[MachineCopyPropagation] Handle COPY with overlapping source/dest.

MachineCopyPropagation::CopyPropagateBlock has a bunch of special
handling for COPY instructions. This handling assumes that COPY
instructions do not modify the source of the copy; this is wrong if
the COPY destination overlaps the source.

To fix the bug, check explicitly for this situation, and fall back to
the generic instruction handling.

This bug can't happen for most register classes because they don't
have this sort of overlap, but there are a few register classes
where this is possible. The testcase uses the AArch64 QQQQ register
class.

Differential Revision: https://reviews.llvm.org/D44911

llvm-svn: 328851

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 4c1717e1ec3..8b6777f06db 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -399,7 +399,9 @@ void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
     MachineInstr *MI = &*I;
     ++I;
 
-    if (MI->isCopy()) {
+    // Analyze copies (which don't overlap themselves).
+    if (MI->isCopy() && !TRI->regsOverlap(MI->getOperand(0).getReg(),
+                                          MI->getOperand(1).getReg())) {
       unsigned Def = MI->getOperand(0).getReg();
       unsigned Src = MI->getOperand(1).getReg();