Re: [PATCH v2] x86: Generate PLT32 relocation for ".long foo@PLT - .L4"

On Mon, Jul 27, 2026 at 3:04 PM Fangrui Song <[email protected]> wrote:
>
> On Sun, Jul 26, 2026 at 8:51 PM H.J. Lu <[email protected]> wrote:
> >
> > On Sat, Jul 25, 2026 at 10:35 AM H.J. Lu <[email protected]> wrote:
> > >
> > > Add BFD_RELOC_X86_64_PLT32 and use it to generate R_X86_64_PLT32 for
> > > directives like ".long foo@PLT - .L4" so that PLT entries are used to
> > > resolve the PC32 relocation against function symbols for such directives.
> > >
> > > bfd/
> > >
> > > PR gas/34423
> > > * elf64-x86-64.c (x86_64_reloc_map): Add BFD_RELOC_X86_64_PLT32.
> > > * reloc.c (bfd_reloc_code_real): Add BFD_RELOC_X86_64_PLT32.
> > > * bfd-in2.h: Regenerated.
> > > * libbfd.h: Likewise.
> > >
> > > gas/
> > >
> > > PR gas/34423
> > > * config/tc-i386.c (x86_cons): Return BFD_RELOC_X86_64_PLT32 for
> > > directives like ".long foo@PLT - .L4".
> > > (tc_gen_reloc): Generate R_X86_64_PLT32 for BFD_RELOC_X86_64_PLT32
> > > with addend computed like R_X86_64_PC32.
> > > * testsuite/gas/i386/reloc64.l: Updated.
> > > * testsuite/gas/i386/reloc64.s: Replace ".long xtrn@plt - ." with
> > > ".long xtrn@plt - _start".
> > > * testsuite/gas/i386/ilp32/reloc64.l: Updated.
> > > * testsuite/gas/i386/ilp32/reloc64.s: Replace ".long xtrn@plt - ."
> > > with ".long xtrn@plt - ptr".
> > > * testsuite/gas/i386/x86-64-jump-table.d: New file.
> > > * testsuite/gas/i386/x86-64-jump-table.d: Likewise.
> > > * testsuite/gas/i386/x86-64-jump-table.s: Likewise.
> > > * testsuite/gas/i386/x86-64.exp: Run x86-64-jump-table.
> > > * testsuite/gas/i386/ilp32/x86-64-jump-table.d: New file.
> > >
> > > ld/
> > >
> > > PR gas/34423
> > > * testsuite/ld-x86-64/pr34423.c: New file.
> > > * testsuite/ld-x86-64/x86-64-jump-table.s: Likewise.
> > > * testsuite/ld-x86-64/x86-64.exp: Run gas/34423 tests.
> > >
> >
> > Add i386 support like LLVM assembler in the v2 patch.
> >
> >
> > --
> > H.J.
> > ---
> > LLVM assembler supports directives like ".long foo@PLT - .L4" for i386
> > and x86-64.  Implement the same feature to generate PLT32 relocation
> > for directives like ".long foo@PLT - .L4" so that PLT entries are used
> > to resolve the PC32 relocation against function symbols.
> >
> > bfd/
> >
> > PR gas/34423
> > * elf32-i386.c (elf_i386_reloc_type_lookup): Handle
> > BFD_RELOC_386_PC32_TO_PLT32.
> > * elf64-x86-64.c (x86_64_reloc_map): Add
> > BFD_RELOC_X86_64_PC32_TO_PLT32.
> > * reloc.c (bfd_reloc_code_real): Add BFD_RELOC_386_PC32_TO_PLT32
> > and BFD_RELOC_X86_64_PC32_TO_PLT32.
> > * bfd-in2.h: Regenerated.
> > * libbfd.h: Likewise.
> >
> > gas/
> >
> > PR gas/34423
> > * config/tc-i386.c (x86_cons): Return
> > BFD_RELOC_X86_64_PC32_TO_PLT32 or BFD_RELOC_386_PC32_TO_PLT32
> > for directives like ".long foo@PLT - .L4".
> > (md_apply_fix): Compute addend for BFD_RELOC_386_PC32_TO_PLT32.
> > (tc_gen_reloc): Handle BFD_RELOC_X86_64_PC32_TO_PLT32 and
> > BFD_RELOC_386_PC32_TO_PLT32.  Compute addend like
> > BFD_RELOC_32_PCREL for BFD_RELOC_X86_64_PC32_TO_PLT32.
> > * testsuite/gas/i386/i386.exp: Run plt test.
> > * testsuite/gas/i386/ilp32/reloc64.l: Updated.
> > * testsuite/gas/i386/ilp32/reloc64.s: Replace ".long xtrn@plt - ."
> > with ".long xtrn@plt - ptr".
> > * testsuite/gas/i386/ilp32/x86-64-jump-table.d: New file.
> > * testsuite/gas/i386/plt.d: New file.
> > * testsuite/gas/i386/plt.s: Likewise.
> > * testsuite/gas/i386/reloc32.s: Replace ".long xtrn@plt - ."
> > with ".long xtrn@plt - ptr".
> > * testsuite/gas/i386/reloc64.l: Updated.
> > * testsuite/gas/i386/reloc64.s: Replace ".long xtrn@plt - ." with
> > ".long xtrn@plt - _start".
> > * testsuite/gas/i386/x86-64-jump-table.d: New file.
> > * testsuite/gas/i386/x86-64-jump-table.d: Likewise.
> > * testsuite/gas/i386/x86-64-jump-table.s: Likewise.
> > * testsuite/gas/i386/x86-64.exp: Run x86-64-jump-table.
> >
> > ld/
> >
> > PR gas/34423
> > * testsuite/ld-x86-64/pr34423.c: New file.
> > * testsuite/ld-x86-64/x86-64-jump-table.s: Likewise.
> > * testsuite/ld-x86-64/x86-64.exp: Run gas/34423 tests.
>
> Note, the fact that LLVM integrated assembler's x86 port supports
> `.long foo@plt - .` is not really an official decision.
> Having a subtrahend in the expression is awkward to support in assemblers.

GNU assembler change is small.  This feature is used to fold tail calls
into PIC jump table as in

ld/testsuite/ld-x86-64/x86-64-jump-table.s

> In LLVM's aarch64 and riscv ports %pltpcrel(foo)   is used.
>
> https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers
>
> AArch64 uses :specifier:expr, yet PAuth ABI (.quad (g + 7)@AUTH(ia,0))
> cannot use : after data directives due to parsing ambiguity.
> R_AARCH64_PLT32, R_AARCH64_GOTPCREL32, and R_AARCH64_FUNCINIT were
> fixed in llvm/llvm-project#155776 to use %pltpcrel(foo) and
> %gotpcrel(foo) instead of the unofficial foo@plt - . / foo@gotpcrel
> forms.



-- 
H.J.