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

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

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On 12/18/18 3:16 PM, Bernd Edlinger wrote:
> Hi,
> 
> while I looked closely at the asm statement in the gdb,
> I realized that the SP clobber forces the function to use
> the frame pointer, and prevents the red zone.  That
> makes the push / pop sequence in the asm statement safe
> to use, as long as the stack is restored to the original
> value.  That can be a quite useful feature.  And that might
> have been the reason why the rsp clobber was chosen in the
> first place.
> 
> This seems to work for all targets, but it started to work
> this way with gcc-6, all versions before that do ignore
> this clobber stmt (as confirmed by godbolt).
> 
> The clobber stmt make the LRA register allocator switch
> frame_pointer_needed to 1, and therefore in all likelihood,
> all targets should use that consistently.
> 
> On 12/17/18 12:47 PM, Richard Sandiford wrote:
>> Dimitar Dimitrov <dimitar@dinux.eu> writes:
>>> On Sun, Dec 16 2018 at 14:36:26 EET Bernd Edlinger wrote:
>>>> Hi,
>>>>
>>>> if I understood that right, then clobbering sp is and has always been
>>>> ignored.
>>
>> PR77904 was about the clobber not being ignored, so the behaviour
>> hasn't been consistent.
>>
> 
> I think 77904 was a fall-out from the change in the LRA register allocator.
> The patch referenced in the PR does simply honor frame_pointer_needed,
> which changed with gcc-6, and caused a regression on arm.
> 
>> I'm also not sure it was always ignored in recent sources.  The clobber
>> does get added to the associated rtl insn, and it'd be surprising if
>> that never had an effect.
>>
>>>> If that is right, then I would much prefer a warning, that says exactly
>>>> that, because that would also help to understand why removing that clobber
>>>> statement is safe even for old gcc versions.
>>
>> If the asm does leave sp with a different value, then it's never been safe,
>> regardless of the gcc version.  That's why an error seems more appropriate.
>>
>>> Thank you. Looks like general consensus is to have a warning. See attached
>>> patch that switches the error to a warning.
>>
>> I don't think there's a good reason to treat this differently from the
>> preexisting PIC register error.  If the argument for making it a warning
>> rather than an error is that the asm might happen to work by accident,
>> then the same is true for the PIC register.
>>
> 
> In the light of my findings, I believe with a good warning message that
> explains that the SP needs to be restored to the previous value, that
> is a useful feature, that enables the asm statement to push temporary
> values on the stack which would not be safe otherwise.
> 
> Therefore I propose not to rip it out at this time.
> See my proposed patch.  What do you think?
> 
> Is it OK?
> 
> 

Oops, previous version missed the fix of the PR77904 test case, which is
currently broken too.


Bernd-

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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/arm/pr77904.c: Adjust test.
	* 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/arm/pr77904.c
===================================================================
--- gcc/testsuite/gcc.target/arm/pr77904.c	(revision 267164)
+++ 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 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" } } */