[ELF] Make --no-relax disable R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX GOT optimization

[MaskRay]

Summary

This brings back the original version of D81359.
I have found several use cases now.

• Unlike GNU ld, LLD's relocation processing is one pass. If we decide to optimize(relax) R_X86_64_{,REX_}GOTPCRELX, we will suppress GOT generation and cannot undo the decision later. Optimizing R_X86_64_REX_GOTPCRELX can usually make it easy to hit relocation R_X86_64_REX_GOTPCRELX out of range. Without --no-relax, the user has to recompile with -Wa,-mrelax-relocations=no.
• The option would help during my investigationg of the root cause of https://git.kernel.org/linus/09e43968db40c33a73e9ddbfd937f46d5c334924
• There is need for relaxation for AArch64 & RISC-V. Implementing this for x86-64 improves consistency with little target-specific cost (two-line X86_64.cpp change).

[alexander-shaposhnikov]

lld/docs/ld.lld.1

348

I'd probably change the wording a bit, either drop the second part ("This disables R_X86_64 ... ") or indicate that currently it works only for x86_64. I'm not really sure it's that useful, "Disable target-specific relaxations." seems to be sufficient and won't require frequent updates.

[MaskRay]

lld/docs/ld.lld.1

348

This is a manpage where additional information can be useful. It's otherwise unclear --no-relax can help with R_X86_64_REX_GOTPCRELX issues.

[alexander-shaposhnikov]

lld/docs/ld.lld.1

348

I meant the following: "For x86_64 this disables R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX GOT optimization."
(it just sounds a bit more natural, but i do not insist)

[MaskRay]

Add "For x86_64 " to ld.lld.1

lld/ELF/Arch/X86_64.cpp

 }
 
 RelExpr X86_64::adjustGotPcExpr(RelType type, int64_t addend,
                                 const uint8_t *loc) const {
   // Only R_X86_64_[REX_]GOTPCRELX can be relaxed. GNU as may emit GOTPCRELX
   // with addend != -4. Such an instruction does not load the full GOT entry, so
   // we cannot relax the relocation. E.g. movl x@GOTPCREL+4(%rip), %rax
   // (addend=0) loads the high 32 bits of the GOT entry.
-  if ((type != R_X86_64_GOTPCRELX && type != R_X86_64_REX_GOTPCRELX) ||
-      addend != -4)
+  if (!config->relax || addend != -4 ||
+      (type != R_X86_64_GOTPCRELX && type != R_X86_64_REX_GOTPCRELX))
     return R_GOT_PC;
   const uint8_t op = loc[-2];
   const uint8_t modRm = loc[-1];
 
   // FIXME: When PIC is disabled and foo is defined locally in the
   // lower 32 bit address space, memory operand in mov can be converted into
   // immediate operand. Otherwise, mov must be changed to lea. We support only
   // latter relaxation at this moment.

lld/ELF/Config.h

struct Configuration {
   bool optEB = false;
   bool optEL = false;
   bool optimizeBBJumps;
   bool optRemarksWithHotness;
   bool picThunk;
   bool pie;
   bool printGcSections;
   bool printIcfSections;
+  bool relax;
   bool relocatable;
   bool relrPackDynRelocs;
   bool saveTemps;
   std::vector<std::pair<llvm::GlobPattern, uint32_t>> shuffleSections;
   bool singleRoRx;
   bool shared;
   bool symbolic;
   bool isStatic = false;

lld/ELF/Driver.cpp

static void readConfigs(opt::InputArgList &args) {
   config->pie = args.hasFlag(OPT_pie, OPT_no_pie, false);
   config->printIcfSections =
       args.hasFlag(OPT_print_icf_sections, OPT_no_print_icf_sections, false);
   config->printGcSections =
       args.hasFlag(OPT_print_gc_sections, OPT_no_print_gc_sections, false);
   config->printArchiveStats = args.getLastArgValue(OPT_print_archive_stats);
   config->printSymbolOrder =
       args.getLastArgValue(OPT_print_symbol_order);
+  config->relax = args.hasFlag(OPT_relax, OPT_no_relax, true);
   config->rpath = getRpath(args);
   config->relocatable = args.hasArg(OPT_relocatable);
   config->saveTemps = args.hasArg(OPT_save_temps);
   config->searchPaths = args::getStrings(args, OPT_library_path);
   config->sectionStartMap = getSectionStartMap(args);
   config->shared = args.hasArg(OPT_shared);
   config->singleRoRx = !args.hasFlag(OPT_rosegment, OPT_no_rosegment, true);
   config->soName = args.getLastArgValue(OPT_soname);

lld/ELF/Options.td

def pop_state: F<"pop-state">,
   HelpText<"Undo the effect of -push-state">;
 
 def push_state: F<"push-state">,
   HelpText<"Save the current state of -as-needed, -static and -whole-archive">;
 
 def print_map: F<"print-map">,
   HelpText<"Print a link map to the standard output">;
 
