On Mon, Jun 1, 2015 at 1:33 PM, Ramana Radhakrishnan
<ramana....@googlemail.com> wrote:
> On Mon, Jun 1, 2015 at 7:55 PM, Sriraman Tallam <tmsri...@google.com> wrote:
>> On Mon, Jun 1, 2015 at 11:41 AM, Ramana Radhakrishnan
>> <ramana....@googlemail.com> wrote:
>>> On Mon, Jun 1, 2015 at 7:01 PM, Sriraman Tallam <tmsri...@google.com> wrote:
>>>> On Mon, Jun 1, 2015 at 1:24 AM, Ramana Radhakrishnan
>>>> <ramana.radhakrish...@arm.com> wrote:
>>>>>
>>>>>>> Why isn't it just an indirect call in the cases that would require a GOT
>>>>>>> slot and a direct call otherwise ? I'm trying to work out what's so
>>>>>>> different on each target that mandates this to be in the target backend.
>>>>>>> Also it would be better to push the tests into gcc.dg if you can and
>>>>>>> check
>>>>>>> for the absence of a relocation so that folks at least see these as
>>>>>>> being
>>>>>>> UNSUPPORTED on their target.
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> To be even more explicit, shouldn't this be handled similar to the way in
>>>>> which -fno-plt is handled in a target agnostic manner ? After all, if you
>>>>> can handle this for the command line, doing the same for a function which
>>>>> has been decorated with attribute((noplt)) should be simple.
>>>>
>>>> -fno-plt does not work for non-PIC code, having non-PIC code not use
>>>> PLT was my primary motivation. Infact, if you go back in this thread,
>>>> I suggested to HJ if I should piggyback on -fno-plt. I tried using
>>>> the -fno-plt implementation to do this by removing the flag_pic check
>>>> in calls.c, but that does not still work for non-PIC code.
>
> If you want __attribute__ ((noplt)) to work for non-PIC code, we
> should look to code it in the same place surely by making all
> __attribute__((noplt)) calls, indirect calls irrespective of whether
> it's fpic or not.
>
>
>>>
>>> You're missing my point, unless I'm missing something basic here - I
>>> should have been even more explicit and said -fPIC was a given in all
>>> this discussion.
>>>
>>> calls.c:229 has
>>>
>>> else if (flag_pic && !flag_plt && fndecl_or_type
>>> && TREE_CODE (fndecl_or_type) == FUNCTION_DECL
>>> && !targetm.binds_local_p (fndecl_or_type))
>>>
>>> why can't we merge the check in here for the attribute noplt ?
>>
>> We can and and please see this thread, that is the exact patch I proposed :
>> https://gcc.gnu.org/ml/gcc-patches/2015-05/msg02682.html
>>
>> However, there was one caveat. I want this working without -fPIC too.
>> non-PIC code also generates PLT calls and I want them eliminated.
>>
>>>
>>> If a new attribute is added to the "GNU language" in this case, why
>>> isn't this being treated in the same way as the command line option
>>> has been treated ? All this means is that we add an attribute and a
>>> command line option to common code and then not implement it in a
>>> proper target agnostic fashion.
>>
>> You are right. This is the way I wanted it too but I also wanted the
>> attribute to work without PIC. PLT calls are generated without -fPIC
>> and -fPIE too and I wanted a solution for that. On looking at the
>> code in more detail,
>>
>> * -fno-plt is made to work with -fPIC, is there a reason to not make
>> it work for non-PIC code? I can remove the flag_pic check from
>> calls.c
>
> I don't think that's right, you probably have to allow that along with
> (flag_pic || (decl && attribute_no_plt (decl)) - however it seems odd
> to me that the language extension allows this but the flag doesn't.
>
>> * Then, I add the generic attribute "noplt" and everything is fine.
>>
>> There is just one caveat with the above approach, for x86_64
>> (*call_insn) will not generate indirect-calls for *non-PIC* code
>> because constant_call_address_operand in predicates.md will evaluate
>> to false. This can be fixed appropriately in ix86_output_call_insn in
>> i386.c.
>
> Yes, targets need to massage that into place but that's essentially
> the mechanics of retaining indirect calls in each backend. -fno-plt
> doesn't work for ARM / AArch64 with optimizers currently (and I
> suspect on most other targets) because our predicates are too liberal,
> fixed by treating "noplt" or -fno-plt as the equivalent of
> -mlong-calls.
>
>>
>>
>> Is this alright? Sorry for the confusion, but the primary reason why
>> I did not do it the way you suggested is because we wanted "noplt"
>> attribute to work for non-PIC code also.
>
> If that is the case, then this is a slightly more complicated
> condition in the same place. We then always have indirect calls for
> functions that are marked noplt and just have target generate this
> appropriately.
I have now modified this patch.
