From 6f0ab0a9f46a17b68349ff6035aa776bf65f0575 Mon Sep 17 00:00:00 2001
From: Richard Sandiford <[email protected]>
Date: Tue, 12 Sep 2023 16:08:56 +0100
Subject: [PATCH] aarch64: Explicitly record probe registers in frame info

The stack frame is currently divided into three areas:

A: the area above the hard frame pointer
B: the SVE saves below the hard frame pointer
C: the outgoing arguments

If the stack frame is allocated in one chunk, the allocation needs a
probe if the frame size is >= guard_size - 1KiB.  In addition, if the
function is not a leaf function, it must probe an address no more than
1KiB above the outgoing SP.  We ensured the second condition by

(1) using single-chunk allocations for non-leaf functions only if
    the link register save slot is within 512 bytes of the bottom
    of the frame; and

(2) using the link register save as a probe (meaning, for instance,
    that it can't be individually shrink wrapped)

If instead the stack is allocated in multiple chunks, then:

* an allocation involving only the outgoing arguments (C above) requires
  a probe if the allocation size is > 1KiB

* any other allocation requires a probe if the allocation size
  is >= guard_size - 1KiB

* second and subsequent allocations require the previous allocation
  to probe at the bottom of the allocated area, regardless of the size
  of that previous allocation

The final point means that, unlike for single allocations,
it can be necessary to have both a non-SVE register probe and
an SVE register probe.  For example:

* allocate A, probe using a non-SVE register save
* allocate B, probe using an SVE register save
* allocate C

The non-SVE register used in this case was again the link register.
It was previously used even if the link register save slot was some
bytes above the bottom of the non-SVE register saves, but an earlier
patch avoided that by putting the link register save slot first.

As a belt-and-braces fix, this patch explicitly records which
probe registers we're using and allows the non-SVE probe to be
whichever register comes first (as for SVE).

The patch also avoids unnecessary probes in sve/pcs/stack_clash_3.c.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::sve_save_and_probe)
	(aarch64_frame::hard_fp_save_and_probe): New fields.
	* config/aarch64/aarch64.cc (aarch64_layout_frame): Initialize them.
	Rather than asserting that a leaf function saves LR, instead assert
	that a leaf function saves something.
	(aarch64_get_separate_components): Prevent the chosen probe
	registers from being individually shrink-wrapped.
	(aarch64_allocate_and_probe_stack_space): Remove workaround for
	probe registers that aren't at the bottom of the previous allocation.

gcc/testsuite/
	* gcc.target/aarch64/sve/pcs/stack_clash_3.c: Avoid redundant probes.
---
 gcc/config/aarch64/aarch64.cc                 | 68 +++++++++++++++----
 gcc/config/aarch64/aarch64.h                  |  8 +++
 .../aarch64/sve/pcs/stack_clash_3.c           |  6 +-
 3 files changed, 64 insertions(+), 18 deletions(-)

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 8abf3d7a1e2..a8d907df884 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -8210,15 +8210,11 @@ aarch64_layout_frame (void)
 	&& !crtl->abi->clobbers_full_reg_p (regno))
       frame.reg_offset[regno] = SLOT_REQUIRED;
 
-  /* With stack-clash, LR must be saved in non-leaf functions.  The saving of
-     LR counts as an implicit probe which allows us to maintain the invariant
-     described in the comment at expand_prologue.  */
-  gcc_assert (crtl->is_leaf
-	      || maybe_ne (frame.reg_offset[R30_REGNUM], SLOT_NOT_REQUIRED));
 
   poly_int64 offset = crtl->outgoing_args_size;
   gcc_assert (multiple_p (offset, STACK_BOUNDARY / BITS_PER_UNIT));
   frame.bytes_below_saved_regs = offset;
+  frame.sve_save_and_probe = INVALID_REGNUM;
 
