diff options
Diffstat (limited to 'devel/llvm60/files/lld/patch-head-r338251.diff')
| -rw-r--r-- | devel/llvm60/files/lld/patch-head-r338251.diff | 123 |
1 files changed, 123 insertions, 0 deletions
diff --git a/devel/llvm60/files/lld/patch-head-r338251.diff b/devel/llvm60/files/lld/patch-head-r338251.diff new file mode 100644 index 000000000000..1c3df02100cd --- /dev/null +++ b/devel/llvm60/files/lld/patch-head-r338251.diff @@ -0,0 +1,123 @@ +r338251 | markj | 2018-08-23 16:58:19 +0200 (Thu, 23 Aug 2018) | 29 lines + +Add an lld option to emit PC-relative relocations for ifunc calls. + +The current kernel ifunc implementation creates a PLT entry for each +ifunc definition. ifunc calls therefore consist of a call to the +PLT entry followed by an indirect jump. The jump target is written +during boot when the kernel linker resolves R_[*]_IRELATIVE relocations. +This implementation is defined by requirements for userland code, where +text relocations are avoided. This requirement is not present for the +kernel, so the implementation has avoidable overhead (namely, an extra +indirect jump per call). + +Address this for now by adding a special option to the static linker +to inhibit PLT creation for ifuncs. Instead, relocations to ifunc call +sites are passed through to the output file, so the kernel linker can +enumerate such call sites and apply PC-relative relocations directly +to the text section. Thus the overhead of an ifunc call becomes exactly +the same as that of an ordinary function call. This option is only for +use by the kernel and will not work for regular programs. + +The final form of this optimization is up for debate; for now, this +change is simple and static enough to be acceptable as an interim +solution. + +Reviewed by: emaste +Discussed with: arichardson, dim +MFC after: 1 month +Sponsored by: The FreeBSD Foundation +Differential Revision: https://reviews.freebsd.org/D16748 + +Index: tools/lld/ELF/Config.h +=================================================================== +--- tools/lld/ELF/Config.h (revision 338250) ++++ tools/lld/ELF/Config.h (revision 338251) +@@ -155,6 +155,7 @@ struct Configuration { + bool ZCombreloc; + bool ZExecstack; + bool ZHazardplt; ++ bool ZIfuncnoplt; + bool ZNocopyreloc; + bool ZNodelete; + bool ZNodlopen; +Index: tools/lld/ELF/Driver.cpp +=================================================================== +--- tools/lld/ELF/Driver.cpp (revision 338250) ++++ tools/lld/ELF/Driver.cpp (revision 338251) +@@ -669,6 +669,7 @@ void LinkerDriver::readConfigs(opt::InputArgList & + Config->ZCombreloc = !hasZOption(Args, "nocombreloc"); + Config->ZExecstack = hasZOption(Args, "execstack"); + Config->ZHazardplt = hasZOption(Args, "hazardplt"); ++ Config->ZIfuncnoplt = hasZOption(Args, "ifunc-noplt"); + Config->ZNocopyreloc = hasZOption(Args, "nocopyreloc"); + Config->ZNodelete = hasZOption(Args, "nodelete"); + Config->ZNodlopen = hasZOption(Args, "nodlopen"); +Index: tools/lld/ELF/Relocations.cpp +=================================================================== +--- tools/lld/ELF/Relocations.cpp (revision 338250) ++++ tools/lld/ELF/Relocations.cpp (revision 338251) +@@ -374,6 +374,9 @@ static bool isStaticLinkTimeConstant(RelExpr E, Re + R_PPC_PLT_OPD, R_TLSDESC_CALL, R_TLSDESC_PAGE, R_HINT>(E)) + return true; + ++ if (Sym.isGnuIFunc() && Config->ZIfuncnoplt) ++ return false; ++ + // These never do, except if the entire file is position dependent or if + // only the low bits are used. + if (E == R_GOT || E == R_PLT || E == R_TLSDESC) +@@ -921,7 +924,9 @@ static void scanRelocs(InputSectionBase &Sec, Arra + // Strenghten or relax a PLT access. + // + // GNU ifunc symbols must be accessed via PLT because their addresses +- // are determined by runtime. ++ // are determined by runtime. If the -z ifunc-noplt option is specified, ++ // we permit the optimization of ifunc calls by omitting the PLT entry ++ // and preserving relocations at ifunc call sites. + // + // On the other hand, if we know that a PLT entry will be resolved within + // the same ELF module, we can skip PLT access and directly jump to the +@@ -929,7 +934,7 @@ static void scanRelocs(InputSectionBase &Sec, Arra + // all dynamic symbols that can be resolved within the executable will + // actually be resolved that way at runtime, because the main exectuable + // is always at the beginning of a search list. We can leverage that fact. +- if (Sym.isGnuIFunc()) ++ if (Sym.isGnuIFunc() && !Config->ZIfuncnoplt) + Expr = toPlt(Expr); + else if (!Preemptible && Expr == R_GOT_PC && !isAbsoluteValue(Sym)) + Expr = +@@ -1034,6 +1039,16 @@ static void scanRelocs(InputSectionBase &Sec, Arra + continue; + } + ++ // Preserve relocations against ifuncs if we were asked to do so. ++ if (Sym.isGnuIFunc() && Config->ZIfuncnoplt) { ++ if (Config->IsRela) ++ InX::RelaDyn->addReloc({Type, &Sec, Offset, false, &Sym, Addend}); ++ else ++ // Preserve the existing addend. ++ InX::RelaDyn->addReloc({Type, &Sec, Offset, false, &Sym, 0}); ++ continue; ++ } ++ + // If the output being produced is position independent, the final value + // is still not known. In that case we still need some help from the + // dynamic linker. We can however do better than just copying the incoming +Index: tools/lld/ELF/Writer.cpp +=================================================================== +--- tools/lld/ELF/Writer.cpp (revision 338250) ++++ tools/lld/ELF/Writer.cpp (revision 338251) +@@ -1400,8 +1400,11 @@ template <class ELFT> void Writer<ELFT>::finalizeS + applySynthetic({InX::EhFrame}, + [](SyntheticSection *SS) { SS->finalizeContents(); }); + +- for (Symbol *S : Symtab->getSymbols()) ++ for (Symbol *S : Symtab->getSymbols()) { + S->IsPreemptible |= computeIsPreemptible(*S); ++ if (S->isGnuIFunc() && Config->ZIfuncnoplt) ++ S->ExportDynamic = true; ++ } + + // Scan relocations. This must be done after every symbol is declared so that + // we can correctly decide if a dynamic relocation is needed. |