[SCEV] Strengthen variance condition in calculateLoopDisposition

Given loops `L1` and `L2` with AddRecs `AR1` and `AR2` varying in them respectively.
When identifying loop disposition of `AR2` w.r.t. `L1`, we only say that it is varying if
`L1` contains `L2`. But there is also a possible situation where `L1` and `L2` are
consecutive sibling loops within the parent loop. In this case, `AR2` is also varying
w.r.t. `L1`, but we don't correctly identify it.

It can lead, for exaple, to attempt of incorrect folding. Consider:
  AR1 = {a,+,b}<L1>
  AR2 = {c,+,d}<L2>
  EXAR2 = sext(AR1)
  MUL = mul AR1, EXAR2
If we incorrectly assume that `EXAR2` is invariant w.r.t. `L1`, we can end up trying to
construct something like: `{a * {c,+,d}<L2>,+,b * {c,+,d}<L2>}<L1>`, which is incorrect
because `AR2` is not available on entrance of `L1`.

Both situations "`L1` contains `L2`" and "`L1` preceeds sibling loop `L2`" can be handled
with one check: "header of `L1` dominates header of `L2`". This patch replaces the old
insufficient check with this one.

Differential Revision: https://reviews.llvm.org/D39453

llvm-svn: 318819

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 5a88e68dc90..d2a7a545733 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10927,9 +10927,11 @@ ScalarEvolution::computeLoopDisposition(const SCEV *S, const Loop *L) {
     if (!L)
       return LoopVariant;
 
-    // This recurrence is variant w.r.t. L if L contains AR's loop.
-    if (L->contains(AR->getLoop()))
+    // Everything that is not defined at loop entry is variant.
+    if (DT.dominates(L->getHeader(), AR->getLoop()->getHeader()))
       return LoopVariant;
+    assert(!L->contains(AR->getLoop()) && "Containing loop's header does not"
+           " dominate the contained loop's header?");
 
     // This recurrence is invariant w.r.t. L if AR's loop contains L.
     if (AR->getLoop()->contains(L))