[llvm-dev] getting code ranges of multiple blocks and prevent reordering?

[Reid Kleckner via llvm-dev]

The short answer is, no, there is no way to just ask LLVM not to reorder
things. There isn't a good principled way to do this, because it relies on
the assumption that LLVM will lay the code out the same way that you gave
it do it.

At source level, the user usually writes something like this establishing a
scoped try region:

try {
  *p = 42;
  if (p)
    a = fp_x / fp_y;
  if (a)
    might_throw(p, a);
} catch (...) {
}

In LLVM, the backend could easily decide to move that conditional code out
of line to the end of the function if its heuristics identify it as cold
code. That means that it's not sufficient to do things like emitting labels
in inline assembly at the try region start and try region end, or the
equivalent with new intrinsics.

To do this reliably, you need to insert labels around every potentially
throwing operation, which is what CodeGen does for the `invoke` LLVM IR
instruction. Then, after code layout is done, you can walk over the code to
find the labels you inserted before, and coalesce consecutive regions of
potentially throwing instructions. Typically, you build a table called an
LSDA, which contains relocations against your labels.

You can look at the code in llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp,
WinException.cpp, and DwarfCFIException.cpp to find code that implements
this logic.

On Tue, May 29, 2018 at 9:20 PM Jay K via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi. I'm very new to LLVM.
>
> For reasons to do with custom exception handling, we have a need to check
> IP/PC at runtime against code ranges. This can encompass multiple logically
> adjacent blocks.
>
> How to do this?
>
> I'm guessing:
>   insert a label at end of every block, takes it address, store that
> somewhere in our data; preferably as an offset from module or function
> start, but full address with relocation would work
>   take address of start of every block, similarly
>    That is not allowed as I understand for function-entry block, so add an
> extra dummy block as entry, that branches to actual start.
>
> And then, somehow, ask LLVM not to reorder anything, with functions with
> such labels?
>
> Does this make sense and can anyone fill in details?
>
> Also, of course some reordering would be ok, as long as start of first
> block is first, and end of last block is last. The order of the blocks
> between doesn't matter.
>
> I realize, there is another approach, something like assigning our own
> numbers  to scopes, have a local volatile integer, that we assign as we
> enter/leave scopes. i.e. "NT/x86 EH style". But that is a bigger and less
> efficient change compared to what we have (a non-LLVM
> less-optimizing codegen that lets us do what I describe).
>
> Thank you,
>  - Jay
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180530/5a35e49a/attachment.html>