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author | Max Kazantsev <max.kazantsev@azul.com> | 2017-11-22 06:21:39 +0000 |
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committer | Max Kazantsev <max.kazantsev@azul.com> | 2017-11-22 06:21:39 +0000 |
commit | f66b47b32879fceffde1fe1511500ae3ce749667 (patch) | |
tree | 25ac2327091a39251f58000ee92b907d70f22892 /lib/Analysis | |
parent | 882fbe3d4ca592cdc6be84327ff21d691df5d0d3 (diff) |
[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318819 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Analysis')
-rw-r--r-- | lib/Analysis/ScalarEvolution.cpp | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp index 5a88e68dc90..d2a7a545733 100644 --- a/lib/Analysis/ScalarEvolution.cpp +++ b/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)) |