[release/6.0] Port #58410: Fix zero initialization use of "initReg" (#62207)

If the only local is an enregistered must-init struct,
we were setting `initReg` to its register (in this case, `xmm0`).
However, `initReg` is expected to be an integer register. In the
test case, with GS cookie stress, the GS cookie code asserted
that `initReg` was an integer register, but it wasn't.

The fix is the change the condition to use the actual register
assigned to the variable (in this case, `xmm0`), not the variable
type (here, `TYP_STRUCT`).

No spmi asm diffs.

Fixes #57911

Author: BruceForstall

src/coreclr/jit/codegencommon.cpp

@@ -486,13 +486,14 @@ regMaskTP CodeGenInterface::genGetRegMask(const LclVarDsc* varDsc)
 
     assert(varDsc->lvIsInReg());
 
-    if (varTypeUsesFloatReg(varDsc->TypeGet()))
+    regNumber reg = varDsc->GetRegNum();
+    if (genIsValidFloatReg(reg))
     {
-        regMask = genRegMaskFloat(varDsc->GetRegNum(), varDsc->TypeGet());
+        regMask = genRegMaskFloat(reg, varDsc->GetRegisterType());
     }
     else
     {
-        regMask = genRegMask(varDsc->GetRegNum());
+        regMask = genRegMask(reg);
     }
     return regMask;
 }
@@ -7148,8 +7149,9 @@ void CodeGen::genFnProlog()
 
         if (isInReg)
         {
-            regMaskTP regMask = genRegMask(varDsc->GetRegNum());
-            if (!varDsc->IsFloatRegType())
+            regNumber regForVar = varDsc->GetRegNum();
+            regMaskTP regMask   = genRegMask(regForVar);
+            if (!genIsValidFloatReg(regForVar))
             {
                 initRegs |= regMask;
 

src/coreclr/jit/codegenlinear.cpp

@@ -51,19 +51,16 @@ void CodeGen::genInitializeRegisterState()
             continue;
         }
 
-        // Is this a floating-point argument?
-        if (varDsc->IsFloatRegType())
+        if (varDsc->lvAddrExposed)
         {
             continue;
         }
 
-        noway_assert(!varTypeUsesFloatReg(varDsc->TypeGet()));
-
         // Mark the register as holding the variable
-        assert(varDsc->GetRegNum() != REG_STK);
-        if (!varDsc->lvAddrExposed)
+        regNumber reg = varDsc->GetRegNum();
+        if (genIsValidIntReg(reg))
         {
-            regSet.verifyRegUsed(varDsc->GetRegNum());
+            regSet.verifyRegUsed(reg);
         }
     }
 }

src/coreclr/jit/compiler.cpp

@@ -2170,7 +2170,7 @@ VarName Compiler::compVarName(regNumber reg, bool isFloatReg)
             /* If the variable is not in a register, or not in the register we're looking for, quit. */
             /* Also, if it is a compiler generated variable (i.e. slot# > info.compVarScopesCount), don't bother. */
             if ((varDsc->lvRegister != 0) && (varDsc->GetRegNum() == reg) &&
-                (varDsc->IsFloatRegType() || !isFloatReg) && (varDsc->lvSlotNum < info.compVarScopesCount))
+                (varDsc->lvSlotNum < info.compVarScopesCount))
             {
                 /* check if variable in that register is live */
                 if (VarSetOps::IsMember(this, compCurLife, varDsc->lvVarIndex))

src/coreclr/jit/compiler.h

@@ -961,10 +961,6 @@ class LclVarDsc
                     Compiler*            pComp,
                     RefCountState        state     = RCS_NORMAL,
                     bool                 propagate = true);
-    bool IsFloatRegType() const
-    {
-        return varTypeUsesFloatReg(lvType) || lvIsHfaRegArg();
-    }
 
     var_types GetHfaType() const
     {