[PAC][CodeGen] Fix handling of extern_weak symbols with signed GOT

Before executing auth instruction on a signed address from a GOT slot,
we must ensure that the address is not zero since zero addresses are not
signed to preserve zero-checks working as expected. A GOT slot might be
filled with zero when we have an undefined weak symbol.

Author: kovdan01

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -2300,6 +2300,15 @@ void AArch64AsmPrinter::LowerLOADgotAUTH(const MachineInstr &MI) {
                      .addImm(0));
 
   assert(GAMO.isGlobal());
+  MCSymbol *UndefWeakSym;
+  if (GAMO.getGlobal()->hasExternalWeakLinkage()) {
+    UndefWeakSym = createTempSymbol("undef_weak");
+    EmitToStreamer(
+        MCInstBuilder(AArch64::CBZX)
+            .addReg(DstReg)
+            .addExpr(MCSymbolRefExpr::create(UndefWeakSym, OutContext)));
+  }
+
   assert(GAMO.getGlobal()->getValueType() != nullptr);
   unsigned AuthOpcode = GAMO.getGlobal()->getValueType()->isFunctionTy()
                             ? AArch64::AUTIA
@@ -2308,6 +2317,9 @@ void AArch64AsmPrinter::LowerLOADgotAUTH(const MachineInstr &MI) {
                      .addReg(DstReg)
                      .addReg(DstReg)
                      .addReg(AArch64::X16));
+
+  if (GAMO.getGlobal()->hasExternalWeakLinkage())
+    OutStreamer->emitLabel(UndefWeakSym);
 }
 
 const MCExpr *

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -1945,7 +1945,7 @@ let Predicates = [HasPAuth] in {
   def LOADgotAUTH : Pseudo<(outs GPR64common:$dst), (ins i64imm:$addr), []>,
                Sched<[WriteI, ReadI]> {
     let Defs = [X16];
-    let Size = 16;
+    let Size = 20;
   }
 
   // Load a signed global address from a special $auth_ptr$ stub slot.

llvm/test/CodeGen/AArch64/ptrauth-extern-weak.ll

@@ -19,7 +19,9 @@ define ptr @foo() {
 ; CHECK:         adrp  x16, :got_auth:var
 ; CHECK-NEXT:    add   x16, x16, :got_auth_lo12:var
 ; CHECK-NEXT:    ldr   x0,  [x16]
+; CHECK-NEXT:    cbz   x0, .Lundef_weak0
 ; CHECK-NEXT:    autia x0,  x16
+; CHECK-NEXT:  .Lundef_weak0:
 ; CHECK-NEXT:    ret
 }
 
@@ -33,7 +35,9 @@ define ptr @bar() {
 ; CHECK:         adrp  x16, :got_auth:arr_var
 ; CHECK-NEXT:    add   x16, x16, :got_auth_lo12:arr_var
 ; CHECK-NEXT:    ldr   x8,  [x16]
+; CHECK-NEXT:    cbz   x8, .Lundef_weak1
 ; CHECK-NEXT:    autda x8,  x16
+; CHECK-NEXT:  .Lundef_weak1:
 ; CHECK-NEXT:    add   x0,  x8, #20
 ; CHECK-NEXT:    ret
 }