[llvm-dev] Way to guarantee generated indirect call is via memory?

[John Criswell via llvm-dev]

Dear Eugene,

I think the best way to do this is to generate inline assembly code at 
the LLVM IR level that uses the memory-based addressing mode that you 
want.  Any other approach could break with changes/enhancements to the 
LLVM code generator.

The only other way to do this is to write an LLVM MachineFunctionPass, 
but that is probably overkill for what you're trying to do.

Regards,

John Criswell


On 8/17/15 6:41 PM, Eugene Rozenfeld via llvm-dev wrote:
>
> I’d like to generate an indirect call instruction that will end up as 
> a call via memory and not register.
>
> The address of the target is a constant.
>
> For example, something like
>
> %25 = load i64, i64* @get_Now
>
> %26 = inttoptr i64 %25 to i64 ()*
>
> %27 = call i64 %26()
>
> may end up as
>
> mov      rsi,qword ptr [00007FF658381070]
>
> call        rsi
>
> or as
>
> call  qword ptr [00007FF658381070]
>
> If I want to guarantee that the second form is always used, how would 
> I go about that?
>
> One possibility is to use patchpoints 
> (http://llvm.org/docs/StackMaps.html#id9). Are there any other 
> alternatives?
>
> Eugene
>
>
>
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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