[LLVMdev] ARM Jump table pcrelative relaxation in clang (original) (raw)

Eric Bentura ebentura at gmail.com
Sun Jul 5 23🔞59 PDT 2015

Hi Tim, Thank you for your answer.

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

If I wanted to back port that to 3.5 where should I look at? Where in the ARM backend the decision to relax an instruction is taken?

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

This is passing even with -filetype=obj. The transformation I apply are in the optimizer so I must build the new bc to create the object file.

Thanks for your help

Eric

Le dimanche 5 juillet 2015, Tim Northover <t.p.northover at gmail.com> a écrit :

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, .LJTI400 > 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. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150706/0547fdc8/attachment.html>

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