[LLVMdev] Support for per-loop pragma

Devang Patel devang.patel at gmail.com
Wed Jun 2 15:29:46 PDT 2010

On Wed, Jun 2, 2010 at 11:08 AM, Junjie Gu <jgu222 at gmail.com> wrote:
> Thanks for comments/suggestions.
> Yes, attaching metadata to instructions will be good choices for many
> cases. But for loops,
> attaching metadata to back-edges requires that the front end to build
> loops, which is an
> additional task for the front end. And this task is really a backend's
> job, not the front end's.

The front-ends (llvm-gcc and clang) typically lower their internal AST
into llvm IR. The front-end's will  have to do a job of attaching
pragma to respective AST node any way. Which node will it chose to
annotate #pragma, irrespective of how it is annotated ?

> If the only concern is that it is hard for pragma intrinsics to stay
> with their associated code,
> another choice is to process pragma intrinsic at the beginning of
> optimizer and associates
> them to instructions by either attaching metadata to instructions and
> some kind maps (between
> instructions/BB to metadata, etc).

Note, The loop optimizer may not be the first thing run by the
optimizer. If you wait until loop optimizer to form loop then it may
be too late.  Whoever is converting llvm.pragma intrinsic into
attached metadata will likely attach metadata to immediate next
instruction anyway.


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