JIT/x86: Fix reported kill regs for varargs prologs (#113793)

* x86: Fix reported kill regs for varargs prologs

Author: filipnavara

src/coreclr/jit/codegencommon.cpp

@@ -5924,11 +5924,11 @@ void CodeGen::genFnProlog()
 
         // MOV EAX, <VARARGS HANDLE>
         assert(compiler->lvaVarargsHandleArg == compiler->info.compArgsCount - 1);
-        GetEmitter()->emitIns_R_S(ins_Load(TYP_I_IMPL), EA_PTRSIZE, REG_EAX, compiler->lvaVarargsHandleArg, 0);
-        regSet.verifyRegUsed(REG_EAX);
+        GetEmitter()->emitIns_R_S(ins_Load(TYP_I_IMPL), EA_PTRSIZE, REG_SCRATCH, compiler->lvaVarargsHandleArg, 0);
+        regSet.verifyRegUsed(REG_SCRATCH);
 
         // MOV EAX, [EAX]
-        GetEmitter()->emitIns_R_AR(ins_Load(TYP_I_IMPL), EA_PTRSIZE, REG_EAX, REG_EAX, 0);
+        GetEmitter()->emitIns_R_AR(ins_Load(TYP_I_IMPL), EA_PTRSIZE, REG_SCRATCH, REG_SCRATCH, 0);
 
         // EDX might actually be holding something here.  So make sure to only use EAX for this code
         // sequence.
@@ -5940,16 +5940,16 @@ void CodeGen::genFnProlog()
         noway_assert(lastArg->lvFramePointerBased);
 
         // LEA EAX, &<VARARGS HANDLE> + EAX
-        GetEmitter()->emitIns_R_ARR(INS_lea, EA_PTRSIZE, REG_EAX, genFramePointerReg(), REG_EAX, offset);
+        GetEmitter()->emitIns_R_ARR(INS_lea, EA_PTRSIZE, REG_SCRATCH, genFramePointerReg(), REG_SCRATCH, offset);
 
         if (varDsc->lvIsInReg())
         {
-            GetEmitter()->emitIns_Mov(INS_mov, EA_PTRSIZE, varDsc->GetRegNum(), REG_EAX, /* canSkip */ true);
+            GetEmitter()->emitIns_Mov(INS_mov, EA_PTRSIZE, varDsc->GetRegNum(), REG_SCRATCH, /* canSkip */ true);
             regSet.verifyRegUsed(varDsc->GetRegNum());
         }
         else
         {
-            GetEmitter()->emitIns_S_R(ins_Store(TYP_I_IMPL), EA_PTRSIZE, REG_EAX, argsStartVar, 0);
+            GetEmitter()->emitIns_S_R(ins_Store(TYP_I_IMPL), EA_PTRSIZE, REG_SCRATCH, argsStartVar, 0);
         }
     }
 

src/coreclr/jit/lsrabuild.cpp

@@ -2448,7 +2448,13 @@ void LinearScan::buildIntervals()
         // do that in the prolog. We handle registers in the prolog and the
         // stack args in the scratch BB that we have ensured exists. The
         // handling clobbers REG_SCRATCH, so kill it here.
-        if ((block == compiler->fgFirstBB) && compiler->lvaHasAnySwiftStackParamToReassemble())
+        bool prologUsesScratchReg = compiler->lvaHasAnySwiftStackParamToReassemble();
+#ifdef TARGET_X86
+        // On x86, CodeGen::genFnProlog does a varargs preprocessing that uses
+        // the scratch register.
+        prologUsesScratchReg |= compiler->info.compIsVarArgs;
+#endif
+        if ((block == compiler->fgFirstBB) && prologUsesScratchReg)
         {
             addKillForRegs(genRegMask(REG_SCRATCH), currentLoc + 1);
             currentLoc += 2;