[PATCH][RFA/RFC] Stack clash mitigation patch 05/08

[Jeff Law <law at redhat dot com>]

I don't think this has changed in any significant way since V1.

--

The prior patch introduced -fstack-clash-protection prologues for the
x86.  And yet we still saw large allocations in our testing.

It turns out combine-stack-adjustments would take

allocate PROBE_INTERVAL
probe
allocate PROBE_INTERVAL
probe
allocate PROBE_INTERVAL
probe
allocate RESIDUAL

And turn that into

allocate (3 * PROBE_INTERVAL) + residual
probe
probe
probe

Adjusting the address of the probes appropriately.  Ugh.

This patch introduces a new note that the backend can attach to a stack
adjustment which essentially tells c-s-a to not merge it into other
adjustments.  THere's an x86 specific test to verify behavior.

OK for the trunk?

	* combine-stack-adj.c (combine_stack_adjustments_for_block): Do
	nothing for stack adjustments with REG_STACK_CHECK.
	* config/i386/i386.c (pro_epilogue_adjust_stack): Return insn.
	(ix86_adjust_satck_and_probe_stack_clash): Add REG_STACK_NOTEs.
	* reg-notes.def (STACK_CHECK): New note.

testsuite/

	* gcc.target/i386/stack-check-11.c: New test.


diff --git a/gcc/combine-stack-adj.c b/gcc/combine-stack-adj.c
index 9ec14a3..82d6dba 100644
--- a/gcc/combine-stack-adj.c
+++ b/gcc/combine-stack-adj.c
@@ -508,6 +508,8 @@ combine_stack_adjustments_for_block (basic_block bb)
 	continue;
 
       set = single_set_for_csa (insn);
+      if (set && find_reg_note (insn, REG_STACK_CHECK, NULL_RTX))
+	set = NULL_RTX;
       if (set)
 	{
 	  rtx dest = SET_DEST (set);
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index fa10c28..d297e8a 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -13405,7 +13405,7 @@ ix86_add_queued_cfa_restore_notes (rtx insn)
    zero if %r11 register is live and cannot be freely used and positive
    otherwise.  */
 
-static void
+static rtx
 pro_epilogue_adjust_stack (rtx dest, rtx src, rtx offset,
 			   int style, bool set_cfa)
 {
@@ -13496,6 +13496,7 @@ pro_epilogue_adjust_stack (rtx dest, rtx src, rtx offset,
       m->fs.sp_valid = valid;
       m->fs.sp_realigned = realigned;
     }
+  return insn;
 }
 
 /* Find an available register to be used as dynamic realign argument
@@ -13837,9 +13838,11 @@ ix86_adjust_stack_and_probe_stack_clash (const HOST_WIDE_INT size)
       for (i = PROBE_INTERVAL; i <= size; i += PROBE_INTERVAL)
 	{
 	  /* Allocate PROBE_INTERVAL bytes.  */
-	  pro_epilogue_adjust_stack (stack_pointer_rtx, stack_pointer_rtx,
+	  rtx insn
+	    = pro_epilogue_adjust_stack (stack_pointer_rtx, stack_pointer_rtx,
 				     GEN_INT (-PROBE_INTERVAL), -1,
 				     m->fs.cfa_reg == stack_pointer_rtx);
+	  add_reg_note (insn, REG_STACK_CHECK, const0_rtx);
 
 	  /* And probe at *sp.  */
 	  emit_stack_probe (stack_pointer_rtx);
diff --git a/gcc/reg-notes.def b/gcc/reg-notes.def
index 8734d26..18cf7e3 100644
--- a/gcc/reg-notes.def
+++ b/gcc/reg-notes.def
@@ -223,6 +223,10 @@ REG_NOTE (ARGS_SIZE)
    pseudo reg.  */
 REG_NOTE (RETURNED)
 
+/* Indicates the instruction is a stack check probe that should not
+   be combined with other stack adjustments.  */
+REG_NOTE (STACK_CHECK)
+
 /* Used to mark a call with the function decl called by the call.
    The decl might not be available in the call due to splitting of the call
    insn.  This note is a SYMBOL_REF.  */
diff --git a/gcc/testsuite/gcc.target/i386/stack-check-11.c b/gcc/testsuite/gcc.target/i386/stack-check-11.c
new file mode 100644
index 0000000..183103f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/stack-check-11.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fstack-clash-protection" } */
+/* { dg-require-effective-target supports_stack_clash_protection } */
+
+extern void arf (unsigned long int *, unsigned long int *);
+void
+frob ()
+{
+  unsigned long int num[859];
+  unsigned long int den[859];
+  arf (den, num);
+}
+
+/* { dg-final { scan-assembler-times "subq" 4 } } */
+/* { dg-final { scan-assembler-times "orq" 3 } } */
+