[RISC-V] Use common CodeGen::genHomeRegisterParams (#101288)

* Code review from #101114

* Use common genHomeRegisterParams on RISC-V

* Make passSlot integer-only because we know hardware floating-point calling convention passes in registers only

* Make a RISC-V specific routine for homing stack parts of split parameters.

* Move genHomeStackPartOfSplitParameter out of genHomeSwiftStructParameters, share stack segment homing with Swift code

src/coreclr/jit/abi.cpp

@@ -258,14 +258,16 @@ bool ABIPassingInformation::HasExactlyOneStackSegment() const
 //
 bool ABIPassingInformation::IsSplitAcrossRegistersAndStack() const
 {
-    bool anyReg   = false;
-    bool anyStack = false;
-    for (unsigned i = 0; i < NumSegments; i++)
+    if (NumSegments < 2)
+        return false;
+
+    bool isFirstInReg = Segments[0].IsPassedInRegister();
+    for (unsigned i = 1; i < NumSegments; i++)
     {
-        anyReg |= Segments[i].IsPassedInRegister();
-        anyStack |= Segments[i].IsPassedOnStack();
+        if (isFirstInReg != Segments[i].IsPassedInRegister())
+            return true;
     }
-    return anyReg && anyStack;
+    return false;
 }
 
 //-----------------------------------------------------------------------------

src/coreclr/jit/abi.h

@@ -51,8 +51,8 @@ struct ABIPassingInformation
     // - On loongarch64/riscv64, structs can be passed in two registers or
     // can be split out over register and stack, giving
     // multiple register segments and a struct segment.
-    unsigned           NumSegments = 0;
-    ABIPassingSegment* Segments    = nullptr;
+    unsigned           NumSegments;
+    ABIPassingSegment* Segments;
 
     ABIPassingInformation(unsigned numSegments = 0, ABIPassingSegment* segments = nullptr)
         : NumSegments(numSegments)

src/coreclr/jit/codegen.h

@@ -284,7 +284,9 @@ class CodeGen final : public CodeGenInterface
     void genEnregisterOSRArgsAndLocals();
 #endif
 
+    void genHomeStackSegment(unsigned lclNum, const ABIPassingSegment& seg, regNumber initReg, bool* pInitRegZeroed);
     void genHomeSwiftStructParameters(bool handleStack);
+    void genHomeStackPartOfSplitParameter(regNumber initReg, bool* initRegStillZeroed);
 
     void genCheckUseBlockInit();
 #if defined(UNIX_AMD64_ABI) && defined(FEATURE_SIMD)

src/coreclr/jit/codegencommon.cpp

@@ -2802,7 +2802,6 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 */
 
-#if !defined(TARGET_RISCV64)
 struct RegNode;
 
 struct RegNodeEdge
@@ -3346,8 +3345,6 @@ void CodeGen::genHomeRegisterParams(regNumber initReg, bool* initRegStillZeroed)
     }
 }
 
-#endif
-
 // -----------------------------------------------------------------------------
 // genGetParameterHomingTempRegisterCandidates: Get the registers that are
 // usable during register homing.
@@ -4122,15 +4119,69 @@ void CodeGen::genEnregisterOSRArgsAndLocals()
     }
 }
 
+#if defined(SWIFT_SUPPORT) || defined(TARGET_RISCV64)
+//-----------------------------------------------------------------------------
+// genHomeSwiftStructParameters: Move the incoming segment to the local stack frame.
+//
+// Arguments:
+//    lclNum - Number of local variable to home
+//    seg - Stack segment of the local variable to home
+//    initReg - Scratch register to use if needed
+//    initRegStillZeroed - Set to false if the scratch register was needed
+//
+void CodeGen::genHomeStackSegment(unsigned                 lclNum,
+                                  const ABIPassingSegment& seg,
+                                  regNumber                initReg,
+                                  bool*                    initRegStillZeroed)
+{
+    var_types loadType = TYP_UNDEF;
+    switch (seg.Size)
+    {
+        case 1:
+            loadType = TYP_UBYTE;
+            break;
+        case 2:
+            loadType = TYP_USHORT;
+            break;
+        case 3:
+        case 4:
+            loadType = TYP_INT;
+            break;
+        case 5:
+        case 6:
+        case 7:
+        case 8:
+            loadType = TYP_LONG;
+            break;
+        default:
+            assert(!"Unexpected segment size for struct parameter not passed implicitly by ref");
+            return;
+    }
+    emitAttr size = emitTypeSize(loadType);
+
+    int loadOffset =
+        -(isFramePointerUsed() ? genCallerSPtoFPdelta() : genCallerSPtoInitialSPdelta()) + (int)seg.GetStackOffset();
+#ifdef TARGET_XARCH
+    GetEmitter()->emitIns_R_AR(ins_Load(loadType), size, initReg, genFramePointerReg(), loadOffset);
+#else
+    genInstrWithConstant(ins_Load(loadType), size, initReg, genFramePointerReg(), loadOffset, initReg);
+#endif
+    GetEmitter()->emitIns_S_R(ins_Store(loadType), size, initReg, lclNum, seg.Offset);
+
+    if (initRegStillZeroed)
+        *initRegStillZeroed = false;
+}
+#endif // defined(SWIFT_SUPPORT) || defined(TARGET_RISCV64)
+
 #ifdef SWIFT_SUPPORT
 
