Revert r75581: it causes massive breakage in the Ada

testsuite, due to exception handling not working
correctly.  Maybe because the libgcc unwinder is
miscompiled - not sure, and I won't have time to
look into it before leaving on holiday.  Note that
miscompilations of libgcc are not picked up by the
nightly testers, because they dynamically link with
libgcc, so pick up the system version rather than
the version built as part of llvm-gcc.  This is a
nasty flaw in the nightly testers.  (On the other
hand the Ada testsuite links with the just built
libgcc).

llvm-svn: 76895

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86RegisterInfo.cpp b/llvm/lib/Target/X86/X86RegisterInfo.cpp
index 751b20598b2..bf523dde880 100644
--- a/llvm/lib/Target/X86/X86RegisterInfo.cpp
+++ b/llvm/lib/Target/X86/X86RegisterInfo.cpp
@@ -784,6 +784,29 @@ void X86RegisterInfo::emitCalleeSavedFrameMoves(MachineFunction &MF,
     unsigned Reg = I->getReg();
     Offset = MaxOffset - Offset + saveAreaOffset;
 
+    // Don't output a new machine move if we're re-saving the frame
+    // pointer. This happens when the PrologEpilogInserter has inserted an extra
+    // "PUSH" of the frame pointer -- the "emitPrologue" method automatically
+    // generates one when frame pointers are used. If we generate a "machine
+    // move" for this extra "PUSH", the linker will lose track of the fact that
+    // the frame pointer should have the value of the first "PUSH" when it's
+    // trying to unwind.
+    // 
+    // FIXME: This looks inelegant. It's possibly correct, but it's covering up
+    //        another bug. I.e., one where we generate a prolog like this:
+    //
+    //          pushl  %ebp
+    //          movl   %esp, %ebp
+    //          pushl  %ebp
+    //          pushl  %esi
+    //           ...
+    //
+    //        The immediate re-push of EBP is unnecessary. At the least, it's an
+    //        optimization bug. EBP can be used as a scratch register in certain
+    //        cases, but probably not when we have a frame pointer.
+    if (HasFP && FramePtr == Reg)
+      continue;
+
     MachineLocation CSDst(MachineLocation::VirtualFP, Offset);
     MachineLocation CSSrc(Reg);
     Moves.push_back(MachineMove(LabelId, CSDst, CSSrc));