[clang][alias fn attr] error: alias must point to a defined variable or function false negative when optimizations are enabled

[nickdesaulniers]

https://godbolt.org/z/TrGeazYc5

extern inline __attribute__((gnu_inline))
void bar (void) {}

void foo [[gnu::alias("_Z3barv")]] (void);
void foo2 (void) __attribute__((alias("_Z3barv")));
void foo3 (void) asm("_Z3barv");

<source>:4:12: error: alias must point to a defined variable or function
    4 | void foo [[gnu::alias("_Z3barv")]] (void);
      |            ^
<source>:4:12: note: the function or variable specified in an alias must refer to its mangled name
<source>:5:33: error: alias must point to a defined variable or function
    5 | void foo2 (void) __attribute__((alias("_Z3barv")));
      |                                 ^
<source>:5:33: note: the function or variable specified in an alias must refer to its mangled name

we only observe these diagnostics at -O0, not -O2.

via #60481

[shafik]

Worth noting this crashes on asserts build: https://godbolt.org/z/basjzojxE

Alias must point to a definition
ptr @_Z3foov
Alias must point to a definition
ptr @_Z4foo2v
fatal error: error in backend: Broken module found, compilation aborted!

Look related maybe duplicate: #69066

[shafik]

It looks like we diagnose this in codegen:

llvm-project/clang/lib/CodeGen/CodeGenModule.cpp

Lines 591 to 594 in adc4f62

	if (GV->isDeclaration()) {
	Diags.Report(Location, diag::err_alias_to_undefined) << IsIFunc << IsIFunc;
	Diags.Report(Location, diag::note_alias_requires_mangled_name)
	<< IsIFunc << IsIFunc;

and:

llvm-project/clang/lib/CodeGen/CodeGenModule.cpp

Lines 612 to 619 in adc4f62

	if (IsIFunc) {
	// Check resolver function type.
	const auto *F = dyn_cast<llvm::Function>(GV);
	if (!F) {
	Diags.Report(Location, diag::err_alias_to_undefined)
	<< IsIFunc << IsIFunc;
	return false;
	}

I think then it makes more sense now that this is optimization sensitive.

CC @AaronBallman @erichkeane

[AaronBallman]

Both of those checks should always be called:

llvm-project/clang/lib/CodeGen/CodeGenModule.cpp

Line 832 in dc20a0e

void CodeGenModule::Release() {

calls checkAliases() which has no optimization-specific logic. So perhaps optimizations are removing some aliases from the list?