[DAGCombine] Handle big endian correctly in CombineConsecutiveLoads

Summary:
Found out, at code inspection, that there was a fault in
DAGCombiner::CombineConsecutiveLoads for big-endian targets.

A BUILD_PAIR is always having the least significant bits of
the composite value in element 0. So when we are doing the checks
for consecutive loads, for big endian targets, we should check
if the load to elt 1 is at the lower address and the load
to elt 0 is at the higher address.

Normally this bug only resulted in missed oppurtunities for
doing the load combine. I guess that in some rare situation it
could lead to faulty combines, but I've not seen that happen.

Note that this patch actually will trigger load combine for
some big endian regression tests.
One example is test/CodeGen/PowerPC/anon_aggr.ll where we now get
  t76: i64,ch = load<LD8[FixedStack-9]
instead of
  t37: i32,ch = load<LD4[FixedStack-10]>
  t35: i32,ch = load<LD4[FixedStack-9]>
  t41: i64 = build_pair t37, t35
before legalization. Then the legalization will split the LD8
into two loads, so the end result is the same. That should
verify that the transfomation is correct now.

Reviewers: niravd, hfinkel

Reviewed By: niravd

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D40444

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319771 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 2 insertions, 1 deletions

diff --git a/include/llvm/CodeGen/ISDOpcodes.h b/include/llvm/CodeGen/ISDOpcodes.h
index 9e4865ff2c2..d256849be9a 100644
--- a/include/llvm/CodeGen/ISDOpcodes.h
+++ b/include/llvm/CodeGen/ISDOpcodes.h
@@ -186,7 +186,8 @@ namespace ISD {
     /// BUILD_PAIR - This is the opposite of EXTRACT_ELEMENT in some ways.
     /// Given two values of the same integer value type, this produces a value
     /// twice as big.  Like EXTRACT_ELEMENT, this can only be used before
-    /// legalization.
+    /// legalization. The lower part of the composite value should be in
+    /// element 0 and the upper part should be in element 1.
     BUILD_PAIR,
 
     /// MERGE_VALUES - This node takes multiple discrete operands and returns