Summary:
This includes a few changes and corrections to the semantics, to support
the translation. This initial attempt to reason about LLVM -> llair
showed three things that needed repair in the semantics, in addition to
various bugs. We address them as follows.
Refactor llair semantics to have only a single kind of flat value:
integers that fit into specified bit widths. Operations on size values
(e.g., offsets, indices and the like) can just take an integer and
ignore its number of bits. Pointers can just be considered integers that
fit into a certain size given by the constant pointer_size. Later on we
can consider making this a parameter to the model.
Change the generic memory model interface to use numbers rather than
words as the generic encoding of a large value. This makes it more
useful for llair where words are not used.
Pay more careful attention to signed/unsigned issues. Neither LLVM nor
llair have a concept of signed vs unsigned value. Instead individual
operations interpret bit patterns in various ways, some of which are
ambiguous in the LLVM manual. For example, since getelementpointer's
indices are explicitly said to be interpreted as signed 2's complement,
we should probably do the same for insertvalue and extractvalue. However
it is not clear how the argument to alloca is to be interpreted. For now
we assume signed.
Reviewed By: jberdine
Differential Revision: D17164133
fbshipit-source-id: 31a8af635
Summary:
This sketches out how translation can be approached. It is partially
based on the Sledge code.
For basic blocks, isn't based on the Sledge code, but just my own
thoughts as a starting point. Essentially, we are trying to build up
larger expressions, and so not assigning to temporary registers that
don't live past the end of the block. This does remove sharing, so a
fancier approach could check for multiple uses of end-of-block dead
registers, or look at the sizes of expressions. The approach should be
flexible enough to accommodate such changes.
Fix icmp syntax
Using finite maps is elegant in the semantics, but awkward for writing
the translation function. Refactor the mappings from labels to functions
and from labels to blocks to use association lists instead.
To remove phi nodes, the translation takes every edge in the control
flow graph and makes a new basic block that contains a single parallel
move instruction that corresponds to the action of the phi node of the
target block.
Reviewed By: jberdine
Differential Revision: D16831051
fbshipit-source-id: 005663e26
Summary:
Made progress on the sanity checking lemma (that the step relation
preserves some simple invariants on the state). Proved the Ret
instruction case of the state invariant lemma. To do this, I fixed a few
bugs in the definition, and strengthened the invariants.
Reviewed By: jberdine
Differential Revision: D16786900
fbshipit-source-id: 6fa8cb170
Summary:
Global variables need allocating and initialising before the machine can
start. The definition here shouldn't constrain how and where they are
allocated. For example, they don't all need to have separate
allocations. We also tag allocated blocks so that the allocation for a
global can never be deallocated.
Start working on a sanity checking invariant on states.
Reviewed By: jberdine
Differential Revision: D16735068
fbshipit-source-id: 0d5e60e7a
Summary:
Start working on a simple model of LLVM with the ultimate goal of
handling relevant and/or tricky aspects of LLVM and LLAIR and then
formalising the translation from LLVM to LLAIR.
This is a complete initial model of everything that we are interested in
except for exceptions, which should be tricky. Also no thought has gone
into the treatment of poison and the undefined value, so the treatment
is naive, which is at least partially justified because we are
interested in the semantics of LLVM IR after the optimisation passes
have run.
Include some sanity checking theorems.
Reviewed By: jberdine
Differential Revision: D16731885
fbshipit-source-id: fd53949fe
Scott Owens