[AsmPrinter] Fix crash in handleIndirectSymViaGOTPCRel

[bruno]

Summary

Check for symbols in MCValue before using them. Bail out early in case they are null. Fix for PR23779.
Rafael, the changes in this patch checks all the possibilities, but do you know if it's possible for evaluateAsRelocatable to return a MCValue with a null SymbolA?

Thanks,

[bruno]

This was done a long time ago -> r240649

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

static void handleIndirectSymViaGOTPCRel(AsmPrinter &AP, const MCExpr **ME,
   // After canonicalization by evaluateAsRelocatable `ME` turns into:
   //
   //  cstexpr := <gotequiv> - <foo> + gotpcrelcst, where
   //    gotpcrelcst := <offset from @foo base> + <cst>
   //
   MCValue MV;
   if (!(*ME)->evaluateAsRelocatable(MV, nullptr, nullptr) || MV.isAbsolute())
     return;
+  const MCSymbolRefExpr *SymA = MV.getSymA();
+  if (!SymA)
+    return;
 
-  const MCSymbol *GOTEquivSym = &MV.getSymA()->getSymbol();
+  // Check that GOT equivalent symbol is cached.
+  const MCSymbol *GOTEquivSym = &SymA->getSymbol();
   if (!AP.GlobalGOTEquivs.count(GOTEquivSym))
     return;
 
   const GlobalValue *BaseGV = dyn_cast<GlobalValue>(BaseCst);
   if (!BaseGV)
     return;
 
+  // Check for a valid base symbol
   const MCSymbol *BaseSym = AP.getSymbol(BaseGV);
-  if (BaseSym != &MV.getSymB()->getSymbol())
+  const MCSymbolRefExpr *SymB = MV.getSymB();
+
+  if (!SymB || BaseSym != &SymB->getSymbol())
     return;
 
   // Make sure to match:
   //
   //    gotpcrelcst := <offset from @foo base> + <cst>
   //
   // If gotpcrelcst is positive it means that we can safely fold the pc rel
   // displacement into the GOTPCREL. We can also can have an extra offset <cst>

test/MC/MachO/cstexpr-gotpcrel-64.ll

%x = add i32 trunc (i64 sub (i64 ptrtoint (i32** @bargotequiv to i64),
                            to i32), %a
   ret i32 %x
 }
 
 ; Also test direct instruction uses.
 define i32** @t1() {
   ret i32** @bargotequiv
 }
+
+; Do not crash when a pattern cannot be matched as a GOT equivalent
+
+@a = external global i8
+@b = internal unnamed_addr constant i8* @a
+
+; X86-LABEL: _c:
+; X86:   .quad _b
+@c = global i8** @b