[llvm-dev] tricky constraints in backend

[Daniel Sanders via llvm-dev]

Hi,

Internally, multi-character constraint codes need to begin with '^' but there's currently FIXME that will prevent your case from working. The code currently assumes that there's a maximum of two characters after the '^', so '^A3' would work but '^A32' would by the constraints 'A3' and '2'. You should be able to get your case working by fixing that FIXME and updating the documentation (http://llvm.org/docs/LangRef.html#constraint-codes) to match.

By the way, I noticed a mistake in that documentation while I was writing this. The documentation says 'a “^” character followed by two letters (e.g. “^wc”)' but the implementation doesn't restrict it to just letters.

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Konstantin Vladimirov via llvm-dev
> Sent: 09 March 2016 16:19
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] tricky constraints in backend
> 
> Hi,
> 
> I am working on custom LLVM 3.7 backend, already having GCC backend.
> In GCC backend there are perfectly legal address constraints like
> "A32", which are widely used in inline assembler.
> 
> I am trying to support those in LLVM, but there is a problem in
> llvm::InlineAsm::ConstraintInfo::Parse function.
> 
> it tries to understand A32 as two separate constraints: one-letter "A"
> and numeric "32", latter, of course, fails (numeric 32 constraint
> means "32-nd parameter", and there are no 32 parameters).
> 
> It is called very early, from llvm::InlineAsm::InlineAsm ctor, where
> assert fails for this reason, so even llvm IR is not emitted.
> 
> Is there a way to support multi-letter target-specific constraints
> with trailing digits?
> 
> ---
> WIth best regards, Konstantin
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev