Even more explanation.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93841 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/PowerPC/PPCTargetMachine.cpp b/lib/Target/PowerPC/PPCTargetMachine.cpp
index c80a530..c7f7882 100644
--- a/lib/Target/PowerPC/PPCTargetMachine.cpp
+++ b/lib/Target/PowerPC/PPCTargetMachine.cpp
@@ -201,6 +201,18 @@
/// to bugs or other conditions. We will default to a 4-byte encoding unless the
/// system tells us otherwise.
///
+/// The issue is when the CIE says their is an LSDA. That mandates that every
+/// FDE have an LSDA slot. But if the function does not need an LSDA. There
+/// needs to be some way to signify there is none. The LSDA is encoded as
+/// pc-rel. But you don't look for some magic value after adding the pc. You
+/// have to look for a zero before adding the pc. The problem is that the size
+/// of the zero to look for depends on the encoding. The unwinder bug in SL is
+/// that it always checks for a pointer-size zero. So on x86_64 it looks for 8
+/// bytes of zero. If you have an LSDA, it works fine since the 8-bytes are
+/// non-zero so it goes ahead and then reads the value based on the encoding.
+/// But if you use sdata4 and there is no LSDA, then the test for zero gives a
+/// false negative and the unwinder thinks there is an LSDA.
+///
/// FIXME: This call-back isn't good! We should be using the correct encoding
/// regardless of the system. However, there are some systems which have bugs
/// that prevent this from occuring.