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.
2018-01-07 Bernd Edlinger <[email protected]>
* 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 <[email protected]>
* 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" } } */
