[GlobalIsel] Combine G_SELECT

Cleanups, preparation for more combines, add known bits for constant
conditions, combine selects where the false and true register are
constants, and improve support for vector conditions.

AMDGPU supports vector conditions. X86 has a todo for vector
conditions. AArch64 SVE supports SEL for vector conditions. How to
implement vector conditions with NEON (with bsl), see arm64-vselect.ll
? Vector select asserts in the instruction selector.

buildNot does not support scalable vectors. We cannot create scalable
constant vectors of -1 and there is no G_Not. AArch64 SVE has a NOT
and a DUP for broadcasting. Something akin to G_CONSTANT_SPLAT,
G_CONSTANT_VECTOR, G_SPLAT_VECTOR, G_BRODCAST, or G_HOMOGENOUS_VECTOR
that takes an immediate and creates a (fixed or scalable) vector where
all elements are the immediate might solve the buildNot challenge,
facilitates new combines, pattern matching, and new selecting
optimizations.

P.S. We need to support integer and float.

llvm#74502

```c
<vscale x 4 x i32> splat (i32 -1)
```

Author: tschuett

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

@@ -423,16 +423,9 @@ class CombinerHelper {
   /// Return true if a G_STORE instruction \p MI is storing an undef value.
   bool matchUndefStore(MachineInstr &MI);
 
-  /// Return true if a G_SELECT instruction \p MI has an undef comparison.
-  bool matchUndefSelectCmp(MachineInstr &MI);
-
   /// Return true if a G_{EXTRACT,INSERT}_VECTOR_ELT has an out of range index.
   bool matchInsertExtractVecEltOutOfBounds(MachineInstr &MI);
 
-  /// Return true if a G_SELECT instruction \p MI has a constant comparison. If
-  /// true, \p OpIdx will store the operand index of the known selected value.
-  bool matchConstantSelectCmp(MachineInstr &MI, unsigned &OpIdx);
-
   /// Replace an instruction with a G_FCONSTANT with value \p C.
   void replaceInstWithFConstant(MachineInstr &MI, double C);
 
@@ -771,9 +764,6 @@ 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.
@@ -816,6 +806,9 @@ 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;
@@ -906,6 +899,21 @@ 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);
+
+  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);
+
+  /// Try to combine selects with constant conditions.
+  bool tryCombineSelectConstantCondition(GSelect *Select, BuildFnTy &MatchInfo);
+
+  /// Try to combine selects with boolean conditions to logical operators.
+  bool tryFoldBoolSelectToLogic(GSelect *Select, BuildFnTy &MatchInfo);
+
+  /// Try to combine selects where the true and false values are constant.
+  bool tryFoldSelectOfConstants(GSelect *Select, BuildFnTy &MatchInfo);
 };
 } // namespace llvm
 

llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

@@ -1701,11 +1701,11 @@ class MachineIRBuilder {
 
   /// Build and insert a bitwise not,
   /// \p NegOne = G_CONSTANT -1
-  /// \p Res = G_OR \p Op0, NegOne
-  MachineInstrBuilder buildNot(const DstOp &Dst, const SrcOp &Src0) {
-    auto NegOne = buildConstant(Dst.getLLTTy(*getMRI()), -1);
-    return buildInstr(TargetOpcode::G_XOR, {Dst}, {Src0, NegOne});
-  }
+  /// \p Res = G_XOR \p Op0, NegOne
+  /// Or
+  /// \p NegOne = G_BUILD_VECTOR -1, -1, -1, ...
+  /// \p Res = G_XOR \p Op0, NegOne
+  MachineInstrBuilder buildNot(const DstOp &Dst, const SrcOp &Src0);
 
   /// Build and insert integer negation
   /// \p Zero = G_CONSTANT 0

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

@@ -419,31 +419,6 @@ def select_same_val: GICombineRule<
   (apply [{ Helper.replaceSingleDefInstWithOperand(*${root}, 2); }])
 >;
 
-// Fold (undef ? x : y) -> y
-def select_undef_cmp: GICombineRule<
-  (defs root:$dst),
-  (match (G_IMPLICIT_DEF $undef),
-         (G_SELECT $dst, $undef, $x, $y)),
-  (apply (GIReplaceReg $dst, $y))
->;
-
-// Fold (true ? x : y) -> x
-// Fold (false ? x : y) -> y
-def select_constant_cmp_matchdata : GIDefMatchData<"unsigned">;
-def select_constant_cmp: GICombineRule<
-  (defs root:$root, select_constant_cmp_matchdata:$matchinfo),
-  (match (wip_match_opcode G_SELECT):$root,
-    [{ return Helper.matchConstantSelectCmp(*${root}, ${matchinfo}); }]),
-  (apply [{ Helper.replaceSingleDefInstWithOperand(*${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}, ${matchinfo}); }])
->;
-
 // Fold (C op x) -> (x op C)
 // TODO: handle more isCommutable opcodes
 // TODO: handle compares (currently not marked as isCommutable)
@@ -1242,6 +1217,12 @@ def select_to_minmax: GICombineRule<
          [{ return Helper.matchSimplifySelectToMinMax(*${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,
@@ -1281,8 +1262,7 @@ 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]>;
+def select_combines : GICombineGroup<[match_selects]>;
 
 def trivial_combines : GICombineGroup<[copy_prop, mul_to_shl, add_p2i_to_ptradd,
                                        mul_by_neg_one, idempotent_prop]>;