[GlobalMerge] Set the alignment on merged global structs

If no alignment is set, the abi/preferred alignment of structs will be
used which may be higher than required. This can lead to extra padding
and in the end an increase in data size.

Differential Revision: https://reviews.llvm.org/D47633

llvm-svn: 334099

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/GlobalMerge.cpp b/llvm/lib/CodeGen/GlobalMerge.cpp
index fbf94d2bb7f..bebf1d6d4cd 100644
--- a/llvm/lib/CodeGen/GlobalMerge.cpp
+++ b/llvm/lib/CodeGen/GlobalMerge.cpp
@@ -500,6 +500,11 @@ bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
         GlobalVariable::NotThreadLocal, AddrSpace);
 
     const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
+    // Set the alignment of the merged struct as the maximum alignment of the
+    // globals to prevent over-alignment. We don't handle globals that are not
+    // default aligned, so the alignment of the MergedLayout struct is
+    // equivalent.
+    MergedGV->setAlignment(MergedLayout->getAlignment());
 
     for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();