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. > > 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 * 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. 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. 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 >>>> >>>> >>>