   /* Now assign stack slots for the registers.  Start with the predicate
      registers, since predicate LDR and STR have a relatively small
@@ -8226,6 +8222,8 @@ aarch64_layout_frame (void)
   for (regno = P0_REGNUM; regno <= P15_REGNUM; regno++)
     if (known_eq (frame.reg_offset[regno], SLOT_REQUIRED))
       {
+	if (frame.sve_save_and_probe == INVALID_REGNUM)
+	  frame.sve_save_and_probe = regno;
 	frame.reg_offset[regno] = offset;
 	offset += BYTES_PER_SVE_PRED;
       }
@@ -8263,6 +8261,8 @@ aarch64_layout_frame (void)
     for (regno = V0_REGNUM; regno <= V31_REGNUM; regno++)
       if (known_eq (frame.reg_offset[regno], SLOT_REQUIRED))
 	{
+	  if (frame.sve_save_and_probe == INVALID_REGNUM)
+	    frame.sve_save_and_probe = regno;
 	  frame.reg_offset[regno] = offset;
 	  offset += vector_save_size;
 	}
@@ -8272,10 +8272,18 @@ aarch64_layout_frame (void)
   frame.below_hard_fp_saved_regs_size = offset - frame.bytes_below_saved_regs;
   bool saves_below_hard_fp_p
     = maybe_ne (frame.below_hard_fp_saved_regs_size, 0);
+  gcc_assert (!saves_below_hard_fp_p
+	      || (frame.sve_save_and_probe != INVALID_REGNUM
+		  && known_eq (frame.reg_offset[frame.sve_save_and_probe],
+			       frame.bytes_below_saved_regs)));
+
   frame.bytes_below_hard_fp = offset;
+  frame.hard_fp_save_and_probe = INVALID_REGNUM;
 
   auto allocate_gpr_slot = [&](unsigned int regno)
     {
+      if (frame.hard_fp_save_and_probe == INVALID_REGNUM)
+	frame.hard_fp_save_and_probe = regno;
       frame.reg_offset[regno] = offset;
       if (frame.wb_push_candidate1 == INVALID_REGNUM)
 	frame.wb_push_candidate1 = regno;
@@ -8309,6 +8317,8 @@ aarch64_layout_frame (void)
   for (regno = V0_REGNUM; regno <= V31_REGNUM; regno++)
     if (known_eq (frame.reg_offset[regno], SLOT_REQUIRED))
       {
+	if (frame.hard_fp_save_and_probe == INVALID_REGNUM)
+	  frame.hard_fp_save_and_probe = regno;
 	/* If there is an alignment gap between integer and fp callee-saves,
 	   allocate the last fp register to it if possible.  */
 	if (regno == last_fp_reg
@@ -8332,6 +8342,17 @@ aarch64_layout_frame (void)
   offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
 
   frame.saved_regs_size = offset - frame.bytes_below_saved_regs;
+  gcc_assert (known_eq (frame.saved_regs_size,
+			frame.below_hard_fp_saved_regs_size)
+	      || (frame.hard_fp_save_and_probe != INVALID_REGNUM
+		  && known_eq (frame.reg_offset[frame.hard_fp_save_and_probe],
+			       frame.bytes_below_hard_fp)));
+
+  /* With stack-clash, a register must be saved in non-leaf functions.
+     The saving of the bottommost register counts as an implicit probe,
+     which allows us to maintain the invariant described in the comment
+     at expand_prologue.  */
+  gcc_assert (crtl->is_leaf || maybe_ne (frame.saved_regs_size, 0));
 
   offset += get_frame_size ();
   offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
@@ -8462,6 +8483,25 @@ aarch64_layout_frame (void)
       frame.final_adjust = frame.bytes_below_saved_regs;
     }
 
+  /* The frame is allocated in pieces, with each non-final piece
+     including a register save at offset 0 that acts as a probe for
+     the following piece.  In addition, the save of the bottommost register
+     acts as a probe for callees and allocas.  Roll back any probes that
+     aren't needed.
+
+     A probe isn't needed if it is associated with the final allocation
+     (including callees and allocas) that happens before the epilogue is
+     executed.  */
+  if (crtl->is_leaf
+      && !cfun->calls_alloca
+      && known_eq (frame.final_adjust, 0))
+    {
+      if (maybe_ne (frame.sve_callee_adjust, 0))
+	frame.sve_save_and_probe = INVALID_REGNUM;
+      else
+	frame.hard_fp_save_and_probe = INVALID_REGNUM;
+    }
+
   /* Make sure the individual adjustments add up to the full frame size.  */
   gcc_assert (known_eq (frame.initial_adjust
 			+ frame.callee_adjust
@@ -9039,13 +9079,6 @@ aarch64_get_separate_components (void)
 
 	poly_int64 offset = frame.reg_offset[regno];
 