 //-----------------------------------------------------------------------------
 // genHomeSwiftStructParameters:
-//  Reassemble Swift struct parameters if necessary.
+//    Reassemble Swift struct parameters if necessary.
 //
-// Parameters:
-//   handleStack - If true, reassemble the segments that were passed on the stack.
-//                 If false, reassemble the segments that were passed in registers.
+// Arguments:
+//    handleStack - If true, reassemble the segments that were passed on the stack.
+//                  If false, reassemble the segments that were passed in registers.
 //
 void CodeGen::genHomeSwiftStructParameters(bool handleStack)
 {
@@ -4176,59 +4227,54 @@ void CodeGen::genHomeSwiftStructParameters(bool handleStack)
             }
             else
             {
-                var_types loadType = TYP_UNDEF;
-                switch (seg.Size)
-                {
-                    case 1:
-                        loadType = TYP_UBYTE;
-                        break;
-                    case 2:
-                        loadType = TYP_USHORT;
-                        break;
-                    case 3:
-                    case 4:
-                        loadType = TYP_INT;
-                        break;
-                    case 5:
-                    case 6:
-                    case 7:
-                    case 8:
-                        loadType = TYP_LONG;
-                        break;
-                    default:
-                        assert(!"Unexpected segment size for struct parameter not passed implicitly by ref");
-                        continue;
-                }
+                // We can use REG_SCRATCH as a temporary register here as we ensured that during LSRA build.
+                genHomeStackSegment(lclNum, seg, REG_SCRATCH, nullptr);
+            }
+        }
+    }
+}
+#endif
 
-                int offset;
-                if (isFramePointerUsed())
-                {
-                    offset = -genCallerSPtoFPdelta();
-                }
-                else
-                {
-                    offset = -genCallerSPtoInitialSPdelta();
-                }
+//-----------------------------------------------------------------------------
+// genHomeStackPartOfSplitParameter: Home the tail (stack) portion of a split parameter next to where the head
+// (register) portion is homed.
+//
+// Arguments:
+//    initReg - scratch register to use if needed
+//    initRegStillZeroed - set to false if scratch register was needed
+//
+// Notes:
+//    No-op on platforms where argument registers are already homed to form a contiguous space with incoming stack.
+//
+void CodeGen::genHomeStackPartOfSplitParameter(regNumber initReg, bool* initRegStillZeroed)
+{
+#ifdef TARGET_RISCV64
+    unsigned lclNum = 0;
+    for (; lclNum < compiler->info.compArgsCount; lclNum++)
+    {
+        LclVarDsc* var = compiler->lvaGetDesc(lclNum);
+        if (!var->lvIsSplit || !var->lvOnFrame)
+            continue;
 
-                offset += (int)seg.GetStackOffset();
+        JITDUMP("Homing stack part of split parameter V%02u\n", lclNum);
 
-                // Move the incoming segment to the local stack frame. We can
-                // use REG_SCRATCH as a temporary register here as we ensured
-                // that during LSRA build.
-#ifdef TARGET_XARCH
-                GetEmitter()->emitIns_R_AR(ins_Load(loadType), emitTypeSize(loadType), REG_SCRATCH,
-                                           genFramePointerReg(), offset);
-#else
-                genInstrWithConstant(ins_Load(loadType), emitTypeSize(loadType), REG_SCRATCH, genFramePointerReg(),
-                                     offset, REG_SCRATCH);
-#endif
+        assert(varTypeIsStruct(var));
+        assert(!compiler->lvaIsImplicitByRefLocal(lclNum));
+        const ABIPassingInformation& abiInfo = compiler->lvaGetParameterABIInfo(lclNum);
+        assert(abiInfo.NumSegments == 2);
+        assert(abiInfo.Segments[0].GetRegister() == REG_ARG_LAST);
+        const ABIPassingSegment& seg = abiInfo.Segments[1];
 
-                GetEmitter()->emitIns_S_R(ins_Store(loadType), emitTypeSize(loadType), REG_SCRATCH, lclNum, seg.Offset);
-            }
+        genHomeStackSegment(lclNum, seg, initReg, initRegStillZeroed);
+
+        for (lclNum += 1; lclNum < compiler->info.compArgsCount; lclNum++)
+        {
+            assert(!compiler->lvaGetDesc(lclNum)->lvIsSplit); // There should be only one split parameter
         }
+        break;
     }
-}
 #endif
+}
 
 /*-----------------------------------------------------------------------------
  *
@@ -5547,6 +5593,8 @@ void CodeGen::genFnProlog()
     {
         compiler->lvaUpdateArgsWithInitialReg();
 
+        genHomeStackPartOfSplitParameter(initReg, &initRegZeroed);
+
         if ((intRegState.rsCalleeRegArgMaskLiveIn | floatRegState.rsCalleeRegArgMaskLiveIn) != RBM_NONE)
         {
             genHomeRegisterParams(initReg, &initRegZeroed);