Reported in https://reviews.llvm.org/D146415. I rewrote the patch and aded the test case. Per that report, spec2006.483.xalancbmk crashes without this fix.
This models the approach used in LFTR. The short summary is that we need to prove the IV is not dead first, and then we have to either prove the poison flag is valid after the new user or delete it.
There are two key differences between this and LFTR.
First, I allow a non-concrete start to the IV. The goal of LFTR is to canonicalize and IVs with constant starts are canonical, so the very restrictive definition there is mostly okay. Here on the other hand, we're explicitly moving *away* from the canonical form, and thus need to handle non-constant starts.
Second, LFTR bails out instead of removing inbounds on a GEP. This is a pragmatic tradeoff since inbounds is hard to infer and assists aliasing. This pass runs very late, and I think the tradeoff runs the other way.
A different approach we could take for the post-inc check would be to perform a pre-inc check instead of a post-inc check. We would still have to check the pre-inc IV, but that would avoid the need to drop inbounds. Doing the pre-inc check would basically trade killing a whole IV for an extra register move in the loop. I'm open to suggestions on the right approach here.
Note that this analysis is quite expensive compile time wise. I have made no effort to optimize (yet).
Differential Revision: https://reviews.llvm.org/D146464
This is a follow up to one of the side discussions on D146429. There are two semantic changes contained here.
The motivation for the change to the legality condition introduced in D146429 comes from the fact that we only check the post-inc form. As such, as long as the values of the post-inc variable don't self wrap, it's actually okay if we wrap past the starting value of the pre-inc IV.
Second, Nikic noticed during review that the test changes changed behavior for TC=0 (i.e. N=0 in the tests). On more careful inspection, it became apparent that the previous manual expansion code was incorrect in the case where the primary IV could wrap without poison, and started with the limit value (i.e. i8 post-inc starts at 255 for 0 exit test, implying pre-inc starts with 0). See @wrap_around test for an example of the (previous) miscompile.
Differential Revision: https://reviews.llvm.org/D146457
The existing logic was unsound, in two ways.
First, due to wrapping on the trip count computation, it could compute a value which convert a loop which exiting on iteration 256, to one which exited at 255. (With i8 trip counts.)
Second, it allowed rewriting when the trip count implies wrapping around the alternate IV. As a trivial example, it allowed rewriting an i128 exit test in terms of an i64 IV. This is obviously wrong.
Note that the test change is fairly minimal - i.e. only the targeted test - but that's only because I precommitted a change which switched the test from 32 to 64 bit pointers. For 32 bit point architectures with 32 bit primary inductions, this transform is almost always unsound to perform.
Differential Revision: https://reviews.llvm.org/D146429
Main benefit here is making the logic easier to follow, slightly more efficient, and more in line with LFTR. This is not NFC. There are three semantic changes here.
First, we drop handling for constants on the LHS of the comparison. These are non-canonical, and we're very late in the optimization pipeline here, so there's no point in supporting this. I removed a test which covered this case.
Second, we don't need the almost dead IV to be an addrec. We just need SCEV to be able to compute a trip count for it.
Third, we require a simple IV for the almost dead IV. In theory, this removes cases we could have previously handled, but given a) zero testing and b) multiple known correctness issues, I'm adopting an attidute of narrowing this down to something which works correctly, and *then* expanding.
There were two major problems with the tests.
First, with the pointer size being 32 bit and the original IVs also being 32 bit, almost all of the positive tests were actually unsound. An upcoming change will add the appropriate safety check, but the test diffs are really hard to understand without switching the tests to 64 bit pointers first.
Second, checking debug messages for failures is a major bad practice. This should not have been accepted in review at all. The reason is that it makes the *order* of legality checks visibile and modifying any of them becomes annoying and tedious.
Autogen for naming change, and remove comments about C code inspiration. Multiple of these are out of sync with the actual IR, and these are IR tests anyways.
There's no need to require the start value to come directly from the loop predecessor. This was sometimes covering up a latent miscompile in this off-by-default option, but the miscompile needs fixed anyways and the issue has been raised on the original review.
Differential Revision: https://reviews.llvm.org/D142240
When the IV is only used by the terminating condition (say IV-A) and the loop
has a predictable back-edge count and we have another IV (say IV-B) that is an
affine add recursion, we will be able to calculate the terminating value of
IV-B in the loop pre-header. This patch adds attempts to replace IV-B as the
new terminating condition and remove IV-A. It is safe to do so since IV-A is
only used as the terminating condition.
This transformation is suitable to be appended after LSR as it may optimize the
loop into the situation mentioned above. The transformation can reduce number of
IV-s in the loop by one.
A cli option `lsr-term-fold` is added and default disabled.
Reviewed By: mcberg2021, craig.topper
Differential Revision: https://reviews.llvm.org/D132443
