Re: [PATCH] [RFC] PR target/52813 and target/11807

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

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On 1/7/19 10:23 AM, Jakub Jelinek wrote:
> On Sun, Dec 16, 2018 at 06:13:57PM +0200, Dimitar Dimitrov wrote:
>> -  /* Clobbering the STACK POINTER register is an error.  */
>> +  /* Clobbered STACK POINTER register is not saved/restored by GCC,
>> +     which is often unexpected by users.  See PR52813.  */
>>    if (overlaps_hard_reg_set_p (regset, Pmode, STACK_POINTER_REGNUM))
>>      {
>> -      error ("Stack Pointer register clobbered by %qs in %<asm%>", regname);
>> +      warning (0, "Stack Pointer register clobbered by %qs in %<asm%>",
>> +	       regname);
>> +      warning (0, "GCC has always ignored Stack Pointer %<asm%> clobbers");
> 
> Why do we write Stack Pointer rather than stack pointer?  That is really
> weird.  The second warning would be a note based on the first one, i.e.
> if (warning ()) note ();
> and better have some -W* option to silence the warning.
> 

Yes, thanks for this suggestion.

Meanwhile I found out, that the stack clobber has only been ignored up to
gcc-5 (at least with lra targets, not really sure about reload targets).
From gcc-6 on, with the exception of PR arm/77904 which was a regression due
to the underlying lra change, but fixed later, and back-ported to gcc-6.3.0,
this works for all targets I tried so far.

To me, it starts to look like a rather unique and useful feature, that I would
like to keep working.


Attached is an updated version if my patch, using the suggested warning option,
and a note with the details.


Bootstrapped on x86_64-pc-linux-gnu.
Is it OK for trunk?


Thanks
Bernd.


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2018-01-07  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* doc/invoke.texi: Document -Wstack-clobber.
	* common.opt (-Wstack-clobber): New default-enabled warning.
	* cfgexpand.c (asm_clobber_reg_is_valid): Emit only a warning together
	with an informative note when the stack pointer is clobbered.

testsuite:
2018-07-01  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* gcc.target/arm/pr77904.c: Adjust test.
	* gcc.target/i386/pr52813.c: Adjust test.


Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c	(revision 267653)
+++ 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 +2873,22 @@ 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 (OPT_Wstack_clobber,
+		   "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 has 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/common.opt
===================================================================
--- gcc/common.opt	(revision 267653)
+++ gcc/common.opt	(working copy)
@@ -702,6 +702,10 @@ Warn when one local variable shadows another local
 Wshadow-compatible-local
 Common Warning Undocumented Alias(Wshadow=compatible-local)
 
+Wstack-clobber
+Common Warning Var(warn_stack_clobber) Init(1)
+Warn when asm statements try to clobber the stack pointer register.
+
 Wstack-protector
 Common Var(warn_stack_protect) Warning
 Warn when not issuing stack smashing protection for some reason.
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 267653)
+++ gcc/doc/invoke.texi	(working copy)
@@ -339,7 +339,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wshift-count-negative  -Wshift-count-overflow  -Wshift-negative-value @gol
 -Wsign-compare  -Wsign-conversion  -Wfloat-conversion @gol
 -Wno-scalar-storage-order  -Wsizeof-pointer-div @gol
--Wsizeof-pointer-memaccess  -Wsizeof-array-argument @gol
+-Wsizeof-pointer-memaccess  -Wsizeof-array-argument  -Wstack-clobber @gol
 -Wstack-protector  -Wstack-usage=@var{byte-size}  -Wstrict-aliasing @gol
 -Wstrict-aliasing=n  -Wstrict-overflow  -Wstrict-overflow=@var{n} @gol
 -Wstringop-overflow=@var{n}  -Wstringop-truncation  -Wsubobject-linkage @gol
@@ -7560,6 +7560,20 @@ This option is only supported for C and Objective-
 @option{-Wall} and by @option{-Wpedantic}, which can be disabled with
 @option{-Wno-pointer-sign}.
 
+@item -Wstack-clobber
+@opindex Wstack-clobber
+@opindex Wno-stack-clobber
+Warn for asm statements that try to clobber the stack pointer.
+Prior to gcc-6 this clobber was ignored, but from gcc-6 on
+this was changed to force the use of the frame pointer in the
+current function even if @option{-fomit-frame-pointer} is in
+effect, additionally this clobber prevents the use of a red-zone
+on some targets.  The stack pointer register still has 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.  Since this might be unexpected,
+the warning is enabled by default.
+
 @item -Wstack-protector
 @opindex Wstack-protector
 @opindex Wno-stack-protector
Index: gcc/testsuite/gcc.target/arm/pr77904.c
===================================================================
--- gcc/testsuite/gcc.target/arm/pr77904.c	(revision 267653)
+++ gcc/testsuite/gcc.target/arm/pr77904.c	(working copy)
@@ -4,7 +4,7 @@
 __attribute__ ((noinline, noclone)) void
 clobber_sp (void)
 {
-  __asm volatile ("" : : : "sp");
+  __asm volatile ("" : : : "sp"); /* { dg-warning "stack pointer register clobbered" } */
 }
 
 int
Index: gcc/testsuite/gcc.target/i386/pr52813.c
===================================================================
--- gcc/testsuite/gcc.target/i386/pr52813.c	(revision 267653)
+++ 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" } } */