[RISC-V] Optimize redundant sext.w generation in GT_JCMP codegen

src/coreclr/jit/codegenriscv64.cpp

@@ -3188,7 +3188,6 @@ void CodeGen::genCodeForCompare(GenTreeOp* tree)
                     regOp1 = tmpRegOp1;
                 }
             }
-
             if (tree->OperIs(GT_EQ, GT_NE))
             {
                 if ((imm != 0) || (cmpSize == EA_4BYTE))
@@ -3353,8 +3352,14 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
                     regNumber tmpRegOp1 = rsGetRsvdReg();
                     assert(regOp1 != tmpRegOp1);
                     imm = static_cast<int32_t>(imm);
-                    emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
-                    regOp1 = tmpRegOp1;
+
+                    // It might be possible to watch the type of op1 to decide the redundancy.
+                    // But due to the safety reason, only a peephole optimization is done now.
+                    if (!(emit->isRedundantSignExtend(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1)))
+                    {
+                        emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
+                        regOp1 = tmpRegOp1;
+                    }
                     break;
                 }
                 case EA_8BYTE:
@@ -3389,8 +3394,14 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
             {
                 regNumber tmpRegOp1 = rsGetRsvdReg();
                 assert(regOp1 != tmpRegOp1);
-                emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
-                regOp1 = tmpRegOp1;
+
+                // It might be possible to watch the type of op1 to decide the redundancy.
+                // But due to the safety reason, only a peephole optimization is done now.
+                if (!(emit->isRedundantSignExtend(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1)))
+                {
+                    emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
+                    regOp1 = tmpRegOp1;
+                }
             }
         }
 
@@ -3438,10 +3449,23 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
             regNumber tmpRegOp2 = rsGetRsvdReg();
             assert(regOp1 != tmpRegOp2);
             assert(regOp2 != tmpRegOp2);
-            emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
-            emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp2, regOp2);
-            regOp1 = tmpRegOp1;
-            regOp2 = tmpRegOp2;
+
+            // It might be possible to watch the type of op1 to decide the redundancy.
+            // But due to the safety reason, only a peephole optimization is done now.
+            bool signExtOp1 = !(emit->isRedundantSignExtend(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1));
+            bool signExtOp2 = !(emit->isRedundantSignExtend(INS_sext_w, EA_8BYTE, tmpRegOp2, regOp2));
+
+            if (signExtOp1)
+            {
+                emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp1, regOp1);
+                regOp1 = tmpRegOp1;
+            }
+
+            if (signExtOp2)
+            {
+                emit->emitIns_R_R(INS_sext_w, EA_8BYTE, tmpRegOp2, regOp2);
+                regOp2 = tmpRegOp2;
+            }
         }
 
         switch (cond.GetCode())

src/coreclr/jit/emitriscv64.cpp

@@ -640,6 +640,101 @@ void emitter::emitIns_Mov(emitAttr attr, regNumber dstReg, regNumber srcReg, boo
     }
 }
 
+//------------------------------------------------------------------------
+// emitInsIsSignExtend: Determines whether a given instruction sign extends
+//
+// Arguments:
+//    ins       -- The instruction being checked
+//
+bool emitter::emitInsIsSignExtend(instruction ins)
+{
+    switch (ins)
+    {
+        case INS_sext_w: // R_R
+        case INS_lui:    // R_I
+
+        // R_R_I
+        case INS_lb:
+        case INS_lh:
+        case INS_lw:
+
+        // R_R_I
+        case INS_addiw:
+        case INS_slliw:
+        case INS_srliw:
+        case INS_sraiw:
+
+        // R_R_R
+        case INS_addw:
+        case INS_subw:
+        case INS_sllw:
+        case INS_srlw:
+        case INS_sraw:
+
+        // R_R_R
+        case INS_mulw:
+        case INS_divw:
+        case INS_divuw:
+        case INS_remw:
+        case INS_remuw:
+        {
+            return true;
+        }
+        // TODO: Add more sign-extension instructions
+        default:
+        {
+            return false;
+        }
+    }
+}
+
+//------------------------------------------------------------------------
+// isRedundantSignExtend:
+//    Check if the current 'sext.w' instruction is redundant and can be omitted.
+//
+//    A 'sext.w' instruction is redundant if the previous instruction sign extends
+//    the source register of current instruction.
+//
+// Arguments:
+//    ins       -- The instruction being emitted
+//    attr      -- The emit attribute
+//    dst       -- The destination register
+//    src       -- The source register
+//
+bool emitter::isRedundantSignExtend(instruction ins, emitAttr size, regNumber dst, regNumber src)
+{
+    assert(ins == INS_sext_w);
+
+    if (!emitComp->opts.OptimizationEnabled())
+    {
+        return false;
+    }
+
+    const bool canOptimize = emitCanPeepholeLastIns();
+
+    if (!canOptimize)
+    {
+        return false;
+    }
+
+    regNumber prevDst  = emitLastIns->idReg1();
+    regNumber prevSrc  = emitLastIns->idReg2();
+    emitAttr  prevSize = emitLastIns->idOpSize();
+
+    bool isPrevInsSignExtend = emitInsIsSignExtend(emitLastIns->idIns());
+
+    if (isPrevInsSignExtend && (prevDst == src))
+    {
+        JITDUMP("\n -- suppressing 'sext.w reg%u, reg%u' as previous instruction already sign-extended "
+                "the register reg%u.\n",
+                dst, src, prevDst);
+
+        return true;
+    }
+
+    return false;
+}
+
 /*****************************************************************************
  *
  *  Add an instruction referencing two registers

src/coreclr/jit/emitriscv64.h

@@ -19,6 +19,8 @@ struct CnsVal
     bool    cnsReloc;
 };
 
+bool isRedundantSignExtend(instruction ins, emitAttr size, regNumber dst, regNumber src);
+
 #ifdef DEBUG
 
 /************************************************************************/
@@ -61,6 +63,7 @@ instrDesc* emitNewInstrCallInd(int              argCnt,
 bool emitInsIsLoad(instruction ins);
 bool emitInsIsStore(instruction ins);
 bool emitInsIsLoadOrStore(instruction ins);
+bool emitInsIsSignExtend(instruction ins);
 
 void emitDispInsName(
     code_t code, const BYTE* addr, bool doffs, unsigned insOffset, const instrDesc* id, const insGroup* ig);