Fix the root cause of PR15348 by correctly handling alignment 0 on

memory intrinsics in the SDAG builder.

When alignment is zero, the lang ref says that *no* alignment
assumptions can be made. This is the exact opposite of the internal API
contracts of the DAG where alignment 0 indicates that the alignment can
be made to be anything desired.

There is another, more explicit alignment that is better suited for the
role of "no alignment at all": an alignment of 1. Map the intrinsic
alignment to this early so that we don't end up generating aligned DAGs.

It is really terrifying that we've never seen this before, but we
suddenly started generating a large number of alignment 0 memcpys due to
the new code to do memcpy-based copying of POD class members. That patch
contains a bug that rounds bitfield alignments down when they are the
first field. This can in turn produce zero alignments.

This fixes weird crashes I've seen in library users of LLVM on 32-bit
hosts, etc.

llvm-svn: 176022

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 0640311a77b..81b0ea7e7bc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3867,6 +3867,7 @@ SDValue SelectionDAG::getMemcpy(SDValue Chain, DebugLoc dl, SDValue Dst,
                                 unsigned Align, bool isVol, bool AlwaysInline,
                                 MachinePointerInfo DstPtrInfo,
                                 MachinePointerInfo SrcPtrInfo) {
+  assert(Align && "The SDAG layer expects explicit alignment and reservers 0");
 
   // Check to see if we should lower the memcpy to loads and stores first.
   // For cases within the target-specified limits, this is the best choice.
@@ -3934,6 +3935,7 @@ SDValue SelectionDAG::getMemmove(SDValue Chain, DebugLoc dl, SDValue Dst,
                                  unsigned Align, bool isVol,
                                  MachinePointerInfo DstPtrInfo,
                                  MachinePointerInfo SrcPtrInfo) {
+  assert(Align && "The SDAG layer expects explicit alignment and reservers 0");
 
   // Check to see if we should lower the memmove to loads and stores first.
   // For cases within the target-specified limits, this is the best choice.
@@ -3988,6 +3990,7 @@ SDValue SelectionDAG::getMemset(SDValue Chain, DebugLoc dl, SDValue Dst,
                                 SDValue Src, SDValue Size,
                                 unsigned Align, bool isVol,
                                 MachinePointerInfo DstPtrInfo) {
+  assert(Align && "The SDAG layer expects explicit alignment and reservers 0");
 
   // Check to see if we should lower the memset to stores first.
   // For cases within the target-specified limits, this is the best choice.