MCP: Fixed bug with dest overlapping copy source

In MachineCopyPropagation, when propagating the source of a copy into
the operand of a later instruction, bail if a destination overlaps
(partly defines) the copy source. If the instruction where the
substitution is happening is also a copy, allowing the propagation
confuses the tracking mechanism.

Differential Revision: https://reviews.llvm.org/D69953

Change-Id: Ic570754f878f2d91a4a50a9bdcf96fbaa240726d

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index ebe76e31dca..d45dfd065fc 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -432,6 +432,15 @@ void MachineCopyPropagation::forwardUses(MachineInstr &MI) {
     if (hasImplicitOverlap(MI, MOUse))
       continue;
 
+    // Check that the instruction is not a copy that partially overwrites the
+    // original copy source that we are about to use. The tracker mechanism
+    // cannot cope with that.
+    if (MI.isCopy() && MI.modifiesRegister(CopySrcReg, TRI) &&
+        !MI.definesRegister(CopySrcReg)) {
+      LLVM_DEBUG(dbgs() << "MCP: Copy source overlap with dest in " << MI);
+      continue;
+    }
+
     if (!DebugCounter::shouldExecute(FwdCounter)) {
       LLVM_DEBUG(dbgs() << "MCP: Skipping forwarding due to debug counter:\n  "
                         << MI);