[prev in list] [next in list] [prev in thread] [next in thread] 

List:       cfe-dev
Subject:    Re: [cfe-dev] Converting pointers to fat pointers
From:       David Chisnall <csdavec () swan ! ac ! uk>
Date:       2012-05-26 12:17:24
Message-ID: 064138E7-C110-4581-AA97-B1AB330EE001 () swan ! ac ! uk
[Download RAW message or body]

On 15 May 2012, at 04:04, John Criswell wrote:

> On 5/14/12 9:57 PM, Sai Charan wrote:
> > In the interest of time & effort, I am leaning on working at the LLVM IR level. 
> > 
> > The code listing in section 3.1 of the SoftBound paper is precisely what I am \
> > looking to do. However, the listing is at the C source level, while section 6 \
> > says that the implementation has been done on the LLVM IR; I don't see how I can \
> > figure out pointer de-references in LLVM IR. Every alloca/load/store is via \
> > <ty>*. 
> > In summary, how do I figure out pointer de-references in LLVM IR.
> 
> Ignoring intrinsic functions, the only LLVM IR instructions that dereference \
> pointers are load and store.

While true, this only tells half of the story.  You also need to be careful of \
pointer arithmetic, which can be done either via GEPs or via int-to-pointer casts.  \
If your fat pointers contain, for example, bounds information then you will need to \
track all of these.

We (cl.cam.ac.uk) are currently in the process of adding LLVM support for a custom \
MIPS-based processor that has hardware support for capabilities, meaning that we have \
64-bit and 256-bit pointers in the system.  64-bit pointers are just numbers, but \
256-bit (capability) pointers include start, bounds, and a set of permissions.  Both \
can be dereferenced, although via different instructions (64-bit pointers are \
implicitly checked against a specific capability register, depending on how they are \
used).

We're hitting a few issues in that LLVM IR assumes that you only have one pointer \
size and so, for example, does things like fold a 64-bit pointer to int to 256-bit \
pointer sequence into a bitcast, and assumptions in the back end that pointers are \
integers.  

David


_______________________________________________
cfe-dev mailing list
cfe-dev@cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev


[prev in list] [next in list] [prev in thread] [next in thread]