Summary:
Previously, the LLVM semantics could be stuck where the LLAIR semantics
was not yet stuck, but would become stuck (at the same place) after
taking a step. This was due to LLVM using the traditional definition of
stuck states: any state from which there are no transitions. However,
LLAIR cannot do that because it might get stuck in the middle of a block
that contains several visible stores. We don't want to consider the
whole block stuck, nor can we finish it. Thus, the LLAIR definition of
stuckness is when the state has the stuck flag set which happens when
stopping in the middle of a block after encountering a stuck
instruction. Now LLVM takes the same approach.
Reviewed By: jberdine
Differential Revision: D17855085
fbshipit-source-id: a094d25d5
Summary:
Add an argument to the Exit instruction. Update the LLVM semantics to
execute the Exit instruction and store the result in an "exited"
component of the state. (Previously it just noticed that it was stuck
about to do an Exit.)
With exiting treated uniformly, now in the proof that for every LLVM
trace, there is a llair trace that simulates it, all of the cheats
except for 1 are just cases that I haven't got to yet. However, the last
cheat is for the situation where the LLVM program gets stuck and the
llair program doesn't. For example, the following two line LLVM program
gets stuck because r2 is not assigned (ignoring for the moment the static
restriction that LLVM is in SSA form).
r1 := r2
Exit(0)
The compilation to llair omits the assignment and so we get a llair
program that doesn't get stuck:
Exit(0)
The key question is whether the static restrictions are sufficient to
ensure that no expression that might be omitted can get stuck.
Reviewed By: jberdine
Differential Revision: D17737589
fbshipit-source-id: bc6c01a1b
Summary:
If the LLVM to llair translation keeps a mapping from register r to
expression e, then for each register r' mentioned in e, there must be an
assignment to r' that dominates the entire live range of r. Thus, where
ever r might be replaced by e, the value of r' will be the same as it
was when the initial assignment to r occurred. Maintaining this
invariant relies on the LLVM being in SSA form.
Reviewed By: jberdine
Differential Revision: D17710288
fbshipit-source-id: fd3eaa57d
Summary:
This is work in progress; many of the cheats aren't true. In particular,
the definition of stuck/complete/partial traces in LLVM and llair don't
quite match up and need some modification. Also, the state relation
isn't strong enough; it will need to include information about registers
used in the expressions of the LLVM register to llair expression
mapping. But the overall shape of the proof is ok and so it can be
used to poke at various local aspects of the translation, such as
individual instructions.
Reviewed By: jberdine
Differential Revision: D17631604
fbshipit-source-id: 743b5d64d
Summary:
Give the llair semantics observable side effects (writes to global
variables) and a semantic function mirroring the LLVM semantics. Start
sketching out the LLVM/llair translation equivalence proof in a top-down
way from the obvious statement of equality of the semantics.
Reviewed By: jberdine
Differential Revision: D17399654
fbshipit-source-id: 2170678a8
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
Scott Owens