This patch extends the condition collection logic to allow adding
conditions from pre-headers to loop headers, by allowing cases where the
target block dominates some of its predecessors.
A == B map to A >= B && A <= B
(https://alive2.llvm.org/ce/z/_dwxKn).
This extends the constraint construction to return a list of
constraints, which can be used to properly de-compose nested AND & OR.
Instead of using ConstraintSystem::negate when adding new constraints,
flip the condition in IR.
The main advantage is that EQ predicates can be represented by 2
constraints, which makes negating based on the constraint tricky. The IR
condition can easily negated.
This patch adds support for select form of and/or.
Currently there is an ongoing effort for moving towards using `select a, b, false` instead of `and i1 a, b` and
`select a, true, b` instead of `or i1 a, b` as well.
D93065 has links to relevant changes.
Alive2 proof: (undef input was disabled due to timeout :( )
- and: https://alive2.llvm.org/ce/z/AgvFbQ
- or: https://alive2.llvm.org/ce/z/KjLJyb
Differential Revision: https://reviews.llvm.org/D93935
This patch generalizes the extraction of a constraint for a given
condition. It allows decompose to return a vector of c * X pairs, which
allows de-composing multiple instructions in the future.
It also adds more clarifying comments.
When processing conditional branches, if the condition is an AND of 2 compares
and the true successor only has the current block as predecessor, queue both
conditions for the true successor.
When processing conditional branches, if the condition is an OR of 2 compares
and the false successor only has the current block as predecessor, queue both
negated conditions for the false successor
If -enable-constraint-elimination is specified, add it to the -O2/-O3 pipeline.
(-O1 uses a separate function now.)
Reviewed By: fhahn, aeubanks
Differential Revision: https://reviews.llvm.org/D88365
This patch is a first draft of a new pass that adds a more flexible way
to eliminate compares based on more complex constraints collected from
dominating conditions.
In particular, it aims at simplifying conditions of the forms below
using a forward propagation approach, rather than instcomine-style
ad-hoc backwards walking of def-use chains.
if (x < y)
if (y < z)
if (x < z) <- simplify
or
if (x + 2 < y)
if (x + 1 < y) <- simplify assuming no wraps
The general approach is to collect conditions and blocks, sort them by
dominance and then iterate over the sorted list. Conditions are turned
into a linear inequality and add it to a system containing the linear
inequalities that hold on entry to the block. For blocks, we check each
compare against the system and see if it is implied by the constraints
in the system.
We also keep a stack of processed conditions and remove conditions from
the stack and the constraint system once they go out-of-scope (= do not
dominate the current block any longer).
Currently there still are the least the following areas for improvements
* Currently large unsigned constants cannot be added to the system
(coefficients must be represented as integers)
* The way constraints are managed currently is not very optimized.
Reviewed By: spatel
Differential Revision: https://reviews.llvm.org/D84547