This patch does two things:
1) Adds new generic function attribute "no_plt" that is similar in
functionality to -fno-plt except that it applies only to calls to
functions that are marked with this attribute.
2) For x86_64, it makes -fno-plt(and the attribute) also work for
non-PIC code by directly generating an indirect call via a GOT entry.
For PIC code, no_plt merely shadows the implementation of -fno-plt, no
surprises here.
* c-family/c-common.c (no_plt): New attribute.
(handle_no_plt_attribute): New handler.
* calls.c (prepare_call_address): Check for no_plt
attribute.
* config/i386/i386.c (ix86_function_ok_for_sibcall): Check
for no_plt attribute.
(ix86_expand_call): Ditto.
(nopic_no_plt_attribute): New function.
(ix86_output_call_insn): Output indirect call for non-pic
no plt calls.
* doc/extend.texi (no_plt): Document new attribute.
* testsuite/gcc.target/i386/noplt-1.c: New test.
* testsuite/gcc.target/i386/noplt-2.c: New test.
* testsuite/gcc.target/i386/noplt-3.c: New test.
* testsuite/gcc.target/i386/noplt-4.c: New test.
Please review.
Thanks
Sri
>
> To be honest, this is trivial to implement in the ARM backend as one
> would just piggy back on the longcalls work - despite that, IMNSHO
> it's best done in a target independent manner.
>
> regards
> Ramana
>
>>
>> Thanks
>> Sri
>>
>>>
>>> regards
>>> Ramana
>>>
>>>
>>>>
>>>>>
>>>>>> I am not familiar with PLT calls for other targets. I can move the
>>>>>> tests to gcc.dg but what relocation are you suggesting I check for?
>>>>>
>>>>>
>>>>> Move the test to gcc.dg, add a target_support_no_plt function in
>>>>> testsuite/lib/target-supports.exp and mark this as being supported only on
>>>>> x86 and use scan-assembler to scan for PLT relocations for x86. Other
>>>>> targets can add things as they deem fit.
>>>>
>>>>>
>>>>> In any case, on a large number of elf/ linux targets I would have thought
>>>>> the absence of a JMP_SLOT relocation would be good enough to check that
>>>>> this
>>>>> is working correctly.
>>>>>
>>>>> regards
>>>>> Ramana
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> Thanks
>>>>>> Sri
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Ramana
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Also I think the PLT calls have EBX in call fusage wich is added by
>>>>>>>>> ix86_expand_call.
>>>>>>>>> else
>>>>>>>>> {
>>>>>>>>> /* Static functions and indirect calls don't need the pic
>>>>>>>>> register. */
>>>>>>>>> if (flag_pic
>>>>>>>>> && (!TARGET_64BIT
>>>>>>>>> || (ix86_cmodel == CM_LARGE_PIC
>>>>>>>>> && DEFAULT_ABI != MS_ABI))
>>>>>>>>> && GET_CODE (XEXP (fnaddr, 0)) == SYMBOL_REF
>>>>>>>>> && ! SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0)))
>>>>>>>>> {
>>>>>>>>> use_reg (&use, gen_rtx_REG (Pmode,
>>>>>>>>> REAL_PIC_OFFSET_TABLE_REGNUM));
>>>>>>>>> if (ix86_use_pseudo_pic_reg ())
>>>>>>>>> emit_move_insn (gen_rtx_REG (Pmode,
>>>>>>>>> REAL_PIC_OFFSET_TABLE_REGNUM),
>>>>>>>>> pic_offset_table_rtx);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> I think you want to take that away from FUSAGE there just like we do
>>>>>>>>> for
>>>>>>>>> local calls
>>>>>>>>> (and in fact the code should already check flag_pic && flag_plt I
>>>>>>>>> suppose.
>>>>>>>>
>>>>>>>>
>>>>>>>> Done that now and patch attached.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>> Sri
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Honza
>>>>>>
>>>>>>
>>>>>
* c-family/c-common.c (no_plt): New attribute.
(handle_no_plt_attribute): New handler.
* calls.c (prepare_call_address): Check for no_plt
attribute.
* config/i386/i386.c (ix86_function_ok_for_sibcall): Check
for no_plt attribute.
(ix86_expand_call): Ditto.
(nopic_no_plt_attribute): New function.
(ix86_output_call_insn): Output indirect call for non-pic
no plt calls.
* doc/extend.texi (no_plt): Document new attribute.
* testsuite/gcc.target/i386/noplt-1.c: New test.
* testsuite/gcc.target/i386/noplt-2.c: New test.
* testsuite/gcc.target/i386/noplt-3.c: New test.
* testsuite/gcc.target/i386/noplt-4.c: New test.
This patch does two things:
* Adds new generic function attribute "no_plt" that is similar in functionality
to -fno-plt except that it applies only to calls to functions that are marked
with this attribute.
