[GISel] Combine (neg (min/max x, (neg x))) into (max/min x, (neg x))

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

@@ -864,6 +864,9 @@ class CombinerHelper {
   /// Combine select to integer min/max.
   bool matchSelectIMinMax(const MachineOperand &MO, BuildFnTy &MatchInfo) const;
 
+  /// Tranform (neg (min/max x, (neg x))) into (max/min x, (neg x)).
+  bool matchSimplifyNegMinMax(MachineInstr &MI, BuildFnTy &MatchInfo) const;
+
   /// Combine selects.
   bool matchSelect(MachineInstr &MI, BuildFnTy &MatchInfo) const;
 

llvm/include/llvm/CodeGen/GlobalISel/Utils.h

@@ -171,6 +171,10 @@ void reportGISelWarning(MachineFunction &MF, const TargetPassConfig &TPC,
                         MachineOptimizationRemarkEmitter &MORE,
                         MachineOptimizationRemarkMissed &R);
 
+/// Returns the inverse opcode of \p MinMaxOpc, which is a generic min/max
+/// opcode like G_SMIN.
+unsigned getInverseGMinMaxOpcode(unsigned MinMaxOpc);
+
 /// If \p VReg is defined by a G_CONSTANT, return the corresponding value.
 std::optional<APInt> getIConstantVRegVal(Register VReg,
                                          const MachineRegisterInfo &MRI);

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

@@ -1372,6 +1372,12 @@ def select_to_iminmax: GICombineRule<
          [{ return Helper.matchSelectIMinMax(${root}, ${info}); }]),
   (apply [{ Helper.applyBuildFnMO(${root}, ${info}); }])>;
 
+def simplify_neg_minmax : GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_SUB):$root,
+         [{ return Helper.matchSimplifyNegMinMax(*${root}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
 def match_selects : GICombineRule<
   (defs root:$root, build_fn_matchinfo:$matchinfo),
   (match (wip_match_opcode G_SELECT):$root,
@@ -2008,7 +2014,7 @@ def all_combines : GICombineGroup<[integer_reassoc_combines, trivial_combines,
     and_or_disjoint_mask, fma_combines, fold_binop_into_select,
     sub_add_reg, select_to_minmax,
     fsub_to_fneg, commute_constant_to_rhs, match_ands, match_ors,
-    combine_concat_vector,
+    simplify_neg_minmax, combine_concat_vector,
     sext_trunc, zext_trunc, prefer_sign_combines, shuffle_combines,
     combine_use_vector_truncate, merge_combines, overflow_combines]>;
 

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -7062,6 +7062,36 @@ bool CombinerHelper::matchSelectIMinMax(const MachineOperand &MO,
   }
 }
 
+// (neg (min/max x, (neg x))) --> (max/min x, (neg x))
+bool CombinerHelper::matchSimplifyNegMinMax(MachineInstr &MI,
+                                            BuildFnTy &MatchInfo) const {
+  assert(MI.getOpcode() == TargetOpcode::G_SUB);
+  Register DestReg = MI.getOperand(0).getReg();
+  LLT DestTy = MRI.getType(DestReg);
+  if (!isLegal({TargetOpcode::G_SUB, {DestTy}}))
+    return false;
+
+  Register X;
+  Register Sub0;
+  auto NegPattern = m_all_of(m_Neg(m_DeferredReg(X)), m_Reg(Sub0));
+  if (mi_match(DestReg, MRI,
+               m_Neg(m_OneUse(m_any_of(m_GSMin(m_Reg(X), NegPattern),
+                                       m_GSMax(m_Reg(X), NegPattern),
+                                       m_GUMin(m_Reg(X), NegPattern),
+                                       m_GUMax(m_Reg(X), NegPattern)))))) {
+    MachineInstr *MinMaxMI = MRI.getVRegDef(MI.getOperand(2).getReg());
+    unsigned NewOpc = getInverseGMinMaxOpcode(MinMaxMI->getOpcode());
+    if (isLegal({NewOpc, {DestTy}})) {
+      MatchInfo = [=](MachineIRBuilder &B) {
+        B.buildInstr(NewOpc, {DestReg}, {X, Sub0});
+      };
+      return true;
+    }
+  }
+
+  return false;
+}
+
 bool CombinerHelper::matchSelect(MachineInstr &MI, BuildFnTy &MatchInfo) const {
   GSelect *Select = cast<GSelect>(&MI);
 

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -276,6 +276,21 @@ void llvm::reportGISelFailure(MachineFunction &MF, const TargetPassConfig &TPC,
   reportGISelFailure(MF, TPC, MORE, R);
 }
 
+unsigned llvm::getInverseGMinMaxOpcode(unsigned MinMaxOpc) {
+  switch (MinMaxOpc) {
+  case TargetOpcode::G_SMIN:
+    return TargetOpcode::G_SMAX;
+  case TargetOpcode::G_SMAX:
+    return TargetOpcode::G_SMIN;
+  case TargetOpcode::G_UMIN:
+    return TargetOpcode::G_UMAX;
+  case TargetOpcode::G_UMAX:
+    return TargetOpcode::G_UMIN;
+  default:
+    llvm_unreachable("unrecognized opcode");
+  }
+}
+
 std::optional<APInt> llvm::getIConstantVRegVal(Register VReg,
                                                const MachineRegisterInfo &MRI) {
   std::optional<ValueAndVReg> ValAndVReg = getIConstantVRegValWithLookThrough(

llvm/lib/Target/RISCV/RISCVCombine.td

@@ -25,5 +25,5 @@ def RISCVPostLegalizerCombiner
     : GICombiner<"RISCVPostLegalizerCombinerImpl",
                  [sub_to_add, combines_for_extload, redundant_and,
                   identity_combines, shift_immed_chain,
-                  commute_constant_to_rhs]> {
+                  commute_constant_to_rhs, simplify_neg_minmax]> {
 }