Allow prologues and epilogues to be inserted later

Arm's SME adds a new processor mode called streaming mode.
This mode enables some new (matrix-oriented) instructions and
disables several existing groups of instructions, such as most
Advanced SIMD vector instructions and a much smaller set of SVE
instructions.  It can also change the current vector length.

There are instructions to switch in and out of streaming mode.
However, their effect on the ISA and vector length can't be represented
directly in RTL, so they need to be emitted late in the pass pipeline,
close to md_reorg.

It's sometimes the responsibility of the prologue and epilogue to
switch modes, which means we need to emit the prologue and epilogue
sequences late as well.  (This loses shrink-wrapping and scheduling
opportunities, but that's a price worth paying.)

This patch therefore adds a target hook for forcing prologue
and epilogue insertion to happen later in the pipeline.

gcc/
	* target.def (use_late_prologue_epilogue): New hook.
	* doc/tm.texi.in: Add TARGET_USE_LATE_PROLOGUE_EPILOGUE.
	* doc/tm.texi: Regenerate.
	* passes.def (pass_late_thread_prologue_and_epilogue): New pass.
	* tree-pass.h (make_pass_late_thread_prologue_and_epilogue): Declare.
	* function.cc (pass_thread_prologue_and_epilogue::gate): New function.
	(pass_data_late_thread_prologue_and_epilogue): New pass variable.
	(pass_late_thread_prologue_and_epilogue): New pass class.
	(make_pass_late_thread_prologue_and_epilogue): New function.

gcc/doc/tm.texi

@@ -11879,6 +11879,25 @@ of the if-block in the @code{struct ce_if_block} structure that is pointed
 to by @var{ce_info}.
 @end defmac
 
+@deftypefn {Target Hook} bool TARGET_USE_LATE_PROLOGUE_EPILOGUE ()
+Return true if the current function's prologue and epilogue should
+be emitted late in the pass pipeline, instead of at the usual point.
+
+Normally, the prologue and epilogue sequences are introduced soon after
+register allocation is complete.  The advantage of this approach is that
+it allows the prologue and epilogue instructions to be optimized and
+scheduled with other code in the function.  However, some targets
+require the prologue and epilogue to be the first and last sequences
+executed by the function, with no variation allowed.  This hook should
+return true on such targets.
+
+The default implementation returns false, which is correct for most
+targets.  The hook should only return true if there is a specific
+target limitation that cannot be described in RTL.  For example,
+the hook might return true if the prologue and epilogue need to switch
+between instruction sets.
+@end deftypefn
+
 @deftypefn {Target Hook} void TARGET_MACHINE_DEPENDENT_REORG (void)
 If non-null, this hook performs a target-specific pass over the
 instruction stream.  The compiler will run it at all optimization levels,

gcc/doc/tm.texi.in

@@ -7784,6 +7784,8 @@ of the if-block in the @code{struct ce_if_block} structure that is pointed
 to by @var{ce_info}.
 @end defmac
 
+@hook TARGET_USE_LATE_PROLOGUE_EPILOGUE
+
 @hook TARGET_MACHINE_DEPENDENT_REORG
 
 @hook TARGET_INIT_BUILTINS

gcc/function.cc

@@ -84,6 +84,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "function-abi.h"
 #include "value-range.h"
 #include "gimple-range.h"
+#include "insn-attr.h"
 
 /* So we can assign to cfun in this file.  */
 #undef cfun
@@ -6629,6 +6630,11 @@ public:
   {}
 
   /* opt_pass methods: */
+  bool gate (function *) final override
+    {
+      return !targetm.use_late_prologue_epilogue ();
+    }
+
   unsigned int execute (function * fun) final override
     {
       rest_of_handle_thread_prologue_and_epilogue (fun);
@@ -6637,6 +6643,44 @@ public:
 
