crosstool-ng/packages/gcc/11.4.0/0025-aarch64-Put-LR-save-probe-in-first-16-bytes.patch

From 174a9747491e591ef2abb3e20a0332303f11003a Mon Sep 17 00:00:00 2001
From: Richard Sandiford <richard.sandiford@arm.com>
Date: Tue, 12 Sep 2023 16:19:49 +0100
Subject: [PATCH 25/29] aarch64: Put LR save probe in first 16 bytes

-fstack-clash-protection uses the save of LR as a probe for the next
allocation.  The next allocation could be:

* another part of the static frame, e.g. when allocating SVE save slots
  or outgoing arguments

* an alloca in the same function

* an allocation made by a callee function

However, when -fomit-frame-pointer is used, the LR save slot is placed
above the other GPR save slots.  It could therefore be up to 80 bytes
above the base of the GPR save area (which is also the hard fp address).

aarch64_allocate_and_probe_stack_space took this into account when
deciding how much subsequent space could be allocated without needing
a probe.  However, it interacted badly with:

      /* If doing a small final adjustment, we always probe at offset 0.
	 This is done to avoid issues when LR is not at position 0 or when
	 the final adjustment is smaller than the probing offset.  */
      else if (final_adjustment_p && rounded_size == 0)
	residual_probe_offset = 0;

which forces any allocation that is smaller than the guard page size
to be probed at offset 0 rather than the usual offset 1024.  It was
therefore possible to construct cases in which we had:

* a probe using LR at SP + 80 bytes (or some other value >= 16)
* an allocation of the guard page size - 16 bytes
* a probe at SP + 0

which allocates guard page size + 64 consecutive unprobed bytes.

This patch requires the LR probe to be in the first 16 bytes of the
save area when stack clash protection is active.  Doing it
unconditionally would cause code-quality regressions, but a later
patch deals with that.

The new comment doesn't say that the probe register is required
to be LR, since a later patch removes that restriction.

gcc/
	* config/aarch64/aarch64.c (aarch64_layout_frame): Ensure that
	the LR save slot is in the first 16 bytes of the register save area.
	(aarch64_allocate_and_probe_stack_space): Remove workaround for
	when LR was not in the first 16 bytes.

gcc/testsuite/
	* gcc.target/aarch64/stack-check-prologue-18.c: New test.
---
 gcc/config/aarch64/aarch64.c                  |  61 ++++-------
 .../aarch64/stack-check-prologue-18.c         | 100 ++++++++++++++++++
 2 files changed, 123 insertions(+), 38 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/aarch64/stack-check-prologue-18.c

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index bfd248761951..3f2b10de987d 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -7532,26 +7532,34 @@ aarch64_layout_frame (void)
   bool saves_below_hard_fp_p
     = maybe_ne (frame.below_hard_fp_saved_regs_size, 0);
   frame.bytes_below_hard_fp = offset;
+
+  auto allocate_gpr_slot = [&](unsigned int regno)
+    {
+      frame.reg_offset[regno] = offset;
+      if (frame.wb_candidate1 == INVALID_REGNUM)
+	frame.wb_candidate1 = regno;
+      else if (frame.wb_candidate2 == INVALID_REGNUM)
+	frame.wb_candidate2 = regno;
+      offset += UNITS_PER_WORD;
+    };
+
   if (frame.emit_frame_chain)
     {
       /* FP and LR are placed in the linkage record.  */
-      frame.reg_offset[R29_REGNUM] = offset;
-      frame.wb_candidate1 = R29_REGNUM;
-      frame.reg_offset[R30_REGNUM] = offset + UNITS_PER_WORD;
-      frame.wb_candidate2 = R30_REGNUM;
-      offset += 2 * UNITS_PER_WORD;
+      allocate_gpr_slot (R29_REGNUM);
+      allocate_gpr_slot (R30_REGNUM);
     }
+  else if (flag_stack_clash_protection
+	   && known_eq (frame.reg_offset[R30_REGNUM], SLOT_REQUIRED))
+    /* Put the LR save slot first, since it makes a good choice of probe
+       for stack clash purposes.  The idea is that the link register usually
+       has to be saved before a call anyway, and so we lose little by
+       stopping it from being individually shrink-wrapped.  */
+    allocate_gpr_slot (R30_REGNUM);
 
   for (regno = R0_REGNUM; regno <= R30_REGNUM; regno++)
     if (known_eq (frame.reg_offset[regno], SLOT_REQUIRED))
-      {
-	frame.reg_offset[regno] = offset;
-	if (frame.wb_candidate1 == INVALID_REGNUM)
-	  frame.wb_candidate1 = regno;
-	else if (frame.wb_candidate2 == INVALID_REGNUM)
-	  frame.wb_candidate2 = regno;
-	offset += UNITS_PER_WORD;
-      }
+      allocate_gpr_slot (regno);
 
