Add support to the Xtensa target for creating trampolines for out-of-range branches.

    * tc-xtensa.c (xtensa_check_frag_count, xtensa_create_trampoline_frag)
    (xtensa_maybe_create_trampoline_frag, init_trampoline_frag)
    (find_trampoline_seg, search_trampolines, get_best_trampoline)
    (check_and_update_trampolines, add_jump_to_trampoline)
    (dump_trampolines): New function.
    (md_parse_option): Add cases for --[no-]trampolines options.
    (md_assemble, finish_vinsn, xtensa_end): Add call to
    xtensa_check_frag_count.
    (xg_assemble_vliw_tokens): Add call to
    xtensa_maybe_create_trampoline_frag.
    (xtensa_relax_frag): Relax fragments with RELAX_TRAMPOLINE state.
    (relax_frag_immed): Relax jump instructions that cannot reach its
    target.
    * tc-xtensa.h (xtensa_relax_statesE::RELAX_TRAMPOLINE): New relax
    state.

    * as.texinfo: Document --[no-]trampolines command-line options.
    * c-xtensa.texi: Document trampolines relaxation and command line
    options.

    * frags.c (get_frag_count, clear_frag_count): New function.
    (frag_alloc): Increment totalfrags counter.
    * frags.h (get_frag_count, clear_frag_count): New function.

    * all.exp: Add test for trampoline relaxation.
    * trampoline.d: Trampoline relaxation expected dump.
    * trampoline.s: Trampoline relaxation test source.

2 files changed, 93 insertions, 0 deletions

diff --git a/gas/doc/as.texinfo b/gas/doc/as.texinfo
index 5c8b000..dc97175 100644
--- a/gas/doc/as.texinfo
+++ b/gas/doc/as.texinfo
@@ -543,6 +543,7 @@ gcc(1), ld(1), and the Info entries for @file{binutils} and @file{ld}.
  [@b{--[no-]target-align}] [@b{--[no-]longcalls}]
  [@b{--[no-]transform}]
  [@b{--rename-section} @var{oldname}=@var{newname}]
+ [@b{--[no-]trampolines}]
 @end ifset
 
 @ifset Z80
diff --git a/gas/doc/c-xtensa.texi b/gas/doc/c-xtensa.texi
index 406e635..e763e36 100644
--- a/gas/doc/c-xtensa.texi
+++ b/gas/doc/c-xtensa.texi
@@ -91,6 +91,16 @@ instruction operands to be errors.
 @kindex --rename-section
 Rename the @var{oldname} section to @var{newname}.  This option can be used
 multiple times to rename multiple sections.
+
+@item --trampolines | --no-trampolines
+@kindex --trampolines
+@kindex --no-trampolines
+Enable or disable transformation of jump instructions to allow jumps
+across a greater range of addresses.  @xref{Xtensa Jump Relaxation,
+,Jump Trampolines}.  This option should be used when jump targets can
+potentially be out of range.  In the absence of such jumps this option
+does not affect code size or performance.  The default is
+@samp{--trampolines}.
 @end table
 
 @c man end
@@ -311,6 +321,7 @@ fields.
 @menu
 * Xtensa Branch Relaxation::        Relaxation of Branches.
 * Xtensa Call Relaxation::          Relaxation of Function Calls.
+* Xtensa Jump Relaxation::          Relaxation of Jumps.
 * Xtensa Immediate Relaxation::     Relaxation of other Immediate Fields.
 @end menu
 
@@ -398,6 +409,87 @@ and some of the calls are out of range, function call relaxation can be
 enabled using the @samp{--longcalls} command-line option or the
 @code{longcalls} directive (@pxref{Longcalls Directive, ,longcalls}).
 
+@node Xtensa Jump Relaxation
+@subsection Jump Relaxation
+@cindex relaxation of jump instructions
+@cindex jump instructions, relaxation
+
+Jump instruction may require relaxation because the Xtensa jump instruction
+(@code{J}) provide a PC-relative offset of only 128 Kbytes in either
+direction.  One option is to use jump long (@code{J.L}) instruction, which
+depending on jump distance may be assembled as jump (@code{J}) or indirect
+jump (@code{JX}).  However it needs a free register.  When there's no spare
+register it is possible to plant intermediate jump sites (trampolines)
+between the jump instruction and its target.  These sites may be located in
+areas unreachable by normal code execution flow, in that case they only
+contain intermediate jumps, or they may be inserted in the middle of code
+block, in which case there's an additional jump from the beginning of the
+trampoline to the instruction past its end.  So, for example:
+
+@smallexample
+@group
+    j 1f
+    ...
+    retw
+    ...
+    mov a10, a2
+    call8 func
+    ...
+1:
+    ...
+@end group
+@end smallexample
+
+might be relaxed to:
+
+@smallexample
+@group
+    j .L0_TR_1
+    ...
+    retw
+.L0_TR_1:
+    j 1f
+    ...
+    mov a10, a2
+    call8 func
+    ...
+1:
+    ...
+@end group
+@end smallexample
+
+or to:
+
+@smallexample
+@group
+    j .L0_TR_1
+    ...
+    retw
+    ...
+    mov a10, a2
+    j .L0_TR_0
+.L0_TR_1:
+    j 1f
+.L0_TR_0:
+    call8 func
+    ...
+1:
+    ...
+@end group
+@end smallexample
+
+The Xtensa assempler uses trampolines with jump around only when it cannot
+find suitable unreachable trampoline.  There may be multiple trampolines
+between the jump instruction and its target.
+
+This relaxation does not apply to jumps to undefined symbols, assuming they
+will reach their targets once resolved.
+
+Jump relaxation is enabled by default because it does not affect code size
+or performance while the code itself is small.  This relaxation may be
+disabled completely with @samp{--no-trampolines} or @samp{--no-transform}
+command-line options (@pxref{Xtensa Options, ,Command Line Options}).
+
 @node Xtensa Immediate Relaxation
 @subsection Other Immediate Field Relaxation
 @cindex immediate fields, relaxation