Passing information from analysis pass to assembly code

[Timothy M Jones]

Hi everyone,

I've been looking at the internals of gcc for a while now, trying to 
figure out the best way to get some information out of it at the 
assembly code level, but still can't work out what's best. I'm wondering 
if anyone can help.

Background: I'm a postdoc at Edinburgh university looking to use gcc for 
some cache optimisation research. I'm using gcc 4.2.0 cross-compiled for 
a PowerPC 405.

The grand scheme is to be able to mark certain memory accesses and later 
use the information in a simulator where I'll implement some sort of 
cache optimisation scheme. For now I'm interested in arrays. For 
example, a piece of C code like this:

     for (i = 0; i < RANDOM_CHUNKS; i++) {
         random_text[i] = RANDOM_TEXT;
     }

I'm interested in capturing the access pattern of the stores to 
'random_text', and I've already created a pass based on prefetch 
insertion in 'tree-ssa-loop-prefetch.c', since this seems to get the 
information I want.

Now, the main problem is that I want to tie this information to the 
assembly code store that performs this operation. So, it seems to me 
that I have to pass the information through the tree structures, down 
into RTL and the out when the assembly code is printed.

I've been considering how to do this, but can't see what's best. I've 
thought about creating a new tree annotation, but then I'll have to 
modify any passes that might alter the tree and not copy it across 
(because they don't know about it). In the RTL, I'm thinking of using 
notes too.

I have several questions about this:

1) Is the tree SSA format the best place to gather this information?

2) Are annotations the best way to keep the information in the trees?

3) Are notes the best way to keep the information in the RTL?

4) Are there any pitfalls I might not be aware of just at the moment 
that could trip me up later?

Thanks very much for your time.

Cheers
Tim

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