+defm relax: BB<"relax",
+  "Enable target-specific relaxations if supported (default)",
+  "Disable target-specific relaxations">;
+
 defm reproduce:
   EEq<"reproduce",
      "Write tar file containing inputs and command to reproduce link">;
 
 defm rosegment: BB<"rosegment",
   "Put read-only non-executable sections in their own segment (default)",
   "Do not put read-only non-executable sections in their own segment">;
 

 
 // Options listed below are silently ignored for now for compatibility.
 def: F<"detect-odr-violations">;
 def: Flag<["-"], "g">;
 def: F<"long-plt">;
 def: F<"no-copy-dt-needed-entries">;
 def: F<"no-ctors-in-init-array">;
 def: F<"no-keep-memory">;
-def: F<"no-relax">;
 def: F<"no-warn-mismatch">;
 def: Separate<["--", "-"], "rpath-link">;
 def: J<"rpath-link=">;
 def: F<"secure-plt">;
 def: F<"sort-common">;
 def: F<"stats">;
 def: F<"warn-execstack">;
 def: F<"warn-once">;

lld/docs/ReleaseNotes.rst

 * ``e_entry`` no longer falls back to the address of ``.text`` if the entry symbol does not exist.
   Instead, a value of 0 will be written.
   (`D110014 <https://reviews.llvm.org/D110014>`_)
 
 Architecture specific changes:
 
 * The x86-32 port now supports TLSDESC (``-mtls-dialect=gnu2``).
   (`D112582 <https://reviews.llvm.org/D112582>`_)
+* For x86-64, ``--no-relax`` now suppresses ``R_X86_64_GOTPCRELX`` and
+  ``R_X86_64_REX_GOTPCRELX`` GOT optimization
+  (`D113615 <https://reviews.llvm.org/D113615>`_)
 
 Breaking changes
 ----------------
 
 * ...
 
 COFF Improvements
 -----------------

lld/docs/ld.lld.1

 Disable STB_GNU_UNIQUE symbol binding.
 .It Fl -no-merge-exidx-entries
 Disable merging .ARM.exidx entries.
 .It Fl -no-nmagic
 Page align sections.
 .It Fl -no-omagic
 Do not set the text data sections to be writable, page align sections.
 .It Fl -no-relax
-Disable target-specific relaxations. This is currently a no-op.
+Disable target-specific relaxations. For x86-64 this disables R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX GOT optimization.
 .It Fl -no-rosegment
 Do not put read-only non-executable sections in their own segment.
 .It Fl -no-undefined-version
 Report version scripts that refer undefined symbols.
 .It Fl -no-undefined
 Report unresolved symbols even if the linker is creating a shared library.
 .It Fl -no-warn-symbol-ordering
 Do not warn about problems with the symbol ordering file or call graph profile.

lld/test/ELF/x86-64-gotpc-relax.s

 # REQUIRES: x86
+## Test R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX GOT optimization.
+
 # RUN: llvm-mc -filetype=obj -relax-relocations -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t1 --no-apply-dynamic-relocs
 # RUN: llvm-readobj -x .got.plt -r %t1 | FileCheck --check-prefixes=RELOC,NO-APPLY-DYNAMIC-RELOCS %s
 # RUN: ld.lld %t.o -o %t1 --apply-dynamic-relocs
 # RUN: llvm-readobj -x .got.plt -r %t1 | FileCheck --check-prefixes=RELOC,APPLY-DYNAMIC-RELOCS %s
 # RUN: ld.lld %t.o -o %t1
 # RUN: llvm-objdump -d %t1 | FileCheck --check-prefix=DISASM %s
 
+## --no-relax disables GOT optimization.
+# RUN: ld.lld --no-relax %t.o -o %t2
+# RUN: llvm-objdump -d %t2 | FileCheck --check-prefix=NORELAX %s
+
 ## There is one R_X86_64_IRELATIVE relocations.
 # RELOC-LABEL: Relocations [
 # RELOC-NEXT:    Section (1) .rela.dyn {
 # RELOC-NEXT:      0x202220 R_X86_64_IRELATIVE - 0x201172
 # RELOC-NEXT:    }
 # RELOC-NEXT:  ]
 # RELOC-LABEL: Hex dump of section '.got.plt':
 # NO-APPLY-DYNAMIC-RELOCS-NEXT:  0x00202220 00000000 00000000

 # DISASM-NEXT: nop
 # DISASM-NEXT: jmp   0x201171 <hid>
 # DISASM-NEXT: nop
 # DISASM-NEXT: jmp   0x201171 <hid>
 # DISASM-NEXT: nop
 # DISASM-NEXT: jmpq  *4119(%rip)
 # DISASM-NEXT: jmpq  *4113(%rip)
 
+# NORELAX-LABEL: <_start>:
+# NORELAX-COUNT-12: movq
+# NORELAX-COUNT-6:  callq *
+# NORELAX-COUNT-6:  jmpq *
+
 .text
 .globl foo
 .type foo, @function
 foo:
  nop
 
 .globl hid
 .hidden hid