 }; // class pass_thread_prologue_and_epilogue
 
+const pass_data pass_data_late_thread_prologue_and_epilogue =
+{
+  RTL_PASS, /* type */
+  "late_pro_and_epilogue", /* name */
+  OPTGROUP_NONE, /* optinfo_flags */
+  TV_THREAD_PROLOGUE_AND_EPILOGUE, /* tv_id */
+  0, /* properties_required */
+  0, /* properties_provided */
+  0, /* properties_destroyed */
+  0, /* todo_flags_start */
+  ( TODO_df_verify | TODO_df_finish ), /* todo_flags_finish */
+};
+
+class pass_late_thread_prologue_and_epilogue : public rtl_opt_pass
+{
+public:
+  pass_late_thread_prologue_and_epilogue (gcc::context *ctxt)
+    : rtl_opt_pass (pass_data_late_thread_prologue_and_epilogue, ctxt)
+  {}
+
+  /* opt_pass methods: */
+  bool gate (function *) final override
+    {
+      return targetm.use_late_prologue_epilogue ();
+    }
+
+  unsigned int execute (function *fn) final override
+    {
+      /* It's not currently possible to have both delay slots and
+	 late prologue/epilogue, since the latter has to run before
+	 the former, and the former won't honor whatever restrictions
+	 the latter is trying to enforce.  */
+      gcc_assert (!DELAY_SLOTS);
+      rest_of_handle_thread_prologue_and_epilogue (fn);
+      return 0;
+    }
+}; // class pass_late_thread_prologue_and_epilogue
+
 } // anon namespace
 
 rtl_opt_pass *
@@ -6645,6 +6689,12 @@ make_pass_thread_prologue_and_epilogue (gcc::context *ctxt)
   return new pass_thread_prologue_and_epilogue (ctxt);
 }
 
+rtl_opt_pass *
+make_pass_late_thread_prologue_and_epilogue (gcc::context *ctxt)
+{
+  return new pass_late_thread_prologue_and_epilogue (ctxt);
+}
+
 namespace {
 
 const pass_data pass_data_zero_call_used_regs =

gcc/passes.def

@@ -533,6 +533,9 @@ along with GCC; see the file COPYING3.  If not see
 	      NEXT_PASS (pass_stack_regs_run);
 	  POP_INSERT_PASSES ()
       POP_INSERT_PASSES ()
+      NEXT_PASS (pass_late_thread_prologue_and_epilogue);
+      /* No target-independent code motion is allowed beyond this point,
+         excepting the legacy delayed-branch pass.  */
       NEXT_PASS (pass_late_compilation);
       PUSH_INSERT_PASSES_WITHIN (pass_late_compilation)
 	  NEXT_PASS (pass_zero_call_used_regs);

gcc/target.def

@@ -4153,6 +4153,27 @@ returns @code{VOIDmode}.",
  machine_mode, (machine_mode m1, machine_mode m2),
  default_cc_modes_compatible)
 
+DEFHOOK
+(use_late_prologue_epilogue,
+ "Return true if the current function's prologue and epilogue should\n\
+be emitted late in the pass pipeline, instead of at the usual point.\n\
+\n\
+Normally, the prologue and epilogue sequences are introduced soon after\n\
+register allocation is complete.  The advantage of this approach is that\n\
+it allows the prologue and epilogue instructions to be optimized and\n\
+scheduled with other code in the function.  However, some targets\n\
+require the prologue and epilogue to be the first and last sequences\n\
+executed by the function, with no variation allowed.  This hook should\n\
+return true on such targets.\n\
+\n\
+The default implementation returns false, which is correct for most\n\
+targets.  The hook should only return true if there is a specific\n\
+target limitation that cannot be described in RTL.  For example,\n\
+the hook might return true if the prologue and epilogue need to switch\n\
+between instruction sets.",
+ bool, (),
+ hook_bool_void_false)
+
 /* Do machine-dependent code transformations.  Called just before
      delayed-branch scheduling.  */
 DEFHOOK

gcc/tree-pass.h

@@ -616,6 +616,8 @@ extern rtl_opt_pass *make_pass_gcse2 (gcc::context *ctxt);
 extern rtl_opt_pass *make_pass_split_after_reload (gcc::context *ctxt);
 extern rtl_opt_pass *make_pass_thread_prologue_and_epilogue (gcc::context
 							     *ctxt);
+extern rtl_opt_pass *make_pass_late_thread_prologue_and_epilogue (gcc::context
+								  *ctxt);
 extern rtl_opt_pass *make_pass_zero_call_used_regs (gcc::context *ctxt);
 extern rtl_opt_pass *make_pass_stack_adjustments (gcc::context *ctxt);
 extern rtl_opt_pass *make_pass_sched_fusion (gcc::context *ctxt);