[PATCH][PR target/83994] Fix stack-clash-protection code generation on x86

[Jeff Law]

pr83994 is a code generation bug in the stack-clash support that affects
openssl (we've turned on stack-clash-protection by default for the F28
builds).

The core problem is stack-clash (like stack-check) will emit a probing
loop if the prologue allocates enough stack space.  When emitting a loop
both implementations will need a scratch register.

They use get_scratch_register_at_entry to find a suitable scratch
register.  This routine assumes that callee registers saves are
completed at the point where the scratch register is going to be used.

In this particular testcase we select %ebx because ax,cx,dx are used for
parameter passing.  That's fine.  The problem is %ebx hasn't been saved yet!

-fstack-check has a bit of code in the frame setup/layout code which
forces the prologue to use pushes rather than reg->mem moves for saving
registers.  There's a gcc_assert in the prologue expander to catch any
case where the registers aren't saved.

-fstack-clash-protection doesn't have that same bit of magic in the
frame setup/layout code and it bypasses the assertion due to a change I
made back in Nov 2017 due to not being aware of this particular issue.

This patch reverts the assertion bypass I added back in Nov 2017 and
adds clarifying comments.  The patch also forces use of push to save
integer registers for a stack-clash protected prologue if probes are
going to be needed.

Bootstrapped and regression tested on x86_64.

While the bug is not marked as a regression, ISTM this needs to be fixed
for gcc-8.

OK for the trunk?

Jeff
-------------- next part --------------
	* i386.c (get_probe_interval): Move to earlier point.
	(ix86_compute_frame_layout): If -fstack-clash-protection and
	the frame is larger than the probe interval, then use pushes
	to save registers rather than reg->mem moves.
	(ix86_expand_prologue): Remove conditional for int_registers_saved
	assertion.

	* gcc.target/i386/pr83994.c: New test.
	

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 72d25ae..4cb55a8 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -11497,6 +11497,18 @@ static void warn_once_call_ms2sysv_xlogues (const char *feature)
     }
 }
 
+/* Return the probing interval for -fstack-clash-protection.  */
+
+static HOST_WIDE_INT
+get_probe_interval (void)
+{
+  if (flag_stack_clash_protection)
+    return (HOST_WIDE_INT_1U
+	    << PARAM_VALUE (PARAM_STACK_CLASH_PROTECTION_PROBE_INTERVAL));
+  else
+    return (HOST_WIDE_INT_1U << STACK_CHECK_PROBE_INTERVAL_EXP);
+}
+
 /* When using -fsplit-stack, the allocation routines set a field in
    the TCB to the bottom of the stack plus this much space, measured
    in bytes.  */
@@ -11773,7 +11785,14 @@ ix86_compute_frame_layout (void)
   to_allocate = offset - frame->sse_reg_save_offset;
 
   if ((!to_allocate && frame->nregs <= 1)
-      || (TARGET_64BIT && to_allocate >= HOST_WIDE_INT_C (0x80000000)))
+      || (TARGET_64BIT && to_allocate >= HOST_WIDE_INT_C (0x80000000))
+      /* If stack clash probing needs a loop, then it needs a
+	 scratch register.  But the returned register is only guaranteed
+	 to be safe to use after register saves are complete.  So if
+	 stack clash protections are enabled and the allocated frame is
+	 larger than the probe interval, then use pushes to save
+	 callee saved registers.  */
+      || (flag_stack_clash_protection && to_allocate > get_probe_interval ()))
     frame->save_regs_using_mov = false;
 
   if (ix86_using_red_zone ()
@@ -12567,18 +12586,6 @@ release_scratch_register_on_entry (struct scratch_reg *sr)
     }
 }
 
-/* Return the probing interval for -fstack-clash-protection.  */
-
-static HOST_WIDE_INT
-get_probe_interval (void)
-{
-  if (flag_stack_clash_protection)
-    return (HOST_WIDE_INT_1U
-	    << PARAM_VALUE (PARAM_STACK_CLASH_PROTECTION_PROBE_INTERVAL));
-  else
-    return (HOST_WIDE_INT_1U << STACK_CHECK_PROBE_INTERVAL_EXP);
-}
-
 /* Emit code to adjust the stack pointer by SIZE bytes while probing it.
 
    This differs from the next routine in that it tries hard to prevent
@@ -13727,12 +13734,11 @@ ix86_expand_prologue (void)
       && (flag_stack_check == STATIC_BUILTIN_STACK_CHECK
 	  || flag_stack_clash_protection))
     {
-      /* This assert wants to verify that integer registers were saved
-	 prior to probing.  This is necessary when probing may be implemented
-	 as a function call (Windows).  It is not necessary for stack clash
-	 protection probing.  */
-      if (!flag_stack_clash_protection)
-	gcc_assert (int_registers_saved);
+      /* We expect the GP registers to be saved when probes are used
+	 as the probing sequences might need a scratch register and
+	 the routine to allocate one assumes the integer registers
+	 have already been saved.  */
+      gcc_assert (int_registers_saved);
 
       if (flag_stack_clash_protection)
 	{
diff --git a/gcc/testsuite/gcc.target/i386/pr83994.c b/gcc/testsuite/gcc.target/i386/pr83994.c
new file mode 100644
index 0000000..b57b04b
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr83994.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -m32 -march=i686 -fpic -fstack-clash-protection" } */
+
+void f1 (char *);
+
+__attribute__ ((regparm (3)))
+int
+f2 (int arg1, int arg2, int arg3)
+{
+  char buf[16384];
+  f1 (buf);
+  f1 (buf);
+  return 0;
+}
+