalizer
 ; ENABLED-O1-NEXT:  MachineDominator Tree Construction
 ; ENABLED-O1-NEXT:  RISCVPostLegalizerCombiner
-; ENABLED-NEXT:  RISCVPostLegalizerLowering
 ; ENABLED-NEXT:  RegBankSelect
 ; ENABLED-NEXT:  Analysis for ComputingKnownBits
 ; ENABLED-O1-NEXT:  Lazy Branch Probability Analysis
