[AArch64][PAC] Do not fold ptrauth wrappers corresponding to extern_weak

When parsing IR, we do some optimizations very early - for example, the
`constantFoldCompareGlobalToNull` function folds comparisons against
null pointer. If the value has `extern_weak` linkage or if it is an
llvm.ptrauth wrapper corresponding to a symbol with `extern_weak`
linkage, the folding is prevented.

When the llvm.ptrauth wrapper was defined after its usage in such
condition, its uses (including the use in the condition) were updated
when the wrapper definition was found, but before its section was
parsed. Since the "llvm.ptrauth" section name indicates that the value
is an llvm.ptrauth wrapper, not having it parsed before updating usages
made the folding code think that this is not an llvm.ptrauth wrapper,
which led to undesired folding.

This patch moves the code updating uses of the global value after the
code parsing its section name.

Author: kovdan01

llvm/lib/AsmParser/LLParser.cpp

@@ -1256,16 +1256,6 @@ bool LLParser::parseGlobal(const std::string &Name, LocTy NameLoc,
   GV->setThreadLocalMode(TLM);
   GV->setUnnamedAddr(UnnamedAddr);
 
-  if (GVal) {
-    if (GVal->getAddressSpace() != AddrSpace)
-      return error(
-          TyLoc,
-          "forward reference and definition of global have different types");
-
-    GVal->replaceAllUsesWith(GV);
-    GVal->eraseFromParent();
-  }
-
   // parse attributes on the global.
   while (Lex.getKind() == lltok::comma) {
     Lex.Lex();
@@ -1303,6 +1293,16 @@ bool LLParser::parseGlobal(const std::string &Name, LocTy NameLoc,
     }
   }
 
+  if (GVal) {
+    if (GVal->getAddressSpace() != AddrSpace)
+      return error(
+          TyLoc,
+          "forward reference and definition of global have different types");
+
+    GVal->replaceAllUsesWith(GV);
+    GVal->eraseFromParent();
+  }
+
   AttrBuilder Attrs(M->getContext());
   LocTy BuiltinLoc;
   std::vector<unsigned> FwdRefAttrGrps;

llvm/test/CodeGen/AArch64/GlobalISel/ptrauth-elf-reloc.ll

@@ -106,10 +106,6 @@ define i8* @test_global_weak() #0 {
   ret i8* %tmp0
 }
 
-; FIXME: if we define this after function definition, the function body
-; transorms to a single call to foo_weak via bl.
-@foo_weak.ptrauth.ia.0 = private constant { ptr, i32, i64, i64 } { ptr @foo_weak, i32 0, i64 0, i64 0 }, section "llvm.ptrauth"
-
 define void @test_foo_weak() #0 {
 ; CHECK-LABEL: test_foo_weak:
 ; CHECK:       // %bb.0:
@@ -152,8 +148,6 @@ define i8* @test_global_strong_def() #0 {
 
 @g_weak = extern_weak global i32
 
-declare extern_weak void @foo_weak() #0
-
 ; For ELF, we should specify dso_local explicitly for strong definition, otherwise the symbol would be assumed preemptible, and GOT load would be used
 @g_strong_def = dso_local constant i32 42
 
@@ -234,6 +228,10 @@ declare extern_weak void @foo_weak() #0
 ; CHECK-NEXT:  g_strong_def.ref.da.0:
 ; CHECK-NEXT:    .xword g_strong_def@AUTH(da,0)
 
+declare extern_weak void @foo_weak() #0
+
+@foo_weak.ptrauth.ia.0 = private constant { ptr, i32, i64, i64 } { ptr @foo_weak, i32 0, i64 0, i64 0 }, section "llvm.ptrauth"
+
 @g_strong_def.ptrauth.da.0 = private constant { i8*, i32, i64, i64 } { i8* bitcast (i32* @g_strong_def to i8*), i32 2, i64 0, i64 0 }, section "llvm.ptrauth"
 
 @g_strong_def.ref.da.0 = dso_local constant i8* bitcast ({ i8*, i32, i64, i64 }* @g_strong_def.ptrauth.da.0 to i8*)