[LLVMdev] ARM Jump table pcrelative relaxation in clang / llc

[Tim Northover]

Hi Eric,

On 5 July 2015 at 07:22, Eric Bentura <ebentura at gmail.com> wrote:
> As a result it seems that the generated machine code is incorrect (as of
> LLVM 3.5): The AsmPrinter generates the following instruction :
> adr r2, .LJTI4_0_0
> when going through the MC streamer, I get a "fatal error: error in backend:
> out of range pc-relative fixup" .

We've fairly recently fixed a bug that looks very similar (r238680,
which was well after 3.6)

> Apparently, the fixup does not hold in the 12 bits we have available. I
> would have expected clang to perform relaxation on this instruction on that
> particular case.

Agreed, whatever's going on it's a bug in the ARM backend.

> Is there a way in the PassManager::runOnFunction to anticipate that so that
> I can generate a IR code that would fit when converted to machine code?

Not as far as I'm aware; since the bug is further back there's no
reason to try and provide such information to earlier passes. The
backend is expected to cope with whatever you give it.

> Strangely enough, this is not happening when using llc to generate the code
> from the bc file, I get the object file.

That's weird. Even with "-filetype=obj" (the bug only occurs when
directly writing an object file)? Not that it really affects anything,
getting the same backend options with llc can be a bit tricky.

> Even though there have been
> improvements since them, I am concerned with the difference of behavior of
> the two tools.

The most common one I find perturbing output is "-mcpu" (even with a
triple), but really there are so many options front-ends can twiddle
that you just have to know what it's doing and copy that.

Cheers.

Tim.