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

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

Hi Chrishophe,

Could you please provide a description of what this patch does and how it achieves that?

On 25/05/18 09:03, Christophe Lyon wrote:
> 2018-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: I4b5127261a9aefa0f0318f110574ec07a856aeb1
>
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 025485d..2434602 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -3967,7 +3967,27 @@ 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 to jump into
> +        the trampoline code just below.  */
> +      if (TARGET_ARM) fprintf (f, "\t.arm\n");
> +      else if (TARGET_THUMB2) fprintf (f, "\t.thumb\n");
> +      else fprintf (f, "\t.code\t16\n");
> +

Please format this in GNU style.

> +      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);

Likewise.
Also, this will use offset 4 for TARGET_THUMB1. Given that you handle TARGET_THUMB1
in the if statement above you expect this code can be entered for TARGET_THUMB1?

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);
> @@ -4008,12 +4028,37 @@ 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));
> +      rtx trampolineCodeStart
> +       = plus_constant (Pmode, XEXP (m_tramp, 0), TARGET_THUMB2 ? 9 : 8);
> +
> +      /* Write initial funcdesc which will jump into trampoline.  */
> +      mem = adjust_address (m_tramp, SImode, 0);
> +      emit_move_insn (mem, trampolineCodeStart);
> +      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"),
> @@ -4027,7 +4072,9 @@ 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 e8ef439..db17ef2 100644
> --- a/gcc/config/arm/arm.h
> +++ b/gcc/config/arm/arm.h
> @@ -1578,7 +1578,7 @@ 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