[GlobalIsel] Combine G_ADD and G_SUB with constants

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

@@ -892,6 +892,16 @@ class CombinerHelper {
 
   bool matchCastOfSelect(const MachineInstr &Cast, const MachineInstr &SelectMI,
                          BuildFnTy &MatchInfo);
+  bool matchFoldAPlusC1MinusC2(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
+  bool matchFoldC2MinusAPlusC1(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
+  bool matchFoldAMinusC1MinusC2(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
+  bool matchFoldC1Minus2MinusC2(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
+  // fold ((A-C1)+C2) -> (A+(C2-C1))
+  bool matchFoldAMinusC1PlusC2(const MachineInstr &MI, BuildFnTy &MatchInfo);
 
 private:
   /// Checks for legality of an indexed variant of \p LdSt.

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

@@ -22,6 +22,7 @@
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/Support/Alignment.h"
 #include "llvm/Support/Casting.h"
+
 #include <cstdint>
 
 namespace llvm {
@@ -178,6 +179,9 @@ std::optional<APInt> getIConstantVRegVal(Register VReg,
 std::optional<int64_t> getIConstantVRegSExtVal(Register VReg,
                                                const MachineRegisterInfo &MRI);
 
+/// \p VReg is defined by a G_CONSTANT, return the corresponding value.
+APInt getIConstantFromReg(Register VReg, const MachineRegisterInfo &MRI);
+
 /// Simple struct used to hold a constant integer value and a virtual
 /// register.
 struct ValueAndVReg {

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

@@ -1747,6 +1747,56 @@ def APlusBMinusCPlusA : GICombineRule<
           (G_ADD $root, $A, $sub1)),
    (apply (G_SUB $root, $B, $C))>;
 
+// fold (A+C1)-C2 -> A+(C1-C2)
+def APlusC1MinusC2: GICombineRule<
+   (defs root:$root, build_fn_matchinfo:$matchinfo),
+   (match (G_CONSTANT $c2, $imm2),
+          (G_CONSTANT $c1, $imm1),
+          (G_ADD $add, $A, $c1),
+          (G_SUB $root, $add, $c2):$root,
+   [{ return Helper.matchFoldAPlusC1MinusC2(*${root}, ${matchinfo}); }]),
+   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
+// fold C2-(A+C1) -> (C2-C1)-A
+def C2MinusAPlusC1: GICombineRule<
+   (defs root:$root, build_fn_matchinfo:$matchinfo),
+   (match (G_CONSTANT $c2, $imm2),
+          (G_CONSTANT $c1, $imm1),
+          (G_ADD $add, $A, $c1),
+          (G_SUB $root, $c2, $add):$root,
+   [{ return Helper.matchFoldC2MinusAPlusC1(*${root}, ${matchinfo}); }]),
+   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
+// fold (A-C1)-C2 -> A-(C1+C2)
+def AMinusC1MinusC2: GICombineRule<
+   (defs root:$root, build_fn_matchinfo:$matchinfo),
+   (match (G_CONSTANT $c2, $imm2),
+          (G_CONSTANT $c1, $imm1),
+          (G_SUB $sub1, $A, $c1),
+          (G_SUB $root, $sub1, $c2):$root,
+   [{ return Helper.matchFoldAMinusC1MinusC2(*${root}, ${matchinfo}); }]),
+   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
+// fold (C1-A)-C2 -> (C1-C2)-A
+def C1Minus2MinusC2: GICombineRule<
+   (defs root:$root, build_fn_matchinfo:$matchinfo),
+   (match (G_CONSTANT $c2, $imm2),
+          (G_CONSTANT $c1, $imm1),
+          (G_SUB $sub1, $c1, $A),
+          (G_SUB $root, $sub1, $c2):$root,
+   [{ return Helper.matchFoldC1Minus2MinusC2(*${root}, ${matchinfo}); }]),
+   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
+// fold ((A-C1)+C2) -> (A+(C2-C1))
+def AMinusC1PlusC2: GICombineRule<
+   (defs root:$root, build_fn_matchinfo:$matchinfo),
+   (match (G_CONSTANT $c2, $imm2),
+          (G_CONSTANT $c1, $imm1),
+          (G_SUB $sub, $A, $c1),
+          (G_ADD $root, $sub, $c2):$root,
+   [{ return Helper.matchFoldAMinusC1PlusC2(*${root}, ${matchinfo}); }]),
+   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
 def integer_reassoc_combines: GICombineGroup<[
   ZeroMinusAPlusB,
   APlusZeroMinusB,
@@ -1755,7 +1805,12 @@ def integer_reassoc_combines: GICombineGroup<[
   AMinusBPlusCMinusA,
   AMinusBPlusBMinusC,
   APlusBMinusAplusC,
-  APlusBMinusCPlusA
+  APlusBMinusCPlusA,
+  APlusC1MinusC2,
+  C2MinusAPlusC1,
+  AMinusC1MinusC2,
+  C1Minus2MinusC2,
+  AMinusC1PlusC2
 ]>;
 