-	/* If the register is saved in the first SVE save slot, we use
-	   it as a stack probe for -fstack-clash-protection.  */
-	if (flag_stack_clash_protection
-	    && maybe_ne (frame.below_hard_fp_saved_regs_size, 0)
-	    && known_eq (offset, frame.bytes_below_saved_regs))
-	  continue;
-
 	/* Get the offset relative to the register we'll use.  */
 	if (frame_pointer_needed)
 	  offset -= frame.bytes_below_hard_fp;
@@ -9080,6 +9113,13 @@ aarch64_get_separate_components (void)
 
   bitmap_clear_bit (components, LR_REGNUM);
   bitmap_clear_bit (components, SP_REGNUM);
+  if (flag_stack_clash_protection)
+    {
+      if (frame.sve_save_and_probe != INVALID_REGNUM)
+	bitmap_clear_bit (components, frame.sve_save_and_probe);
+      if (frame.hard_fp_save_and_probe != INVALID_REGNUM)
+	bitmap_clear_bit (components, frame.hard_fp_save_and_probe);
+    }
 
   return components;
 }
@@ -9616,8 +9656,8 @@ aarch64_epilogue_uses (int regno)
    When probing is needed, we emit a probe at the start of the prologue
    and every PARAM_STACK_CLASH_PROTECTION_GUARD_SIZE bytes thereafter.
 
-   We have to track how much space has been allocated and the only stores
-   to the stack we track as implicit probes are the FP/LR stores.
+   We can also use register saves as probes.  These are stored in
+   sve_save_and_probe and hard_fp_save_and_probe.
 
    For outgoing arguments we probe if the size is larger than 1KB, such that
    the ABI specified buffer is maintained for the next callee.
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index b6135837073..46d4693e206 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -957,6 +957,14 @@ struct GTY (()) aarch64_frame
      This is the register they should use.  */
   unsigned spare_pred_reg;
 
+  /* An SVE register that is saved below the hard frame pointer and that acts
+     as a probe for later allocations, or INVALID_REGNUM if none.  */
+  unsigned sve_save_and_probe;
+
+  /* A register that is saved at the hard frame pointer and that acts
+     as a probe for later allocations, or INVALID_REGNUM if none.  */
+  unsigned hard_fp_save_and_probe;
+
   bool laid_out;
 
   /* True if shadow call stack should be enabled for the current function.  */
diff --git a/gcc/testsuite/gcc.target/aarch64/sve/pcs/stack_clash_3.c b/gcc/testsuite/gcc.target/aarch64/sve/pcs/stack_clash_3.c
index 3e01ec36c3a..3530a0d504b 100644
--- a/gcc/testsuite/gcc.target/aarch64/sve/pcs/stack_clash_3.c
+++ b/gcc/testsuite/gcc.target/aarch64/sve/pcs/stack_clash_3.c
@@ -11,11 +11,10 @@
 **	mov	x11, sp
 **	...
 **	sub	sp, sp, x13
-**	str	p4, \[sp\]
 **	cbz	w0, [^\n]*
+**	str	p4, \[sp\]
 **	...
 **	ptrue	p0\.b, all
-**	ldr	p4, \[sp\]
 **	addvl	sp, sp, #1
 **	ldr	x24, \[sp\], 32
 **	ret
@@ -39,13 +38,12 @@ test_1 (int n)
 **	mov	x11, sp
 **	...
 **	sub	sp, sp, x13
-**	str	p4, \[sp\]
 **	cbz	w0, [^\n]*
+**	str	p4, \[sp\]
 **	str	p5, \[sp, #1, mul vl\]
 **	str	p6, \[sp, #2, mul vl\]
 **	...
 **	ptrue	p0\.b, all
-**	ldr	p4, \[sp\]
 **	addvl	sp, sp, #1
 **	ldr	x24, \[sp\], 32
 **	ret
-- 
2.37.3