When passing a parameter using the 'byval' mechanism, inline code needs to be used

to perform the copy, which may be of lots of memory [*].  It would be good if the
fall-back code generated something reasonable, i.e. did the copy in a loop, rather
than vast numbers of loads and stores.  Add a note about this.  Currently target
specific code seems to always kick in so this is more of a theoretical issue rather
than a practical one now that X86 has been fixed.
[*] It's amazing how often people pass mega-byte long arrays by copy...

llvm-svn: 118275

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 0840bd740be..5477e7bf60d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3283,6 +3283,8 @@ static SDValue getMemcpyLoadsAndStores(SelectionDAG &DAG, DebugLoc dl,
 
   // Expand memcpy to a series of load and store ops if the size operand falls
   // below a certain threshold.
+  // TODO: In the AlwaysInline case, if the size is big then generate a loop
+  // rather than maybe a humongous number of loads and stores.
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   std::vector<EVT> MemOps;
   bool DstAlignCanChange = false;