The widenIVUse avoids generating trunc by evaluating the use as AddRec, this
will not work when:
- SCEV traces back to an instruction inside the loop that SCEV can not
expand, eg. add %indvar, (load %addr)
- SCEV finds a loop variant, eg. add %indvar, %loopvariant
While SCEV fails to avoid trunc, we can still try to use instruction
combining approach to prove trunc is not required. This can be further
extended with other instruction combining checks, but for now we handle the
following case (sub can be "add" and "mul", "nsw + sext" can be "nus + zext")
Src: %c = sub nsw %b, %indvar %d = sext %c to i64 Dst: %indvar.ext1 = sext %indvar to i64 %m = sext %b to i64 %d = sub nsw i64 %m, %indvar.ext1
Therefore, as long as the result of add/sub/mul is extended to wide type with
right extension and overflow wrap combination, no
trunc is required regardless of how %b is generated. This pattern is common
when calculating address in 64 bit architecture.
It extends it by providing proof of why trunc is unnecessary in more general case,
it should also resolve some of the concerns from the following discussion with @reames.