[LLVMdev] building a jump table in LLVM IR

Tom Roeder tmroeder at google.com
Thu Jan 3 22:49:37 PST 2013

Thanks for the followup.

If I understand the suggestion correctly, this doesn't solve the
problem of building a jump table to call into other functions, since,
as you note, indirectbr can only call into blocks in the same
function. Is the conclusion then that there is no way to do this in
LLVM IR? It looks like these kind of restrictions (no branching
between functions and no instructions outside of functions) are
designed into the structure of LLVM IR. Is this correct?



On Thu, Jan 3, 2013 at 6:50 AM, Joshua Cranmer <pidgeot18 at gmail.com> wrote:
> On 1/2/2013 11:05 AM, Tom Roeder wrote:
>> Hi,
>> I'm currently writing an opt module for fast indirect call checks
>> using a table of allowed indirect call targets. The code replaces
>> function pointers with offsets into the table then masks the offset
>> for the table size and restores the function pointer before the call.
>> I have some ways of dealing with some kinds of external code that are
>> sufficient for my use case but not for more general use.
>> I'd like to generalize the table to be a table of jump instructions (a
>> classic jump table) rather than just a table of function pointers,
>> since this would solve some of my problems for the more general cases.
>> I know how to do this using inline assembly in LLVM, but this ties the
>> module to a particular architecture or set of architectures.
>> Is there a way to build a table of jump instructions like this in pure
>> LLVM IR? My limited understanding of LLVM says no, and I haven't been
>> able to find a way to do this, but I would appreciate any pointers you
>> can give me.
> You can use a jump table using indirectbr, although note that this
> instruction requires you to label all the possible target blocks and that
> the targets can only be blocks in the same function.
> --
> Joshua Cranmer
> News submodule owner
> DXR coauthor
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