[PATCH] Allow prologues and epilogues to be inserted later

[Richard Sandiford <richard.sandiford@arm.com>]

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.

Tested on aarch64-linux-gnu (including with a follow-on patch)
and x86_64-linux-gnu.  OK to install?

Richard


gcc/
	* target.def (use_late_prologue_epilogue): New hook.
	* doc/gccint/target-macros/miscellaneous-parameters.rst: Add
	TARGET_USE_LATE_PROLOGUE_EPILOGUE.
	* doc/gccint/target-macros/tm.rst.in: 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.
---
 .../miscellaneous-parameters.rst              |  5 +++
 gcc/doc/gccint/target-macros/tm.rst.in        | 22 ++++++++++
 gcc/function.cc                               | 43 +++++++++++++++++++
 gcc/passes.def                                |  3 ++
 gcc/target.def                                | 21 +++++++++
 gcc/tree-pass.h                               |  2 +
 6 files changed, 96 insertions(+)

diff --git a/gcc/doc/gccint/target-macros/miscellaneous-parameters.rst b/gcc/doc/gccint/target-macros/miscellaneous-parameters.rst
index e4e348c2adc..b48f91d3fd2 100644
--- a/gcc/doc/gccint/target-macros/miscellaneous-parameters.rst
+++ b/gcc/doc/gccint/target-macros/miscellaneous-parameters.rst
@@ -551,6 +551,11 @@ Here are several miscellaneous parameters.
   of the if-block in the ``struct ce_if_block`` structure that is pointed
   to by :samp:`{ce_info}`.
 
+.. include:: tm.rst.in
+  :start-after: [TARGET_USE_LATE_PROLOGUE_EPILOGUE]
+  :end-before: [TARGET_USE_LATE_PROLOGUE_EPILOGUE]
+
+
 .. include:: tm.rst.in
   :start-after: [TARGET_MACHINE_DEPENDENT_REORG]
   :end-before: [TARGET_MACHINE_DEPENDENT_REORG]
diff --git a/gcc/doc/gccint/target-macros/tm.rst.in b/gcc/doc/gccint/target-macros/tm.rst.in
index 44f3a3b2222..2e789f8723d 100644
--- a/gcc/doc/gccint/target-macros/tm.rst.in
+++ b/gcc/doc/gccint/target-macros/tm.rst.in
@@ -3702,6 +3702,28 @@
 
 [TARGET_CC_MODES_COMPATIBLE]
 
+[TARGET_USE_LATE_PROLOGUE_EPILOGUE]
+.. function:: 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.
+
+[TARGET_USE_LATE_PROLOGUE_EPILOGUE]
+
 [TARGET_MACHINE_DEPENDENT_REORG]
 .. function:: void TARGET_MACHINE_DEPENDENT_REORG (void)
 
diff --git a/gcc/function.cc b/gcc/function.cc
index b54a1d81a3b..3b1ab5d09e5 100644
--- a/gcc/function.cc
+++ b/gcc/function.cc
@@ -6641,6 +6641,11 @@ public:
   {}
 
   /* opt_pass methods: */
+  bool gate (function *) final override
+    {
+      return !targetm.use_late_prologue_epilogue ();
+    }
+
   unsigned int execute (function *) final override
     {
       return rest_of_handle_thread_prologue_and_epilogue ();
@@ -6648,6 +6653,38 @@ 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 *) final override
+    {
+      return rest_of_handle_thread_prologue_and_epilogue ();
+    }
+}; // class pass_late_thread_prologue_and_epilogue
+
 } // anon namespace
 
 rtl_opt_pass *
@@ -6656,6 +6693,12 @@ 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 =
diff --git a/gcc/passes.def b/gcc/passes.def
index 193b5794749..822d5713f53 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -518,6 +518,9 @@ 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);
diff --git a/gcc/target.def b/gcc/target.def
index aed1c1d3e22..8b8aef982e8 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -4062,6 +4062,27 @@ returns ``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
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index 8480d41384b..63177764ffa 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -612,6 +612,8 @@ 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);
-- 
2.25.1