* For x86_64, it makes -fno-plt(and the attribute) also work for non-PIC code by
directly generating an indirect call via a GOT entry.
Index: c-family/c-common.c
===================================================================
--- c-family/c-common.c (revision 223720)
+++ c-family/c-common.c (working copy)
@@ -357,6 +357,7 @@ static tree handle_mode_attribute (tree *, tree, t
static tree handle_section_attribute (tree *, tree, tree, int, bool *);
static tree handle_aligned_attribute (tree *, tree, tree, int, bool *);
static tree handle_weak_attribute (tree *, tree, tree, int, bool *) ;
+static tree handle_no_plt_attribute (tree *, tree, tree, int, bool *) ;
static tree handle_alias_ifunc_attribute (bool, tree *, tree, tree, bool *);
static tree handle_ifunc_attribute (tree *, tree, tree, int, bool *);
static tree handle_alias_attribute (tree *, tree, tree, int, bool *);
@@ -706,6 +707,8 @@ const struct attribute_spec c_common_attribute_tab
handle_aligned_attribute, false },
{ "weak", 0, 0, true, false, false,
handle_weak_attribute, false },
+ { "no_plt", 0, 0, true, false, false,
+ handle_no_plt_attribute, false },
{ "ifunc", 1, 1, true, false, false,
handle_ifunc_attribute, false },
{ "alias", 1, 1, true, false, false,
@@ -8185,6 +8188,25 @@ handle_weak_attribute (tree *node, tree name,
return NULL_TREE;
}
+/* Handle a "no_plt" attribute; arguments as in
+ struct attribute_spec.handler. */
+
+static tree
+handle_no_plt_attribute (tree *node, tree name,
+ tree ARG_UNUSED (args),
+ int ARG_UNUSED (flags),
+ bool * ARG_UNUSED (no_add_attrs))
+{
+ if (TREE_CODE (*node) != FUNCTION_DECL)
+ {
+ warning (OPT_Wattributes,
+ "%qE attribute is only applicable on functions", name);
+ *no_add_attrs = true;
+ return NULL_TREE;
+ }
+ return NULL_TREE;
+}
+
/* Handle an "alias" or "ifunc" attribute; arguments as in
struct attribute_spec.handler, except that IS_ALIAS tells us
whether this is an alias as opposed to ifunc attribute. */
Index: calls.c
===================================================================
--- calls.c (revision 223720)
+++ calls.c (working copy)
@@ -226,8 +226,10 @@ prepare_call_address (tree fndecl_or_type, rtx fun
&& targetm.small_register_classes_for_mode_p (FUNCTION_MODE))
? force_not_mem (memory_address (FUNCTION_MODE, funexp))
: memory_address (FUNCTION_MODE, funexp));
- else if (flag_pic && !flag_plt && fndecl_or_type
+ else if (flag_pic && fndecl_or_type
&& TREE_CODE (fndecl_or_type) == FUNCTION_DECL
+ && (!flag_plt
+ || lookup_attribute ("no_plt", DECL_ATTRIBUTES (fndecl_or_type)))
&& !targetm.binds_local_p (fndecl_or_type))
{
funexp = force_reg (Pmode, funexp);
Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c (revision 223720)
+++ config/i386/i386.c (working copy)
@@ -5479,6 +5479,8 @@ ix86_function_ok_for_sibcall (tree decl, tree exp)
&& !TARGET_64BIT
&& flag_pic
&& flag_plt
+ && (TREE_CODE (decl) != FUNCTION_DECL
+ || !lookup_attribute ("no_plt", DECL_ATTRIBUTES (decl)))
&& decl && !targetm.binds_local_p (decl))
return false;
@@ -25497,13 +25499,19 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx call
}
else
{
- /* Static functions and indirect calls don't need the pic register. */
+ /* Static functions and indirect calls don't need the pic register.
Also,
+ check if PLT was explicitly avoided via no-plt or "no_plt" attribute,
making
+ it an indirect call. */
if (flag_pic
&& (!TARGET_64BIT
|| (ix86_cmodel == CM_LARGE_PIC
&& DEFAULT_ABI != MS_ABI))
&& GET_CODE (XEXP (fnaddr, 0)) == SYMBOL_REF
- && ! SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0)))
+ && ! SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0))
+ && flag_plt
+ && (TREE_CODE (SYMBOL_REF_DECL (XEXP(fnaddr, 0))) != FUNCTION_DECL
+ || !lookup_attribute ("no_plt",
+ DECL_ATTRIBUTES (SYMBOL_REF_DECL (XEXP(fnaddr, 0))))))
{
use_reg (&use, gen_rtx_REG (Pmode, REAL_PIC_OFFSET_TABLE_REGNUM));
if (ix86_use_pseudo_pic_reg ())
@@ -25599,6 +25607,34 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx call
return call;
}
+/* Return true if the function being called was marked with attribute
+ "no_plt" or using -fno-plt and we are compiling for no-PIC and x86_64.
