Unrecognized opcodes in module level assembly

[mlund]

Using non-6502 in module level assembler code I get the following compilation error. Inline and stand-alone assembly is OK.

error: instruction requires a CPU feature not currently enabled
  ldz #0

when using e.g. mos-mega65-clang++ -mcpu=mos45gs02 asm.cc and the snippet shown below.

// asm.cc
asm (
 ".text\n"
 ".global func\n"
 "func:\n"
 "ldz #0\n" // not ok
 "rts"
);
void func();

int main() {
  asm volatile ("ldz #0"); // ok
}

[mlund]

This does seem to compile in Compiler Explorer. Not sure if this is related to my setup (git checkout on 15/9).

[mysterymath]

Did a little digging on this, it looks like this may actually be a LTO bug upstream. They're not instantiating the target CPU when scanning the inline assembly for symbols before LTO occurs (it's the first "").

https://github.com/llvm/llvm-project/blob/eb38edc82850c34bbbda7f7ce2235ec882b7d398/llvm/lib/Object/ModuleSymbolTable.cpp#L91

I'll start looking for an existing issue; if I can't find one, I'll try to make a X86 reproducer and file it upstream.

[mysterymath]

Filed llvm/llvm-project#67698 for this upstream.

[mysterymath]

So, I did some digging into this upstream, and it looks like this error is partially spurious; the error is emitted, but the compile and link actually does eventually succeed. The only actual effects of this bug, other than the error message, is that module-level inline assembly is no longer scanned for symbol references to feed into the LTO symbol resolution table. This is already the situation for regular inline assembly, so it isn't a tremendous loss.

It's in a bit of a sticky situation upstream; a lot of code tends to assume that you can scan module-level inline assembly for symbols without knowing the CPU and feature set, and there are tools that operate on bitcode that couldn't possibly have that information, too.

[mysterymath]

Taking this down to p2; it doesn't look like it's likely to be hard blocker on anything, just surprising and annoying, and it's architecturally quite difficult to fix.