[PATCH] Add a jumptable attribute and support for creating jump-instruction tables

[Tom Roeder]

On Wed, May 21, 2014 at 12:01 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> Not quite. The jumptable code creates a declaration (but not a
>> definition) that will later be satisfied by generated assembly in
>> AsmPrinter. It then goes and replaces all address-taken uses of the
>> function symbol with the declaration. So, the problem happens when
>> there's a statement like
>>
>> @falias = alias i32 ()* @f
>>
>> where @f has the jumptable attribute and is being replaced with a
>> declared but not defined function, say @f_JT. Ideally, I'd just be
>> able to replace @f with @f_JT, but since there's no definition for
>> @f_JT, the GlobalAlias code balks, for good reasons, as you explained.
>>
>> It happens that this declaration in this case is going to resolve into
>> a real symbol in the same asm file, but there's no way the code can
>> tell that.
>
> This seems like a representation issue. Can't you create a fake global
> variable (like llvm.used for example) that during codegen gets
> converted in the same way the declaration is currently?

Do you mean: store the names of the functions that need to be replaced
and don't replace them until they need to be written out?

The problem I see in that suggestion is that we don't want to replace
direct calls to the function, only addr-taken uses. So, there need to
be two ways to call each jumptable function: direct calls through the
original symbol and indirect calls through the jumptable symbol.

Also, my work on control-flow integrity that works over jump tables
must know all the replacement symbols to do its job, since it uses
these symbols to generate the check code it inserts at each indirect
call site. So, either there needs to be replacement symbols for it to
use, or everything has to go down into AsmPrinter. The latter options
seems to me like a lot of unnecessary complexity, since function
replacement is very natural and easy in IR.