   poly_int64 max_int_offset = offset;
   offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
@@ -8570,29 +8578,6 @@ aarch64_allocate_and_probe_stack_space (rtx temp1, rtx temp2,
     = (final_adjustment_p
        ? guard_used_by_caller + byte_sp_alignment
        : guard_size - guard_used_by_caller);
-  /* When doing the final adjustment for the outgoing arguments, take into
-     account any unprobed space there is above the current SP.  There are
-     two cases:
-
-     - When saving SVE registers below the hard frame pointer, we force
-       the lowest save to take place in the prologue before doing the final
-       adjustment (i.e. we don't allow the save to be shrink-wrapped).
-       This acts as a probe at SP, so there is no unprobed space.
-
-     - When there are no SVE register saves, we use the store of the link
-       register as a probe.  We can't assume that LR was saved at position 0
-       though, so treat any space below it as unprobed.  */
-  if (final_adjustment_p
-      && known_eq (frame.below_hard_fp_saved_regs_size, 0))
-    {
-      poly_int64 lr_offset = (frame.reg_offset[LR_REGNUM]
-			      - frame.bytes_below_saved_regs);
-      if (known_ge (lr_offset, 0))
-	min_probe_threshold -= lr_offset.to_constant ();
-      else
-	gcc_assert (!flag_stack_clash_protection || known_eq (poly_size, 0));
-    }
-
   poly_int64 frame_size = frame.frame_size;
 
   /* We should always have a positive probe threshold.  */
@@ -8772,8 +8757,8 @@ aarch64_allocate_and_probe_stack_space (rtx temp1, rtx temp2,
       if (final_adjustment_p && rounded_size != 0)
 	min_probe_threshold = 0;
       /* If doing a small final adjustment, we always probe at offset 0.
-	 This is done to avoid issues when LR is not at position 0 or when
-	 the final adjustment is smaller than the probing offset.  */
+	 This is done to avoid issues when the final adjustment is smaller
+	 than the probing offset.  */
       else if (final_adjustment_p && rounded_size == 0)
 	residual_probe_offset = 0;
 
diff --git a/gcc/testsuite/gcc.target/aarch64/stack-check-prologue-18.c b/gcc/testsuite/gcc.target/aarch64/stack-check-prologue-18.c
new file mode 100644
index 000000000000..82447d20fff5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/stack-check-prologue-18.c
@@ -0,0 +1,100 @@
+/* { dg-options "-O2 -fstack-clash-protection -fomit-frame-pointer --param stack-clash-protection-guard-size=12" } */
+/* { dg-final { check-function-bodies "**" "" } } */
+
+void f(int, ...);
+void g();
+
+/*
+** test1:
+**	...
+**	str	x30, \[sp\]
+**	sub	sp, sp, #4064
+**	str	xzr, \[sp\]
+**	cbnz	w0, .*
+**	bl	g
+**	...
+**	str	x26, \[sp, #?4128\]
+**	...
+*/
+int test1(int z) {
+  __uint128_t x = 0;
+  int y[0x400];
+  if (z)
+    {
+      asm volatile ("" :::
+		    "x19", "x20", "x21", "x22", "x23", "x24", "x25", "x26");
+      f(0, 0, 0, 0, 0, 0, 0, &y,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x);
+    }
+  g();
+  return 1;
+}
+
+/*
+** test2:
+**	...
+**	str	x30, \[sp\]
+**	sub	sp, sp, #1040
+**	str	xzr, \[sp\]
+**	cbnz	w0, .*
+**	bl	g
+**	...
+*/
+int test2(int z) {
+  __uint128_t x = 0;
+  int y[0x400];
+  if (z)
+    {
+      asm volatile ("" :::
+		    "x19", "x20", "x21", "x22", "x23", "x24", "x25", "x26");
+      f(0, 0, 0, 0, 0, 0, 0, &y,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x);
+    }
+  g();
+  return 1;
+}
+
+/*
+** test3:
+**	...
+**	str	x30, \[sp\]
+**	sub	sp, sp, #1024
+**	cbnz	w0, .*
+**	bl	g
+**	...
+*/
+int test3(int z) {
+  __uint128_t x = 0;
+  int y[0x400];
+  if (z)
+    {
+      asm volatile ("" :::
+		    "x19", "x20", "x21", "x22", "x23", "x24", "x25", "x26");
+      f(0, 0, 0, 0, 0, 0, 0, &y,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
+	x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x);
+    }
+  g();
+  return 1;
+}
-- 
2.42.0