commit 799fb6fee433891510ab613a024450c926cdb4b3
Author: law <law@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed Nov 15 06:30:31 2017 +0000

            * explow.c (anti_adjust_stack_and_probe_stack_clash): Avoid probing
            the red zone for stack_clash_protection_final_dynamic_probe targets
            when the total dynamic stack size is zero bytes.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254753 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/explow.c b/gcc/explow.c
index 6af6355fe30..53c4a6d7db9 100644
--- a/gcc/explow.c
+++ b/gcc/explow.c
@@ -1978,6 +1978,13 @@ anti_adjust_stack_and_probe_stack_clash (rtx size)
   if (size != CONST0_RTX (Pmode)
       && targetm.stack_clash_protection_final_dynamic_probe (residual))
     {
+      /* SIZE could be zero at runtime and in that case *sp could hold
+	 live data.  Furthermore, we don't want to probe into the red
+	 zone.
+
+	 Go ahead and just guard a probe at *sp on SIZE != 0 at runtime
+	 if SIZE is not a compile time constant.  */
+
       /* Ideally we would just probe at *sp.  However, if SIZE is not
 	 a compile-time constant, but is zero at runtime, then *sp
 	 might hold live data.  So probe at *sp if we know that
@@ -1990,9 +1997,12 @@ anti_adjust_stack_and_probe_stack_clash (rtx size)
 	}
       else
 	{
-	  emit_stack_probe (plus_constant (Pmode, stack_pointer_rtx,
-					   -GET_MODE_SIZE (word_mode)));
+	  rtx label = gen_label_rtx ();
+	  emit_cmp_and_jump_insns (size, CONST0_RTX (GET_MODE (size)),
+				   EQ, NULL_RTX, Pmode, 1, label);
+	  emit_stack_probe (stack_pointer_rtx);
 	  emit_insn (gen_blockage ());
+	  emit_label (label);
 	}
     }
 }