[MIPS] Check if register is non-null before calling isSuperOrSubRegisterEq (NFCI)

[barannikov88]

Summary

D151036 adds an assertions that prohibits iterating over sub- and
super-registers of a null register. This is already the case when
iterating over register units of a null register, and worked by
accident for sub- and super-registers.

[barannikov88]

Rebase onto 8e16365c to make the bot green again

[wzssyqa]

I noticed there are some lines like:

2955     unsigned TmpReg = DstReg;
2956     if (UseSrcReg &&
2957         getContext().getRegisterInfo()->isSuperOrSubRegisterEq(DstReg,
2958                                                                SrcReg)) {
2959       // If $rs is the same as $rd, we need to use AT.
2960       // If it is not available we exit.
2961       unsigned ATReg = getATReg(IDLoc);
2962       if (!ATReg)
2963         return true;
2964       TmpReg = ATReg;
2965     }

Is UseSrcReg OK for here?

[barannikov88]

In D151288#4367391, @wzssyqa wrote:

I noticed there are some lines like:

2955     unsigned TmpReg = DstReg;
2956     if (UseSrcReg &&
2957         getContext().getRegisterInfo()->isSuperOrSubRegisterEq(DstReg,
2958                                                                SrcReg)) {
2959       // If $rs is the same as $rd, we need to use AT.
2960       // If it is not available we exit.
2961       unsigned ATReg = getATReg(IDLoc);
2962       if (!ATReg)
2963         return true;
2964       TmpReg = ATReg;
2965     }

Is UseSrcReg OK for here?

I honestly don't know :) Sounds reasonable, let's try it

[barannikov88]

Check UseSrcReg instead of SrcReg and rebase onto 8e16365c to make the bots happy

[wzssyqa]

I agree with this patch, while I have no approval permission.

@MaskRay

[barannikov88]

@wzssyqa Thanks for the quick review!

[barannikov88]

Whoops, I committed a change with wrong commit message referring to this review (copy&pasted) and the review was closed automatically >_<
Reopening

[barannikov88]

Revert to the correct revision

llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp

if (ABI.ArePtrs64bit() && isGP64bit()) {
     // source register.
 
     const MipsMCExpr *HighestExpr =
         MipsMCExpr::create(MipsMCExpr::MEK_HIGHEST, SymExpr, getContext());
     const MipsMCExpr *HigherExpr =
         MipsMCExpr::create(MipsMCExpr::MEK_HIGHER, SymExpr, getContext());
 
     bool RdRegIsRsReg =
+        SrcReg &&
         getContext().getRegisterInfo()->isSuperOrSubRegisterEq(DstReg, SrcReg);
 
     if (canUseATReg() && UseSrcReg && RdRegIsRsReg) {
       unsigned ATReg = getATReg(IDLoc);
 
       // If $rs is the same as $rd:
       // (d)la $rd, sym($rd) => lui    $at, %highest(sym)
       //                        daddiu $at, $at, %higher(sym)