The document summarizes a presentation on developing a new memory model for expanding LLVM semantics in Isabelle. It discusses the existing memory model and need to change to a sized memory access datatype to allow for non-inbounds traversal with getelementptr. The new model represents raw addresses as words and defines structures like allocation, raw blocks, and assumptions about good allocations and block pairs. The presentation closes by thanking the audience and providing references.

1. Background Existing Memory Model New Memory Model Closing
A Memory Model for Expanded LLVM Semantics
in Isabelle
Presented by: David Bergvelt
University of Illinois
Friday 6 November 2015

2. Background Existing Memory Model New Memory Model Closing
Background

3. Background Existing Memory Model New Memory Model Closing
From last time
Expand LLVM semantics in Isabelle to allow complex data
structures

4. Background Existing Memory Model New Memory Model Closing
From last time
Expand LLVM semantics in Isabelle to allow complex data
structures
getelementptr is used to index through structures

5. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation

6. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation
Pass in pointer and list of indices

7. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation
Pass in pointer and list of indices
Inbounds checking

8. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation
Pass in pointer and list of indices
Inbounds checking
Works as general memory traversal tool when inbounds
checking is disabled

9. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation
Pass in pointer and list of indices
Inbounds checking
Works as general memory traversal tool when inbounds
checking is disabled
Int to pointer, pointer to int bitcasting ”flavor”

10. Background Existing Memory Model New Memory Model Closing
getelementptr
getelementptr
Allows us to index through structures by performing address
calculation
Pass in pointer and list of indices
Inbounds checking
Works as general memory traversal tool when inbounds
checking is disabled
Int to pointer, pointer to int bitcasting ”flavor”
Problem: how do we perform inbounds checks?

11. Background Existing Memory Model New Memory Model Closing
Existing Memory Model

12. Background Existing Memory Model New Memory Model Closing
Memory access datatype

13. Background Existing Memory Model New Memory Model Closing
Base memory model

14. Background Existing Memory Model New Memory Model Closing
New Memory Model

15. Background Existing Memory Model New Memory Model Closing
What do we need to change?
Sized memory access datatype

16. Background Existing Memory Model New Memory Model Closing
What do we need to change?
Sized memory access datatype
Raw addresses represented as ’a words

17. Background Existing Memory Model New Memory Model Closing
What do we need to change?
Sized memory access datatype
Raw addresses represented as ’a words
to allow for non-inbounds traversal with getelementptr

18. Background Existing Memory Model New Memory Model Closing
Implementation (Structures)
allocation::(raw_block set)
raw_block = {start::raw_address, length::(’a uword),
memory::((’a word) -> value)}
raw_address::(’a word)

19. Background Existing Memory Model New Memory Model Closing
Implementation (Assumptions)
good_allocation::(allocation->bool)
good_allocation RBS = (for all rb in RBS and all rb2 in RBS,
good_rb_pair rb1 rb2)
good_rb_pair::(raw_block -> raw_block->bool)
good_rb_pair rb1 rb2 = (good_block rb1) &
((rb1.start = rb2.start) -> (rb1 = rb2)) &
((rb1.start < rb2.start) -> (rb1.start+rb1.length <= rb2.start))
good_block::(raw_block -> bool)
good_block rb = ((rb.start + rb.length) < (max uword)) &
(rb.length>=1)

20. Background Existing Memory Model New Memory Model Closing
Closing

21. Background Existing Memory Model New Memory Model Closing
Thank you!
Sources:
Concrete Semantics
http://www.concrete-semantics.org/concrete-semantics.pdf

22. Background Existing Memory Model New Memory Model Closing
Thank you!
Sources:
Concrete Semantics
http://www.concrete-semantics.org/concrete-semantics.pdf
LLVM Language Reference Manual
http://llvm.org/docs/LangRef.html