Index: doc/rtl.texi
===================================================================
--- doc/rtl.texi        (revision 197532)
+++ doc/rtl.texi        (working copy)
@@ -3099,18 +3099,11 @@ side-effects.

 @findex sequence
 @item (sequence [@var{insns} @dots{}])
-Represents a sequence of insns.  Each of the @var{insns} that appears
-in the vector is suitable for appearing in the chain of insns, so it
-must be an @code{insn}, @code{jump_insn}, @code{call_insn},
-@code{code_label}, @code{barrier} or @code{note}.
+Represents a sequence of insns.  If a @code{sequence} appears in the
+chain of insns, then each of the @var{insns} that appears in the sequence
+must be suitable for appearing in the chain of insns, i.e. must satisfy
+the @code{INSN_P} predicate.

-A @code{sequence} RTX is never placed in an actual insn during RTL
-generation.  It represents the sequence of insns that result from a
-@code{define_expand} @emph{before} those insns are passed to
-@code{emit_insn} to insert them in the chain of insns.  When actually
-inserted, the individual sub-insns are separated out and the
-@code{sequence} is forgotten.
-
 After delay-slot scheduling is completed, an insn and all the insns that
 reside in its delay slots are grouped together into a @code{sequence}.
 The insn requiring the delay slot is the first insn in the vector;
@@ -3123,6 +3116,19 @@ the effect of the insns in the delay slots.  In su
 the branch and should be executed only if the branch is taken; otherwise
 the insn should be executed only if the branch is not taken.
 @xref{Delay Slots}.
+
+Some back ends also use @code{sequence} objects for purposes other than
+delay-slot groups.  This is not supported in the common parts of the
+compiler, which treat such sequences as delay-slot groups.
+
+DWARF2 Call Frame Address (CFA) adjustments are sometimes also expressed
+using @code{sequence} objects as the value of a @code{RTX_FRAME_RELATED_P}
+note.  This only happens if the CFA adjustments cannot be easily derived
+from the pattern of the instruction to which the note is attached.  In
+such cases, the value of the note is used instead of best-guesing the
+semantics of the instruction.  The back end can attach notes containing
+a @code{sequence} of @code{set} patterns that express the effect of the
+parent instruction.
 @end table

 These expression codes appear in place of a side effect, as the body of
Index: rtl.def
===================================================================
--- rtl.def     (revision 197533)
+++ rtl.def     (working copy)
@@ -102,10 +102,24 @@ DEF_RTL_EXPR(EXPR_LIST, "expr_list", "ee", RTX_EXT
    The insns are represented in print by their uids.  */
 DEF_RTL_EXPR(INSN_LIST, "insn_list", "ue", RTX_EXTRA)

-/* SEQUENCE appears in the result of a `gen_...' function
-   for a DEFINE_EXPAND that wants to make several insns.
-   Its elements are the bodies of the insns that should be made.
-   `emit_insn' takes the SEQUENCE apart and makes separate insns.  */
+/* SEQUENCE is used in late passes of the compiler to group insns for
+   one reason or another.
+
+   For example, after delay slot filling, branch instructions with filled
+   delay slots are represented as a SEQUENCE of length 1 + n_delay_slots,
+   with the branch instruction in XEXPVEC(seq, 0, 0) and the instructions
+   occupying the delay slots in the remaining XEXPVEC slots.
+
+   Another place where a SEQUENCE may appear, is in REG_FRAME_RELATED_EXPR
+   notes, to express complex operations that are not obvious from the insn
+   to which the REG_FRAME_RELATED_EXPR note is attached.  In this usage of
+   SEQUENCE, the sequence vector slots do not hold real instructions but
+   only pseudo-instructions that can be translated to DWARF CFA expressions.
+
+   Some back ends also use SEQUENCE to group insns in bundles.
+
+   Much of the compiler infrastructure is not prepared to handle SEQUENCE
+   objects.  Only passes after pass_free_cfg are expected to handle them.  */
 DEF_RTL_EXPR(SEQUENCE, "sequence", "E", RTX_EXTRA)

 /* Represents a non-global base address.  This is only used in alias.c.  */
