[Local] Keep K's range if K does not move when combining metadata.

As K has to dominate I, IIUC I's range metadata must be a subset of
K's. After Eli's recent clarification to the LangRef, loading a value
outside of the range is undefined behavior.
Therefore if I's range contains elements outside of K's range and we would load
one such value, K would cause undefined behavior.

In cases like hoisting/sinking, we still want the most generic range
over all code paths to/from the hoist/sink point. As suggested in the
patches related to D47339, I will refactor the handling of those
scenarios and try to decouple it from this function as follow up, once
we switched to a similar handling of metadata in most of
combineMetadata.

I updated some tests checking mostly the merging of metadata to keep the
metadata of to dominating load. The most interesting one is probably test8 in
test/Transforms/JumpThreading/thread-loads.ll. It contained a comment
about the alias metadata preventing us to eliminate the branch, but it
seem like the actual problem currently is that we merge the ranges of
both loads and cannot eliminate the icmp afterwards. With this patch, we
manage to eliminate the icmp, as the range of the first load excludes 8.

Reviewers: efriedma, nlopes, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D51629

llvm-svn: 345456

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 82fb765842d..0dcd7371210 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -2315,7 +2315,15 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J,
         K->setMetadata(Kind, MDNode::intersect(JMD, KMD));
         break;
       case LLVMContext::MD_range:
-        K->setMetadata(Kind, MDNode::getMostGenericRange(JMD, KMD));
+
+        // If K does move, use most generic range. Otherwise keep the range of
+        // K.
+        if (DoesKMove)
+          // FIXME: If K does move, we should drop the range info and nonnull.
+          //        Currently this function is used with DoesKMove in passes
+          //        doing hoisting/sinking and the current behavior of using the
+          //        most generic range is correct in those cases.
+          K->setMetadata(Kind, MDNode::getMostGenericRange(JMD, KMD));
         break;
       case LLVMContext::MD_fpmath:
         K->setMetadata(Kind, MDNode::getMostGenericFPMath(JMD, KMD));