[RISCV] Update comments in RISCVMatInt to reflect we don't always use ADDIW after LUI now. NFC (llvm#159829)

The simm32 base case only uses lui+addiw when necessary after
3d2650b

The worst case 8 instruction sequence doesn't leave a full 32 bits for
the LUI+ADDI(W) after the 3 12-bit ADDI and SLLI pairs are created. So
we will never generate LUI+ADDIW in the worst case sequence.

Author: topperc

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

@@ -111,18 +111,18 @@ static void generateInstSeqImpl(int64_t Val, const MCSubtargetInfo &STI,
   assert(IsRV64 && "Can't emit >32-bit imm for non-RV64 target");
 
   // In the worst case, for a full 64-bit constant, a sequence of 8 instructions
-  // (i.e., LUI+ADDIW+SLLI+ADDI+SLLI+ADDI+SLLI+ADDI) has to be emitted. Note
-  // that the first two instructions (LUI+ADDIW) can contribute up to 32 bits
+  // (i.e., LUI+ADDI+SLLI+ADDI+SLLI+ADDI+SLLI+ADDI) has to be emitted. Note
+  // that the first two instructions (LUI+ADDI) can contribute up to 32 bits
   // while the following ADDI instructions contribute up to 12 bits each.
   //
   // On the first glance, implementing this seems to be possible by simply
-  // emitting the most significant 32 bits (LUI+ADDIW) followed by as many left
-  // shift (SLLI) and immediate additions (ADDI) as needed. However, due to the
-  // fact that ADDI performs a sign extended addition, doing it like that would
-  // only be possible when at most 11 bits of the ADDI instructions are used.
-  // Using all 12 bits of the ADDI instructions, like done by GAS, actually
-  // requires that the constant is processed starting with the least significant
-  // bit.
+  // emitting the most significant 32 bits (LUI+ADDI(W)) followed by as many
+  // left shift (SLLI) and immediate additions (ADDI) as needed. However, due to
+  // the fact that ADDI performs a sign extended addition, doing it like that
+  // would only be possible when at most 11 bits of the ADDI instructions are
+  // used. Using all 12 bits of the ADDI instructions, like done by GAS,
+  // actually requires that the constant is processed starting with the least
+  // significant bit.
   //
   // In the following, constants are processed from LSB to MSB but instruction
   // emission is performed from MSB to LSB by recursively calling
@@ -145,7 +145,7 @@ static void generateInstSeqImpl(int64_t Val, const MCSubtargetInfo &STI,
     Val >>= ShiftAmount;
 
     // If the remaining bits don't fit in 12 bits, we might be able to reduce
-    // the // shift amount in order to use LUI which will zero the lower 12
+    // the shift amount in order to use LUI which will zero the lower 12
     // bits.
     if (ShiftAmount > 12 && !isInt<12>(Val)) {
       if (isInt<32>((uint64_t)Val << 12)) {
@@ -344,8 +344,9 @@ InstSeq generateInstSeq(int64_t Val, const MCSubtargetInfo &STI) {
 
   // Perform optimization with BSETI in the Zbs extension.
   if (Res.size() > 2 && STI.hasFeature(RISCV::FeatureStdExtZbs)) {
-    // Create a simm32 value for LUI+ADDIW by forcing the upper 33 bits to zero.
-    // Xor that with original value to get which bits should be set by BSETI.
+    // Create a simm32 value for LUI+ADDI(W) by forcing the upper 33 bits to
+    // zero. Xor that with original value to get which bits should be set by
+    // BSETI.
     uint64_t Lo = Val & 0x7fffffff;
     uint64_t Hi = Val ^ Lo;
     assert(Hi != 0);
@@ -372,8 +373,8 @@ InstSeq generateInstSeq(int64_t Val, const MCSubtargetInfo &STI) {
 
   // Perform optimization with BCLRI in the Zbs extension.
   if (Res.size() > 2 && STI.hasFeature(RISCV::FeatureStdExtZbs)) {
-    // Create a simm32 value for LUI+ADDIW by forcing the upper 33 bits to one.
-    // Xor that with original value to get which bits should be cleared by
+    // Create a simm32 value for LUI+ADDI(W) by forcing the upper 33 bits to
+    // one. Xor that with original value to get which bits should be cleared by
     // BCLRI.
     uint64_t Lo = Val | 0xffffffff80000000;
     uint64_t Hi = Val ^ Lo;