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Re: [ARM/FDPIC v5 09/21] [ARM] FDPIC: Add support for taking address of nested function


On Tue, 16 Jul 2019 at 14:42, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
>
>
> On 7/16/19 12:18 PM, Kyrill Tkachov wrote:
> > Hi Christophe
> >
> > On 5/15/19 1:39 PM, Christophe Lyon wrote:
> > > In FDPIC mode, the trampoline generated to support pointers to nested
> > > functions looks like:
> > >
> > >            .word trampoline address
> > >            .word trampoline GOT address
> > >            ldr            r12, [pc, #8]
> > >            ldr            r9, [pc, #8]
> > >            ldr           pc, [pc, #8]
> > >            .word static chain value
> > >            .word GOT address
> > >            .word function's address
> > >
> > > because in FDPIC function pointers are actually pointers to function
> > > descriptors, we have to actually generate a function descriptor for
> > > the trampoline.
> > >
> > > 2019-XX-XX  Christophe Lyon <christophe.lyon@st.com>
> > >         Mickaël Guêné <mickael.guene@st.com>
> > >
> > >         gcc/
> > >         * config/arm/arm.c (arm_asm_trampoline_template): Add FDPIC
> > >         support.
> > >         (arm_trampoline_init): Likewise.
> > >         (arm_trampoline_init): Likewise.
> > >         * config/arm/arm.h (TRAMPOLINE_SIZE): Likewise.
> > >
> > > Change-Id: Idc4d5f629ae4f8d79bdf9623517481d524a0c144
> > >
> > > diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> > > index 40e3f3b..99d13bf 100644
> > > --- a/gcc/config/arm/arm.c
> > > +++ b/gcc/config/arm/arm.c
> > > @@ -3976,13 +3976,50 @@ arm_warn_func_return (tree decl)
> > >             .word static chain value
> > >             .word function's address
> > >     XXX FIXME: When the trampoline returns, r8 will be clobbered.  */
> > > +/* In FDPIC mode, the trampoline looks like:
> > > +          .word trampoline address
> > > +          .word trampoline GOT address
> > > +          ldr            r12, [pc, #8] ; #4 for Thumb2
> > > +          ldr            r9,  [pc, #8] ; #4 for Thumb2
> > > +          ldr           pc,  [pc, #8] ; #4 for Thumb2
> > > +          .word static chain value
> > > +          .word GOT address
> > > +          .word function's address
> > > +*/
> >
> >
> > I think this comment is not right for Thumb2.
> >
> > These load instructionshave 32-bit encodings, even in Thumb2 (they use
> > high registers).
>
> Andre and Wilco pointed out to me offline that the offset should be #4
> for Arm mode.
>
> The Arm ARM at E1.2.3 says:
>
> PC, the program counter
>
> * When executing an A32 instruction, PC reads as the address of the
> current instruction plus 8.
>
> * When executing a T32 instruction, PC reads as the address of the
> current instruction plus 4.
>

Yes, it looks like the code is right, and the comment is wrong:
- offset 8 for thumb2 mode
- offset 4 for arm mode

