Fold %x umin_seq %y to %x if %x ule %y. This also subsumes the
special handling for constant operands, as if %y is constant this
folds to umin via implied poison reasoning, and if %x is constant
then either %x is not zero and it folds to umin, or it is known
zero, in which case it is ule anything.
Fold %x umin_seq %y to %x umin %y if %x cannot be zero. They only
differ in semantics for %x==0.
More generally %x *_seq %y folds to %x * %y if %x cannot be the
saturation fold (though currently we only have umin_seq).
Similar to how we convert logical and/or to bitwise and/or, we should
also convert umin_seq to umin based on implied poison reasoning. In
%x umin_seq %y, if %y being poison implies %x being poison, then we
don't need the sequential evaluation: Having %y contribute towards
the result will never make the result more poisonous. An important
corollary of this is that if %y is never poison, we also don't need
the sequential evaluation.
This avoids some of the regressions in D124910.
Differential Revision: https://reviews.llvm.org/D124921
Since we don't merge/expand non-sequential umin exprs into umin_seq exprs,
we may have umin_seq(umin(umin_seq())) chain, and the innermost umin_seq
can have duplicate operands still.
Let's consider sequential min/max expression family
to be more complex than their non-sequential counterparts,
preserving internal ordering within them.
We could just merge all umin into umin_seq, but that is likely
a pessimization, so don't do that, but pretend that we did
for the purpose of deduplication.
Having the same operand more than once doesn't change the outcome here,
neither reduction-wise nor poison-wise.
We must keep the first instance specifically though.
Two crashes have been reported. This change disables the new logic while leaving the new node in tree. Hopefully, that's enough to allow investigation without breakage while avoiding massive churn.
As discussed in https://github.com/llvm/llvm-project/issues/53020 / https://reviews.llvm.org/D116692,
SCEV is forbidden from reasoning about 'backedge taken count'
if the branch condition is a poison-safe logical operation,
which is conservatively correct, but is severely limiting.
Instead, we should have a way to express those
poison blocking properties in SCEV expressions.
The proposed semantics is:
```
Sequential/in-order min/max SCEV expressions are non-commutative variants
of commutative min/max SCEV expressions. If none of their operands
are poison, then they are functionally equivalent, otherwise,
if the operand that represents the saturation point* of given expression,
comes before the first poison operand, then the whole expression is not poison,
but is said saturation point.
```
* saturation point - the maximal/minimal possible integer value for the given type
The lowering is straight-forward:
```
compare each operand to the saturation point,
perform sequential in-order logical-or (poison-safe!) ordered reduction
over those checks, and if reduction returned true then return
saturation point else return the naive min/max reduction over the operands
```
https://alive2.llvm.org/ce/z/Q7jxvH (2 ops)
https://alive2.llvm.org/ce/z/QCRrhk (3 ops)
Note that we don't need to check the last operand: https://alive2.llvm.org/ce/z/abvHQS
Note that this is not commutative: https://alive2.llvm.org/ce/z/FK9e97
That allows us to handle the patterns in question.
Reviewed By: nikic, reames
Differential Revision: https://reviews.llvm.org/D116766
Nikita Popov