On Mon, Jun 1, 2015 at 1:33 PM, Ramana Radhakrishnan wrote: > On Mon, Jun 1, 2015 at 7:55 PM, Sriraman Tallam wrote: >> On Mon, Jun 1, 2015 at 11:41 AM, Ramana Radhakrishnan >> wrote: >>> On Mon, Jun 1, 2015 at 7:01 PM, Sriraman Tallam wrote: >>>> On Mon, Jun 1, 2015 at 1:24 AM, Ramana Radhakrishnan >>>> 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 >>>>>> >>>>>> >>>>>