[cfe-dev] Block Local Variables and Alloca's

[Daniel Dunbar]

Hi Prakash,

On Sun, Mar 28, 2010 at 8:41 PM, Prakash Prabhu
<prakash.prabhu at gmail.com> wrote:
> Hi,
> I have a question with respect to relating the alloca's of local variables
> in the LLVM bit-
> code to their actual declarations in the source code. Consider for instance
> the following
> declaration of a variable called 'digest' within a loop (iteration local
> variable):
> int main()
> {
>   ...
>   for (i=0; i < n; i++)
>   {
>     unsigned char digest[16];
>     ...
>   }
> }
> The LLVM bit-code looks as follows:
>
> define i32 @main(i32 %argc, i8** %argv) noreturn nounwind {
> entry:
>   ...
>   %digest = alloca [16 x i8], align 1             ; <[16 x i8]*> [#uses=2]
>   ...
> for.cond:
>   ...
> }
> What I would like to is to transform the code above, for certain selective
> loops,
> to pre-allocate n copies of digest,  using alloca's or malloc/free before
> the loop.
> I would like modify clang to detect and annotate variables like digest as
> being local
> to a loop/compound statement, which can then be used by some pass in LLVM's
> opt to
> find the corresponding alloca and apply the transformation above. Right now,
> one crude
> way of doing this would be to add calls to annotation functions at places
> where the
> variables are declared, by detecting VarDecl instances in clang. The later
> pass in opt can then
> parse the annotation function and figure out the nearest enclosing loop to
> do the pre-allocation
>  transformation. Is there is better/easier way to do this ?

In general, this seems like something you should do solely on the
basis of the LLVM IR, not at the Clang AST level. The backend already
has lots of infrastructure for dealing with loops, although I'm not
personally familiar with it. You might try mailing llvm-dev instead.

 - Daniel

> thanks,
> Prakash
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