[RISCV] Add early out to generateInstSeq when the initial sequence is 1 or 2 instructions.

[craig.topper]

Summary

This avoids checking the size of the sequence repeatedly for each
special case. Especially on RV32 where none of the special cases
apply.

[reames]

LGTM

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp

if ((Val & 0xfff) != 0 && (Val & 1) == 0 && Res.size() >= 2) {
     generateInstSeqImpl(ShiftedVal, ActiveFeatures, TmpSeq);
     TmpSeq.emplace_back(RISCV::SLLI, TrailingZeros);
 
     // Keep the new sequence if it is an improvement.
     if (TmpSeq.size() < Res.size() || IsShiftedCompressible)
       Res = TmpSeq;
   }
 
-  // If the constant is positive we might be able to generate a shifted constant
-  // with no leading zeros and use a final SRLI to restore them.
-  if (Val > 0 && Res.size() > 2) {
-    assert(ActiveFeatures[RISCV::Feature64Bit] &&
-           "Expected RV32 to only need 2 instructions");
+  // If we have a 1 or 2 instruction sequence this is the best we can do. This
+  // will always be true for RV32 and will often be true for RV64.
+  if (Res.size() <= 2)
+    return Res;
+
+
+  // If the constant is positive we might be able to generate a shifted constant
+  // with no leading zeros and use a final SRLI to restore them.
+  if (Val > 0) {
+    assert(Res.size() > 2 && "Expected longer sequence");
     unsigned LeadingZeros = llvm::countl_zero((uint64_t)Val);
     uint64_t ShiftedVal = (uint64_t)Val << LeadingZeros;
     // Fill in the bits that will be shifted out with 1s. An example where this
     // helps is trailing one masks with 32 or more ones. This will generate
     // ADDI -1 and an SRLI.
     ShiftedVal |= maskTrailingOnes<uint64_t>(LeadingZeros);
 
     RISCVMatInt::InstSeq TmpSeq;
if (LeadingZeros == 32 && ActiveFeatures[RISCV::FeatureStdExtZba]) {
       // Keep the new sequence if it is an improvement.
       if (TmpSeq.size() < Res.size())
         Res = TmpSeq;
     }
   }
 
   // Perform optimization with BCLRI/BSETI in the Zbs extension.
   if (Res.size() > 2 && ActiveFeatures[RISCV::FeatureStdExtZbs]) {
-    assert(ActiveFeatures[RISCV::Feature64Bit] &&
-           "Expected RV32 to only need 2 instructions");
-
     // 1. For values in range 0xffffffff 7fffffff ~ 0xffffffff 00000000,
     //    call generateInstSeqImpl with Val|0x80000000 (which is expected be
     //    an int32), then emit (BCLRI r, 31).
     // 2. For values in range 0x80000000 ~ 0xffffffff, call generateInstSeqImpl
     //    with Val&~0x80000000 (which is expected to be an int32), then
     //    emit (BSETI r, 31).
     int64_t NewVal;
     unsigned Opc;
if (Opc > 0) {
       }
       if (TmpSeq.size() < Res.size())
         Res = TmpSeq;
     }
   }
 
   // Perform optimization with SH*ADD in the Zba extension.
   if (Res.size() > 2 && ActiveFeatures[RISCV::FeatureStdExtZba]) {
-    assert(ActiveFeatures[RISCV::Feature64Bit] &&
-           "Expected RV32 to only need 2 instructions");
     int64_t Div = 0;
     unsigned Opc = 0;
     RISCVMatInt::InstSeq TmpSeq;
     // Select the opcode and divisor.
     if ((Val % 3) == 0 && isInt<32>(Val / 3)) {
       Div = 3;
       Opc = RISCV::SH1ADD;
     } else if ((Val % 5) == 0 && isInt<32>(Val / 5)) {