Thanks,

Christophe

> Thanks,
>
> Kyrill
>
>
> >
> > Also, please merge this comment with the one above (no separate /**/)
> >
> > >
> > >  static void
> > >  arm_asm_trampoline_template (FILE *f)
> > >  {
> > >    fprintf (f, "\t.syntax unified\n");
> > >
> > > -  if (TARGET_ARM)
> > > +  if (TARGET_FDPIC)
> > > +    {
> > > +      /* The first two words are a function descriptor pointing to the
> > > +        trampoline code just below.  */
> > > +      if (TARGET_ARM)
> > > +       fprintf (f, "\t.arm\n");
> > > +      else if (TARGET_THUMB2)
> > > +       fprintf (f, "\t.thumb\n");
> > > +      else
> > > +       /* Only ARM and Thumb-2 are supported.  */
> > > +       gcc_unreachable ();
> > > +
> > > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > > +      /* Trampoline code which sets the static chain register but also
> > > +        PIC register before jumping into real code. */
> > > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > > +                  STATIC_CHAIN_REGNUM, PC_REGNUM,
> > > +                  TARGET_THUMB2 ? 8 : 4);
> > > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > > +                  PIC_OFFSET_TABLE_REGNUM, PC_REGNUM,
> > > +                  TARGET_THUMB2 ? 8 : 4);
> > > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > > +                  PC_REGNUM, PC_REGNUM,
> > > +                  TARGET_THUMB2 ? 8 : 4);
> >
> >
> > As above, I think the offset should be 8 for both Arm and Thumb2.
> >
> > Thanks,
> >
> > Kyrill
> >
> >
> > > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > > +    }
> > > +  else if (TARGET_ARM)
> > >      {
> > >        fprintf (f, "\t.arm\n");
> > >        asm_fprintf (f, "\tldr\t%r, [%r, #0]\n", STATIC_CHAIN_REGNUM,
> > > PC_REGNUM);
> > > @@ -4023,12 +4060,40 @@ arm_trampoline_init (rtx m_tramp, tree fndecl,
> > > rtx chain_value)
> > >    emit_block_move (m_tramp, assemble_trampoline_template (),
> > >                     GEN_INT (TRAMPOLINE_SIZE), BLOCK_OP_NORMAL);
> > >
> > > -  mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 8 : 12);
> > > -  emit_move_insn (mem, chain_value);
> > > +  if (TARGET_FDPIC)
> > > +    {
> > > +      rtx funcdesc = XEXP (DECL_RTL (fndecl), 0);
> > > +      rtx fnaddr = gen_rtx_MEM (Pmode, funcdesc);
> > > +      rtx gotaddr = gen_rtx_MEM (Pmode, plus_constant (Pmode,
> > > funcdesc, 4));
> > > +      /* The function start address is at offset 8, but in Thumb mode
> > > +        we want bit 0 set to 1 to indicate Thumb-ness, hence 9
> > > +        below.  */
> > > +      rtx trampoline_code_start
> > > +       = plus_constant (Pmode, XEXP (m_tramp, 0), TARGET_THUMB2 ? 9
> > : 8);
> > > +
> > > +      /* Write initial funcdesc which points to the trampoline.  */
> > > +      mem = adjust_address (m_tramp, SImode, 0);
> > > +      emit_move_insn (mem, trampoline_code_start);
> > > +      mem = adjust_address (m_tramp, SImode, 4);
> > > +      emit_move_insn (mem, gen_rtx_REG (Pmode,
> > PIC_OFFSET_TABLE_REGNUM));
> > > +      /* Setup static chain.  */
> > > +      mem = adjust_address (m_tramp, SImode, 20);
> > > +      emit_move_insn (mem, chain_value);
> > > +      /* GOT + real function entry point.  */
> > > +      mem = adjust_address (m_tramp, SImode, 24);
> > > +      emit_move_insn (mem, gotaddr);
> > > +      mem = adjust_address (m_tramp, SImode, 28);
> > > +      emit_move_insn (mem, fnaddr);
> > > +    }
> > > +  else
> > > +    {
> > > +      mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 8 : 12);
> > > +      emit_move_insn (mem, chain_value);
> > >
> > > -  mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 12 : 16);
> > > -  fnaddr = XEXP (DECL_RTL (fndecl), 0);
> > > -  emit_move_insn (mem, fnaddr);
> > > +      mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 12 : 16);
> > > +      fnaddr = XEXP (DECL_RTL (fndecl), 0);
> > > +      emit_move_insn (mem, fnaddr);
> > > +    }
> > >
> > >    a_tramp = XEXP (m_tramp, 0);
> > >    emit_library_call (gen_rtx_SYMBOL_REF (Pmode, "__clear_cache"),
> > > @@ -4042,7 +4107,9 @@ arm_trampoline_init (rtx m_tramp, tree fndecl,
> > > rtx chain_value)
> > >  static rtx
> > >  arm_trampoline_adjust_address (rtx addr)
> > >  {
> > > -  if (TARGET_THUMB)
> > > +  /* For FDPIC don't fix trampoline address since it's a function
> > > +     descriptor and not a function address.  */
> > > +  if (TARGET_THUMB && !TARGET_FDPIC)
> > >      addr = expand_simple_binop (Pmode, IOR, addr, const1_rtx,
> > >                                  NULL, 0, OPTAB_LIB_WIDEN);
> > >    return addr;
> > > diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
> > > index 45c0e2b..f80df63 100644
> > > --- a/gcc/config/arm/arm.h
> > > +++ b/gcc/config/arm/arm.h
> > > @@ -1602,7 +1602,7 @@ typedef struct
> > >  #define INIT_EXPANDERS  arm_init_expanders ()
> > >
> > >  /* Length in units of the trampoline for entering a nested
> > function.  */
> > > -#define TRAMPOLINE_SIZE  (TARGET_32BIT ? 16 : 20)
> > > +#define TRAMPOLINE_SIZE  (TARGET_FDPIC ? 32 : (TARGET_32BIT ? 16 : 20))
> > >
> > >  /* Alignment required for a trampoline in bits.  */
> > >  #define TRAMPOLINE_ALIGNMENT  32
> > > --
> > > 2.6.3
> > >


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