[RegAllocFast] Properly track the physical register definitions on calls.

PR26485

llvm-svn: 261384

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/RegAllocFast.cpp b/llvm/lib/CodeGen/RegAllocFast.cpp
index f4c076fea0e..8d7a7213ba0 100644
--- a/llvm/lib/CodeGen/RegAllocFast.cpp
+++ b/llvm/lib/CodeGen/RegAllocFast.cpp
@@ -1002,11 +1002,13 @@ void RAFast::AllocateBasicBlock() {
 
     unsigned DefOpEnd = MI->getNumOperands();
     if (MI->isCall()) {
-      // Spill all virtregs before a call. This serves two purposes: 1. If an
+      // Spill all virtregs before a call. This serves one purpose: If an
       // exception is thrown, the landing pad is going to expect to find
-      // registers in their spill slots, and 2. we don't have to wade through
-      // all the <imp-def> operands on the call instruction.
-      DefOpEnd = VirtOpEnd;
+      // registers in their spill slots.
+      // Note: although this is appealing to just consider all definitions
+      // as call-clobbered, this is not correct because some of those
+      // definitions may be used later on and we do not want to reuse
+      // those for virtual registers in between.
       DEBUG(dbgs() << "  Spilling remaining registers before call.\n");
       spillAll(MI);