[cfe-dev] Determining if it is a bug in Clang or my code

[Reid Kleckner]

Can you see if the the invalid opcode is ud2a? Clang sometimes emits those
after encountering certain kinds of UB.

I think the most common is falling off the end of a function that is
supposed to return a value. If you compile your code with -Wreturn-type (it
should be on by default), you should see a warning for it, but not if the
code is in a system header. There are other more obscure ways to trigger
it, like passing non-POD objects through a vararg pack, but my money is on
-Wreturn-type.

On Thu, Apr 30, 2015 at 12:31 PM, Chris Smith <chrsmith at google.com> wrote:

> Hello,
>
> I've been working on an operating-system as a hobby project for a few
> months now, and finally tried converting the codebase to Clang. While the
> code compiles fine, I now get a surprising Interrupt 0x06 "Invalid Opcode"
> being fired when executing some C++ code. (Running either under bochs and
> qemu.)
>
> The same codebase works fine when compiled under GCC, the faulty code(?)
> only happens when built under Clang. The part of the code isn't invoking
> any assembly (inline or otherwise), and the C++ itself is fairly straight
> forward. (See below.)
>
> My questions are:
>
> - Is the fact this interrupt firing while executing pure C++ code proof of
> a compiler bug? Or is it possible to generate invalid opcodes through using
> undefined C++ behavior, etc.
>
> - How likely is it that this is actually a Clang codegen bug? I worked on
> the F# compiler at Microsoft, and know quite well that "I found a bug in
> the compiler" is latin for "I don't understand how this language works";
> though the fact the code is triggering a CPU interrupt is concerning.
>
> - Would it be worth while to distill my os-project down and try to produce
> a minimal repro? If so, where should I send it?
>
> As for the code itself, the problem seems to be occurring in my
> implementation of printf. I'm using variadic template arguments to do it in
> a typesafe way. Is "variadic template codegen for 32-bit" a particularly
> rough area of the Clang/LLVM codebase?
>
> Any insight would be appreciated.
>
> Thanks,
> -Chris
>
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>
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