[RISC-V] Fix initReg usage in genPushCalleeSavedRegisters (#99353)

Author: sirntar

src/coreclr/jit/codegenriscv64.cpp

@@ -7782,6 +7782,11 @@ void CodeGen::genPushCalleeSavedRegisters(regNumber initReg, bool* pInitRegZeroe
 {
     assert(compiler->compGeneratingProlog);
 
+    // The 'initReg' could have been calculated as one of the callee-saved registers (let's say T0, T1 and T2 are in
+    // use, so the next possible register is S1, which should be callee-save register). This is fine, as long as we
+    // save callee-saved registers before using 'initReg' for the first time. Instead, we can use REG_SCRATCH
+    // beforehand. We don't care if REG_SCRATCH will be overwritten, so we'll skip 'RegZeroed check'.
+    //
     // Unlike on x86/x64, we can also push float registers to stack
     regMaskTP rsPushRegs = regSet.rsGetModifiedRegsMask() & RBM_CALLEE_SAVED;
 
@@ -7894,11 +7899,11 @@ void CodeGen::genPushCalleeSavedRegisters(regNumber initReg, bool* pInitRegZeroe
             calleeSaveSPDelta = AlignUp((UINT)offset, STACK_ALIGN);
             offset            = calleeSaveSPDelta - offset;
 
-            genStackPointerAdjustment(-calleeSaveSPDelta, initReg, pInitRegZeroed, /* reportUnwindData */ true);
+            genStackPointerAdjustment(-calleeSaveSPDelta, REG_SCRATCH, nullptr, /* reportUnwindData */ true);
         }
         else
         {
-            genStackPointerAdjustment(-totalFrameSize, initReg, pInitRegZeroed, /* reportUnwindData */ true);
+            genStackPointerAdjustment(-totalFrameSize, REG_SCRATCH, nullptr, /* reportUnwindData */ true);
         }
     }
 
@@ -7907,6 +7912,8 @@ void CodeGen::genPushCalleeSavedRegisters(regNumber initReg, bool* pInitRegZeroe
     genSaveCalleeSavedRegistersHelp(rsPushRegs, offset, 0);
     offset += (int)(genCountBits(rsPushRegs) << 3); // each reg has 8 bytes
 
+    // From now on, we can safely use initReg.
+
     emit->emitIns_R_R_I(INS_sd, EA_PTRSIZE, REG_RA, REG_SPBASE, offset);
     compiler->unwindSaveReg(REG_RA, offset);