 def freeze_of_non_undef_non_poison : GICombineRule<

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -7433,3 +7433,118 @@ void CombinerHelper::applyExpandFPowI(MachineInstr &MI, int64_t Exponent) {
   Builder.buildCopy(Dst, *Res);
   MI.eraseFromParent();
 }
+
+bool CombinerHelper::matchFoldAPlusC1MinusC2(const MachineInstr &MI,
+                                             BuildFnTy &MatchInfo) {
+  // fold (A+C1)-C2 -> A+(C1-C2)
+  const GSub *Sub = cast<GSub>(&MI);
+  GAdd *Add = cast<GAdd>(MRI.getVRegDef(Sub->getLHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Add->getReg(0)))
+    return false;
+
+  APInt C2 = getIConstantFromReg(Sub->getRHSReg(), MRI);
+  APInt C1 = getIConstantFromReg(Add->getRHSReg(), MRI);
+
+  Register Dst = Sub->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, C1 - C2);
+    B.buildAdd(Dst, Add->getLHSReg(), Const);
+  };
+
+  return true;
+}
+
+bool CombinerHelper::matchFoldC2MinusAPlusC1(const MachineInstr &MI,
+                                             BuildFnTy &MatchInfo) {
+  // fold C2-(A+C1) -> (C2-C1)-A
+  const GSub *Sub = cast<GSub>(&MI);
+  GAdd *Add = cast<GAdd>(MRI.getVRegDef(Sub->getRHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Add->getReg(0)))
+    return false;
+
+  APInt C2 = getIConstantFromReg(Sub->getLHSReg(), MRI);
+  APInt C1 = getIConstantFromReg(Add->getRHSReg(), MRI);
+
+  Register Dst = Sub->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, C2 - C1);
+    B.buildSub(Dst, Const, Add->getLHSReg());
+  };
+
+  return true;
+}
+
+bool CombinerHelper::matchFoldAMinusC1MinusC2(const MachineInstr &MI,
+                                              BuildFnTy &MatchInfo) {
+  // fold (A-C1)-C2 -> A-(C1+C2)
+  const GSub *Sub1 = cast<GSub>(&MI);
+  GSub *Sub2 = cast<GSub>(MRI.getVRegDef(Sub1->getLHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Sub2->getReg(0)))
+    return false;
+
+  APInt C2 = getIConstantFromReg(Sub1->getRHSReg(), MRI);
+  APInt C1 = getIConstantFromReg(Sub2->getRHSReg(), MRI);
+
+  Register Dst = Sub1->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, C1 + C2);
+    B.buildSub(Dst, Sub2->getLHSReg(), Const);
+  };
+
+  return true;
+}
+
+bool CombinerHelper::matchFoldC1Minus2MinusC2(const MachineInstr &MI,
+                                              BuildFnTy &MatchInfo) {
+  // fold (C1-A)-C2 -> (C1-C2)-A
+  const GSub *Sub1 = cast<GSub>(&MI);
+  GSub *Sub2 = cast<GSub>(MRI.getVRegDef(Sub1->getLHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Sub2->getReg(0)))
+    return false;
+
+  APInt C2 = getIConstantFromReg(Sub1->getRHSReg(), MRI);
+  APInt C1 = getIConstantFromReg(Sub2->getLHSReg(), MRI);
+
+  Register Dst = Sub1->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, C1 - C2);
+    B.buildSub(Dst, Const, Sub2->getRHSReg());
+  };
+
+  return true;
+}
+
+bool CombinerHelper::matchFoldAMinusC1PlusC2(const MachineInstr &MI,
+                                             BuildFnTy &MatchInfo) {
+  // fold ((A-C1)+C2) -> (A+(C2-C1))
+  const GAdd *Add = cast<GAdd>(&MI);
+  GSub *Sub = cast<GSub>(MRI.getVRegDef(Add->getLHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Sub->getReg(0)))
+    return false;
+
+  APInt C2 = getIConstantFromReg(Add->getRHSReg(), MRI);
+  APInt C1 = getIConstantFromReg(Sub->getRHSReg(), MRI);
+
+  Register Dst = Add->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, C2 - C1);
+    B.buildAdd(Dst, Sub->getLHSReg(), Const);
+  };
+
+  return true;
+}

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -303,6 +303,13 @@ std::optional<APInt> llvm::getIConstantVRegVal(Register VReg,
   return ValAndVReg->Value;
 }
 
+APInt llvm::getIConstantFromReg(Register Reg, const MachineRegisterInfo &MRI) {
+  MachineInstr *Const = MRI.getVRegDef(Reg);
+  assert((Const && Const->getOpcode() == TargetOpcode::G_CONSTANT) &&
+         "expected a G_CONSTANT on Reg");
+  return Const->getOperand(1).getCImm()->getValue();
+}
+
 std::optional<int64_t>
 llvm::getIConstantVRegSExtVal(Register VReg, const MachineRegisterInfo &MRI) {
   std::optional<APInt> Val = getIConstantVRegVal(VReg, MRI);