Loop unrolling tends to produce chains of
`%x1 = add %x0, 1; %x2 = add %x1, 1; ...` with one add per unrolled
iteration. This patch simplifies these adds to `%xN = add %x0, N`
directly during unrolling, rather than waiting for InstCombine to do so.
The motivation for this is that having a single add (rather than
an add chain) on the induction variable makes it a simple recurrence,
which we specially recognize in a number of places. This allows
InstCombine to directly perform folds with that knowledge, instead
of first folding the add chains, and then doing other folds in another
InstCombine iteration.
Due to the reduced number of InstCombine iterations, this also
results in a small compile-time improvement.
Differential Revision: https://reviews.llvm.org/D153540
Fixes https://github.com/llvm/llvm-project/issues/58565
The previous implementation visits operands in pre-order, but this does
not guarantee an instruction is visited before its uses. This can cause
instructions to be copied in the incorrect order. For example:
```
a = ...
b = add a, 1
c = add a, b
d = add b, a
```
Pre-order visits does not guarantee the order in which `a` and `b` are
visited. LoopUnrollAndJam may incorrectly insert `b` before `a`.
This patch implements post-order visits. By visiting dependencies first,
we guarantee that an instruction's dependencies are visited first.
Differential Revision: https://reviews.llvm.org/D140255
