[libunwind] improve x86_64 comments in compact_unwind_encoding.h

llvm-svn: 224657

1 files changed, 7 insertions, 3 deletions

diff --git a/libcxxabi/include/mach-o/compact_unwind_encoding.h b/libcxxabi/include/mach-o/compact_unwind_encoding.h
index 8d5ee7d..b71c2c8 100644
--- a/libcxxabi/include/mach-o/compact_unwind_encoding.h
+++ b/libcxxabi/include/mach-o/compact_unwind_encoding.h
@@ -133,6 +133,10 @@ enum {
 //    linker in final linked images which have only dwarf unwind info for a
 //    function.
 //
+// The permutation encoding is a Lehmer code sequence encoded into a
+// single variable-base number so we can encode the ordering of up to
+// six registers in a 10-bit space.
+//
 // The following is the algorithm used to create the permutation encoding used
 // with frameless stacks.  It is passed the number of registers to be saved and
 // an array of the register numbers saved.
@@ -234,7 +238,7 @@ enum {
 //    EPB value, then RBP is restored by popping off the stack, and the return 
 //    is done by popping the stack once more into the pc.
 //    All non-volatile registers that need to be restored must have been saved
-//    in a small range in the stack that starts RBP-8 to RBP-1020.  The offset/4 
+//    in a small range in the stack that starts RBP-8 to RBP-2040.  The offset/8 
 //    is encoded in the UNWIND_X86_64_RBP_FRAME_OFFSET bits.  The registers saved
 //    are encoded in the UNWIND_X86_64_RBP_FRAME_REGISTERS bits as five 3-bit entries.
 //    Each entry contains which register to restore.  
@@ -244,8 +248,8 @@ enum {
 //    unwind encoding) is added to the RSP. Then the return is done by 
 //    popping the stack into the pc.
 //    All non-volatile registers that need to be restored must have been saved
-//    on the stack immediately after the return address.  The stack_size/4 is
-//    encoded in the UNWIND_X86_64_FRAMELESS_STACK_SIZE (max stack size is 1024).
+//    on the stack immediately after the return address.  The stack_size/8 is
+//    encoded in the UNWIND_X86_64_FRAMELESS_STACK_SIZE (max stack size is 2048).
 //    The number of registers saved is encoded in UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT.
 //    UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION constains which registers were
 //    saved and their order.