[RISCV] Implement convertSelectOfConstantsToMath

[lenary]

Summary

The current lowering of SELECT on RISC-V uses a branch instruction to load a
register with one or other value. This is inefficient, especially in the case of
small constants that can be computed easily.

By implementing the TargetLowering::convertSelectOfConstantsToMath hook, some of
the simpler cases are covered that let us avoid introducing a branch in these
cases.

[luismarques]

LGTM.

llvm/lib/Target/RISCV/RISCVISelLowering.h

EmitInstrWithCustomInserter(MachineInstr &MI,
                               MachineBasicBlock *BB) const override;
 
   EVT getSetCCResultType(const DataLayout &DL, LLVMContext &Context,
                          EVT VT) const override;
 
   bool convertSetCCLogicToBitwiseLogic(EVT VT) const override {
     return VT.isScalarInteger();
   }
+  bool convertSelectOfConstantsToMath(EVT VT) const override { return true; }
 
   bool shouldInsertFencesForAtomic(const Instruction *I) const override {
     return isa<LoadInst>(I) || isa<StoreInst>(I);
   }
   Instruction *emitLeadingFence(IRBuilder<> &Builder, Instruction *Inst,
                                 AtomicOrdering Ord) const override;
   Instruction *emitTrailingFence(IRBuilder<> &Builder, Instruction *Inst,
                                  AtomicOrdering Ord) const override;

llvm/test/CodeGen/RISCV/select-const.ll

 ; RV64IFD-NEXT:    ret
   %1 = select i1 %a, i32 1, i32 0
   ret i32 %1
 }
 
 define signext i32 @select_const_int_one_away(i1 zeroext %a) nounwind {
 ; RV32I-LABEL: select_const_int_one_away:
 ; RV32I:       # %bb.0:
-; RV32I-NEXT:    mv a1, a0
-; RV32I-NEXT:    addi a0, zero, 3
-; RV32I-NEXT:    bnez a1, .LBB1_2
-; RV32I-NEXT:  # %bb.1:
-; RV32I-NEXT:    addi a0, zero, 4
-; RV32I-NEXT:  .LBB1_2:
+; RV32I-NEXT:    addi a1, zero, 4
+; RV32I-NEXT:    sub a0, a1, a0
 ; RV32I-NEXT:    ret
 ;
 ; RV32IF-LABEL: select_const_int_one_away:
 ; RV32IF:       # %bb.0:
-; RV32IF-NEXT:    mv a1, a0
-; RV32IF-NEXT:    addi a0, zero, 3
-; RV32IF-NEXT:    bnez a1, .LBB1_2
-; RV32IF-NEXT:  # %bb.1:
-; RV32IF-NEXT:    addi a0, zero, 4
-; RV32IF-NEXT:  .LBB1_2:
+; RV32IF-NEXT:    addi a1, zero, 4
+; RV32IF-NEXT:    sub a0, a1, a0
 ; RV32IF-NEXT:    ret
 ;
 ; RV64I-LABEL: select_const_int_one_away:
 ; RV64I:       # %bb.0:
-; RV64I-NEXT:    mv a1, a0
-; RV64I-NEXT:    addi a0, zero, 3
-; RV64I-NEXT:    bnez a1, .LBB1_2
-; RV64I-NEXT:  # %bb.1:
-; RV64I-NEXT:    addi a0, zero, 4
-; RV64I-NEXT:  .LBB1_2:
+; RV64I-NEXT:    addi a1, zero, 4
+; RV64I-NEXT:    sub a0, a1, a0
 ; RV64I-NEXT:    ret
 ;
 ; RV64IFD-LABEL: select_const_int_one_away:
 ; RV64IFD:       # %bb.0:
-; RV64IFD-NEXT:    mv a1, a0
-; RV64IFD-NEXT:    addi a0, zero, 3
-; RV64IFD-NEXT:    bnez a1, .LBB1_2
-; RV64IFD-NEXT:  # %bb.1:
-; RV64IFD-NEXT:    addi a0, zero, 4
-; RV64IFD-NEXT:  .LBB1_2:
+; RV64IFD-NEXT:    addi a1, zero, 4
+; RV64IFD-NEXT:    sub a0, a1, a0
 ; RV64IFD-NEXT:    ret
   %1 = select i1 %a, i32 3, i32 4
   ret i32 %1
 }
 
 define signext i32 @select_const_int_pow2_zero(i1 zeroext %a) nounwind {
 ; RV32I-LABEL: select_const_int_pow2_zero:
 ; RV32I:       # %bb.0:
-; RV32I-NEXT:    mv a1, a0
-; RV32I-NEXT:    addi a0, zero, 4
-; RV32I-NEXT:    bnez a1, .LBB2_2
-; RV32I-NEXT:  # %bb.1:
-; RV32I-NEXT:    mv a0, zero
-; RV32I-NEXT:  .LBB2_2:
+; RV32I-NEXT:    slli a0, a0, 2
 ; RV32I-NEXT:    ret
 ;
 ; RV32IF-LABEL: select_const_int_pow2_zero:
 ; RV32IF:       # %bb.0:
-; RV32IF-NEXT:    mv a1, a0
-; RV32IF-NEXT:    addi a0, zero, 4
-; RV32IF-NEXT:    bnez a1, .LBB2_2
-; RV32IF-NEXT:  # %bb.1:
-; RV32IF-NEXT:    mv a0, zero
-; RV32IF-NEXT:  .LBB2_2:
+; RV32IF-NEXT:    slli a0, a0, 2
 ; RV32IF-NEXT:    ret
 ;
 ; RV64I-LABEL: select_const_int_pow2_zero:
 ; RV64I:       # %bb.0:
-; RV64I-NEXT:    mv a1, a0
-; RV64I-NEXT:    addi a0, zero, 4
-; RV64I-NEXT:    bnez a1, .LBB2_2
-; RV64I-NEXT:  # %bb.1:
-; RV64I-NEXT:    mv a0, zero
-; RV64I-NEXT:  .LBB2_2:
+; RV64I-NEXT:    slli a0, a0, 2
 ; RV64I-NEXT:    ret
 ;
 ; RV64IFD-LABEL: select_const_int_pow2_zero:
 ; RV64IFD:       # %bb.0:
-; RV64IFD-NEXT:    mv a1, a0
-; RV64IFD-NEXT:    addi a0, zero, 4
-; RV64IFD-NEXT:    bnez a1, .LBB2_2
-; RV64IFD-NEXT:  # %bb.1:
-; RV64IFD-NEXT:    mv a0, zero
-; RV64IFD-NEXT:  .LBB2_2:
+; RV64IFD-NEXT:    slli a0, a0, 2
 ; RV64IFD-NEXT:    ret
   %1 = select i1 %a, i32 4, i32 0
   ret i32 %1
 }
 
 define signext i32 @select_const_int_harder(i1 zeroext %a) nounwind {
 ; RV32I-LABEL: select_const_int_harder:
 ; RV32I:       # %bb.0: