Summary:
for.outer:
br for.inner
for.inner:
LI <loop invariant load instruction>
for.inner.latch:
br for.inner, for.outer.latch
for.outer.latch:
br for.outer, for.outer.exit
LI is a loop invariant load instruction that post dominate for.outer, so LI should be able to move out of the loop nest. However, there is a bug in allLoopPathsLeadToBlock().
Current algorithm of allLoopPathsLeadToBlock()
1. get all the transitive predecessors of the basic block LI belongs to (for.inner) ==> for.outer, for.inner.latch
2. if any successors of any of the predecessors are not for.inner or for.inner's predecessors, then return false
3. return true
Although for.inner.latch is for.inner's predecessor, but for.inner dominates for.inner.latch, which means if for.inner.latch is ever executed, for.inner should be as well. It should not return false for cases like this.
Author: Whitney (committed by xingxue)
Reviewers: kbarton, jdoerfert, Meinersbur, hfinkel, fhahn
Reviewed By: jdoerfert
Subscribers: hiraditya, jsji, llvm-commits, etiotto, bmahjour
Tags: #LLVM
Differential Revision: https://reviews.llvm.org/D62418
llvm-svn: 361762
The description of `isGuaranteedToExecute` does not correspond to its implementation.
According to description, it should return `true` if an instruction is executed under the
assumption that its loop is *entered*. However there is a sophisticated alrogithm inside
that tries to prove that the instruction is executed if the loop is *exited*, which is not the
same thing for infinite loops. There is an attempt to protect from dealing with infinite loops
by prohibiting loops without exit blocks, however an infinite loop can have exit blocks.
As result of that, MustExecute can falsely consider some blocks that are never entered as
mustexec, and LICM can hoist dangerous instructions out of them basing on this fact.
This may introduce UB to programs which did not contain it initially.
This patch removes the problematic algorithm and replaced it with a one which tries to
prove what is required in description.
Differential Revision: https://reviews.llvm.org/D50558
Reviewed By: reames
llvm-svn: 339984
CanProveNotTakenFirstIteration utility does not handle the case when
condition of the branch is a constant. Add its handling.
Reviewers: reames, anna, mkazantsev
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D46996
llvm-svn: 332695
Most basic possible test for the logic used by LICM.
Also contains a speculative build fix for compiles which complain about a definition of a stuct K; followed by a declaration as class K;
llvm-svn: 328058
As suggested in the original review (https://reviews.llvm.org/D44524), use an annotation style printer instead.
Note: The switch from -analyze to -disable-output in tests was driven by the fact that seems to be the idiomatic style used in annoation passes. I tried to keep both working, but the old style pass API for printers really doesn't make this easy. It invokes (runOnFunction, print(Module)) repeatedly. I decided the extra state wasn't worth it given the old pass manager is going away soonish anyway.
llvm-svn: 328015
Many of our loop passes make use of so called "must execute" or "guaranteed to execute" facts to prove the legality of code motion. The basic notion is that we know (by assumption) an instruction didn't fault at it's original location, so if the location we move it to is strictly post dominated by the original, then we can't have introduced a new fault.
At the moment, the testing for this logic is somewhat adhoc and done mostly through LICM. Since I'm working on that code, I want to improve the testing. This patch is the first step in that direction. It doesn't actually test the variant used by the loop passes - I need to move that to the Analysis library first - but instead exercises an alternate implementation used by SCEV. (I plan on merging both implementations.)
Note: I'll be replacing the printing logic within this with an annotation based version in the near future. Anna suggested this in review, and it seems like a strictly better format.
Differential Revision: https://reviews.llvm.org/D44524
llvm-svn: 328004
Xing Xue