[GlobalIsel] Combine selects with constants (#76089)

A first small step at combining selects.

Author: Thorsten Schütt

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

@@ -769,9 +769,6 @@ class CombinerHelper {
   bool matchCombineFSubFpExtFNegFMulToFMadOrFMA(MachineInstr &MI,
                                                 BuildFnTy &MatchInfo);
 
-  /// Fold boolean selects to logical operations.
-  bool matchSelectToLogical(MachineInstr &MI, BuildFnTy &MatchInfo);
-
   bool matchCombineFMinMaxNaN(MachineInstr &MI, unsigned &Info);
 
   /// Transform G_ADD(x, G_SUB(y, x)) to y.
@@ -814,6 +811,9 @@ class CombinerHelper {
   // Given a binop \p MI, commute operands 1 and 2.
   void applyCommuteBinOpOperands(MachineInstr &MI);
 
+  /// Combine selects.
+  bool matchSelect(MachineInstr &MI, BuildFnTy &MatchInfo);
+
 private:
   /// Checks for legality of an indexed variant of \p LdSt.
   bool isIndexedLoadStoreLegal(GLoadStore &LdSt) const;
@@ -904,6 +904,18 @@ class CombinerHelper {
   /// select (fcmp uge x, 1.0) 1.0, x -> fminnm x, 1.0
   bool matchFPSelectToMinMax(Register Dst, Register Cond, Register TrueVal,
                              Register FalseVal, BuildFnTy &MatchInfo);
+
+  /// Try to fold selects to logical operations.
+  bool tryFoldBoolSelectToLogic(GSelect *Select, BuildFnTy &MatchInfo);
+
+  bool tryFoldSelectOfConstants(GSelect *Select, BuildFnTy &MatchInfo);
+
+  bool isOneOrOneSplat(Register Src, bool AllowUndefs);
+  bool isZeroOrZeroSplat(Register Src, bool AllowUndefs);
+  bool isConstantSplatVector(Register Src, int64_t SplatValue,
+                             bool AllowUndefs);
+
+  std::optional<APInt> getConstantOrConstantSplatVector(Register Src);
 };
 } // namespace llvm
 

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

@@ -437,13 +437,6 @@ def select_constant_cmp: GICombineRule<
   (apply [{ Helper.replaceSingleDefInstWithOperand(*${root}, ${matchinfo}); }])
 >;
 
-def select_to_logical : GICombineRule<
-  (defs root:$root, build_fn_matchinfo:$matchinfo),
-  (match (wip_match_opcode G_SELECT):$root,
-    [{ return Helper.matchSelectToLogical(*${root}, ${matchinfo}); }]),
-  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])
->;
-
 // Fold (C op x) -> (x op C)
 // TODO: handle more isCommutable opcodes
 // TODO: handle compares (currently not marked as isCommutable)
@@ -1242,6 +1235,12 @@ def select_to_minmax: GICombineRule<
          [{ return Helper.matchSimplifySelectToMinMax(*${root}, ${info}); }]),
   (apply [{ Helper.applyBuildFn(*${root}, ${info}); }])>;
 
+def match_selects : GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_SELECT):$root,
+        [{ return Helper.matchSelect(*${root}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,
@@ -1282,7 +1281,7 @@ def width_reduction_combines : GICombineGroup<[reduce_shl_of_extend,
 def phi_combines : GICombineGroup<[extend_through_phis]>;
 
 def select_combines : GICombineGroup<[select_undef_cmp, select_constant_cmp,
-                                      select_to_logical]>;
+                                      match_selects]>;
 
 def trivial_combines : GICombineGroup<[copy_prop, mul_to_shl, add_p2i_to_ptradd,
                                        mul_by_neg_one, idempotent_prop]>;