[GlobalISel] Try to commute G_CONSTANT_FOLD_BARRIER LHS operands to RHS.

[Pierre-vh]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

6028–6032

llvm/test/CodeGen/AArch64/GlobalISel/combine-const-fold-barrier-rhs.mir

6

is alignment necessary?

[tschuett]

Please remove all has_value() they have no value. Options have boolean conversion. You may use them in GTest.

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

public:
   ///   (X + Y) != X -> Y != 0
   ///   (X - Y) != X -> Y != 0
   ///   (X ^ Y) != X -> Y != 0
   bool matchRedundantBinOpInEquality(MachineInstr &MI, BuildFnTy &MatchInfo);
 
   /// Match shifts greater or equal to the bitwidth of the operation.
   bool matchShiftsTooBig(MachineInstr &MI);
 
+  /// Match constant LHS ops that should be commuted.
+  bool matchCommuteConstantToRHS(MachineInstr &MI);
+
 private:
   /// Given a non-indexed load or store instruction \p MI, find an offset that
   /// can be usefully and legally folded into it as a post-indexing operation.
   ///
   /// \returns true if a candidate is found.
   bool findPostIndexCandidate(MachineInstr &MI, Register &Addr, Register &Base,
                               Register &Offset);
 

llvm/include/llvm/Target/GlobalISel/Combine.td

def select_to_logical : GICombineRule<
   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])
 >;
 
 // Fold (C op x) -> (x op C)
 // TODO: handle more isCommutable opcodes
 // TODO: handle compares (currently not marked as isCommutable)
 def commute_constant_to_rhs : GICombineRule<
   (defs root:$root),
-  (match (wip_match_opcode G_ADD, G_MUL, G_AND, G_OR, G_XOR):$root, [{
-    return getIConstantVRegVal(${root}->getOperand(1).getReg(), MRI).has_value();
-  }]),
+  (match (wip_match_opcode G_ADD, G_MUL, G_AND, G_OR, G_XOR):$root,
+    [{ return Helper.matchCommuteConstantToRHS(*${root}); }]),
   (apply [{
     Observer.changingInstr(*${root});
     Register LHSReg = ${root}->getOperand(1).getReg();
     Register RHSReg = ${root}->getOperand(2).getReg();
     ${root}->getOperand(1).setReg(RHSReg);
     ${root}->getOperand(2).setReg(LHSReg);
     Observer.changedInstr(*${root});
   }])

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

bool CombinerHelper::matchShiftsTooBig(MachineInstr &MI) {
   LLT ResTy = MRI.getType(MI.getOperand(0).getReg());
   auto IsShiftTooBig = [&](const Constant *C) {
     auto *CI = dyn_cast<ConstantInt>(C);
     return CI && CI->uge(ResTy.getScalarSizeInBits());
   };
   return matchUnaryPredicate(MRI, ShiftReg, IsShiftTooBig);
 }
 
+bool CombinerHelper::matchCommuteConstantToRHS(MachineInstr &MI) {
+  Register LHS = MI.getOperand(1).getReg();
+  Register RHS = MI.getOperand(2).getReg();
+  auto *LHSDef = MRI.getVRegDef(LHS);
+  if (getIConstantVRegVal(LHS, MRI).has_value())
+    return true;
+
+  // LHS may be a G_CONSTANT_FOLD_BARRIER. If so we commute
+  // as long as we don't already have a constant on the RHS.
+  if (LHSDef->getOpcode() != TargetOpcode::G_CONSTANT_FOLD_BARRIER)
+    return false;
+  return MRI.getVRegDef(RHS)->getOpcode() !=
+             TargetOpcode::G_CONSTANT_FOLD_BARRIER &&
+         !getIConstantVRegVal(RHS, MRI);
+}
+
 bool CombinerHelper::tryCombine(MachineInstr &MI) {
   if (tryCombineCopy(MI))
     return true;
   if (tryCombineExtendingLoads(MI))
     return true;
   if (tryCombineIndexedLoadStore(MI))
     return true;
   return false;
 }

llvm/test/CodeGen/AArch64/GlobalISel/combine-const-fold-barrier-rhs.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 2
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+---
+name:            cfb_lhs
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $w0
+
+    ; CHECK-LABEL: name: cfb_lhs
+    ; CHECK: liveins: $w0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK-NEXT: %cst:_(s32) = G_CONSTANT i32 1
+    ; CHECK-NEXT: %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    ; CHECK-NEXT: %add:_(s32) = G_ADD [[COPY]], %cfb
+    ; CHECK-NEXT: $w0 = COPY %add(s32)
+    ; CHECK-NEXT: RET_ReallyLR
+    %0:_(s32) = COPY $w0
+    %cst:_(s32) = G_CONSTANT i32 1
+    %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    %cst2:_(s32) = G_CONSTANT i32 2
+    %add:_(s32) = G_ADD %cfb, %0
+    $w0 = COPY %add
+    RET_ReallyLR
+
+...
+---
+name:            cfb_lhs_cfb_already_rhs
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $w0
+
+    ; CHECK-LABEL: name: cfb_lhs_cfb_already_rhs
+    ; CHECK: liveins: $w0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %cst:_(s32) = G_CONSTANT i32 1
+    ; CHECK-NEXT: %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    ; CHECK-NEXT: %cst2:_(s32) = G_CONSTANT i32 2
+    ; CHECK-NEXT: %cfb2:_(s32) = G_CONSTANT_FOLD_BARRIER %cst2
+    ; CHECK-NEXT: %add:_(s32) = G_ADD %cfb, %cfb2
+    ; CHECK-NEXT: $w0 = COPY %add(s32)
+    ; CHECK-NEXT: RET_ReallyLR
+    %0:_(s32) = COPY $w0
+    %cst:_(s32) = G_CONSTANT i32 1
+    %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    %cst2:_(s32) = G_CONSTANT i32 2
+    %cfb2:_(s32) = G_CONSTANT_FOLD_BARRIER %cst2
+    %add:_(s32) = G_ADD %cfb, %cfb2
+    $w0 = COPY %add
+    RET_ReallyLR
+
+...
+---
+name:            cfb_lhs_cst_on_rhs
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $w0
+
+    ; CHECK-LABEL: name: cfb_lhs_cst_on_rhs
+    ; CHECK: liveins: $w0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %cst:_(s32) = G_CONSTANT i32 1
+    ; CHECK-NEXT: %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    ; CHECK-NEXT: %cst2:_(s32) = G_CONSTANT i32 2
+    ; CHECK-NEXT: %add:_(s32) = G_ADD %cfb, %cst2
+    ; CHECK-NEXT: $w0 = COPY %add(s32)
+    ; CHECK-NEXT: RET_ReallyLR
+    %0:_(s32) = COPY $w0
+    %cst:_(s32) = G_CONSTANT i32 1
+    %cfb:_(s32) = G_CONSTANT_FOLD_BARRIER %cst
+    %cst2:_(s32) = G_CONSTANT i32 2
+    %add:_(s32) = G_ADD %cfb, %cst2
+    $w0 = COPY %add
+    RET_ReallyLR
+
+...