2018-12-18  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* cfgexpand.c (asm_clobber_reg_is_valid): Emit only a warning together
	with an information message when the stack pointer is clobbered.

testsuite:
2018-12-18  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* gcc.target/i386/pr52813.c: Adjust test.

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c	(revision 267164)
+++ gcc/cfgexpand.c	(working copy)
@@ -2854,6 +2854,7 @@ tree_conflicts_with_clobbers_p (tree t, HARD_REG_S
    asm clobber operand.  Some HW registers cannot be
    saved/restored, hence they should not be clobbered by
    asm statements.  */
+
 static bool
 asm_clobber_reg_is_valid (int regno, int nregs, const char *regname)
 {
@@ -2872,11 +2873,23 @@ asm_clobber_reg_is_valid (int regno, int nregs, co
       error ("PIC register clobbered by %qs in %<asm%>", regname);
       is_valid = false;
     }
-  /* Clobbering the STACK POINTER register is an error.  */
+  /* Clobbering the STACK POINTER register is likely an error.
+     However it is useful to force the use of frame pointer and prevent
+     the use of red zone.  Thus without this clobber, pushing temporary
+     values onto the stack might clobber the red zone or make stack based
+     memory references invalid.  */
   if (overlaps_hard_reg_set_p (regset, Pmode, STACK_POINTER_REGNUM))
     {
-      error ("Stack Pointer register clobbered by %qs in %<asm%>", regname);
-      is_valid = false;
+      if (warning (0, "Stack Pointer register clobbered by %qs in %<asm%>",
+		   regname))
+	{
+	  inform (input_location,
+		  "This does likely not do what you would expect."
+		  " The Stack Pointer register still needs to be restored to"
+		  " the previous value, however it is safe to push values onto"
+		  " the stack, when they are popped again from the stack"
+		  " before the asm statement terminates");
+	}
     }
 
   return is_valid;
Index: gcc/testsuite/gcc.target/i386/pr52813.c
===================================================================
--- gcc/testsuite/gcc.target/i386/pr52813.c	(revision 267164)
+++ gcc/testsuite/gcc.target/i386/pr52813.c	(working copy)
@@ -1,9 +1,10 @@
 /* Ensure that stack pointer cannot be an asm clobber.  */
 /* { dg-do compile { target { ! ia32 } } } */
-/* { dg-options "-O2" } */
+/* { dg-options "-O3 -fomit-frame-pointer" } */
 
 void
 test1 (void)
 {
-  asm volatile ("" : : : "%esp"); /* { dg-error "Stack Pointer register clobbered" } */
+  asm volatile ("" : : : "%rsp"); /* { dg-warning "Stack Pointer register clobbered" } */
 }
+/* { dg-final { scan-assembler "(?n)pushq.*%rbp" } } */