+ This is currently used only with 64-bit ELF targets to call the function
+ marked "no_plt" indirectly. */
+
+static bool
+nopic_no_plt_attribute (rtx call_op)
+{
+ if (flag_pic)
+ return false;
+
+ if (!TARGET_64BIT || TARGET_MACHO|| TARGET_SEH || TARGET_PECOFF)
+ return false;
+
+ if (SYMBOL_REF_LOCAL_P (call_op))
+ return false;
+
+ tree symbol_decl = SYMBOL_REF_DECL (call_op);
+
+ if (symbol_decl != NULL_TREE
+ && TREE_CODE (symbol_decl) == FUNCTION_DECL
+ && (!flag_plt
+ || lookup_attribute ("no_plt", DECL_ATTRIBUTES (symbol_decl))))
+ return true;
+
+ return false;
+}
+
/* Output the assembly for a call instruction. */
const char *
@@ -25610,7 +25646,9 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
if (SIBLING_CALL_P (insn))
{
- if (direct_p)
+ if (direct_p && nopic_no_plt_attribute (call_op))
+ xasm = "%!jmp\t*%p0@GOTPCREL(%%rip)";
+ else if (direct_p)
xasm = "%!jmp\t%P0";
/* SEH epilogue detection requires the indirect branch case
to include REX.W. */
@@ -25653,7 +25691,9 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
seh_nop_p = true;
}
- if (direct_p)
+ if (direct_p && nopic_no_plt_attribute (call_op))
+ xasm = "%!call\t*%p0@GOTPCREL(%%rip)";
+ else if (direct_p)
xasm = "%!call\t%P0";
else
xasm = "%!call\t%A0";
Index: doc/extend.texi
===================================================================
--- doc/extend.texi (revision 223720)
+++ doc/extend.texi (working copy)
@@ -2916,6 +2916,15 @@ the standard C library can be guaranteed not to th
with the notable exceptions of @code{qsort} and @code{bsearch} that
take function pointer arguments.
+@item no_plt
+@cindex @code{no_plt} function attribute
+The @code{no_plt} attribute is used to inform the compiler that a calls
+to the function should not use the PLT. For example, external functions
+defined in shared objects are called from the executable using the PLT.
+This attribute on the function declaration calls these functions indirectly
+rather than going via the PLT. This is similar to @option{-fno-plt} but
+is only applicable to calls to the function marked with this attribute.
+
@item optimize
@cindex @code{optimize} function attribute
The @code{optimize} attribute is used to specify that a function is to
Index: testsuite/gcc.target/i386/noplt-1.c
===================================================================
--- testsuite/gcc.target/i386/noplt-1.c (revision 0)
+++ testsuite/gcc.target/i386/noplt-1.c (working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile { target x86_64-*-linux* } } */
+/* { dg-options "-fno-pic" } */
+
+__attribute__ ((no_plt))
+void foo();
+
+int main()
+{
+ foo();
+ return 0;
+}
+
+/* { dg-final { scan-assembler "call\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } }
*/
Index: testsuite/gcc.target/i386/noplt-2.c
===================================================================
--- testsuite/gcc.target/i386/noplt-2.c (revision 0)
+++ testsuite/gcc.target/i386/noplt-2.c (working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile { target x86_64-*-linux* } } */
+/* { dg-options "-O2 -fno-pic" } */
+
+
+__attribute__ ((no_plt))
+int foo();
+
+int main()
+{
+ return foo();
+}
+
+/* { dg-final { scan-assembler "jmp\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } }
*/
Index: testsuite/gcc.target/i386/noplt-3.c
===================================================================
--- testsuite/gcc.target/i386/noplt-3.c (revision 0)
+++ testsuite/gcc.target/i386/noplt-3.c (working copy)
@@ -0,0 +1,12 @@
+/* { dg-do compile { target x86_64-*-linux* } } */
+/* { dg-options "-fno-pic -fno-plt" } */
+
+void foo();
+
+int main()
+{
+ foo();
+ return 0;
+}
+
+/* { dg-final { scan-assembler "call\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } }
*/
Index: testsuite/gcc.target/i386/noplt-4.c
===================================================================
--- testsuite/gcc.target/i386/noplt-4.c (revision 0)
+++ testsuite/gcc.target/i386/noplt-4.c (working copy)
@@ -0,0 +1,11 @@
+/* { dg-do compile { target x86_64-*-linux* } } */
+/* { dg-options "-O2 -fno-pic -fno-plt" } */
+
+int foo();
+
+int main()
+{
+ return foo();
+}
+
+/* { dg-final { scan-assembler "jmp\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } } */
