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

[Christophe Lyon <christophe dot lyon at linaro dot org>]

On 8 June 2018 at 12:33, Kyrill  Tkachov <kyrylo.tkachov@foss.arm.com> wrote:
> Hi Chrishophe,
>
> Could you please provide a description of what this patch does and how it
> achieves that?
>

OK, I'l try to do some archaeology and formulate a description.


>
> 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.

Sorry for missing that. I did visually inspect all the patches and ran
check_GNU_style (both .sh and .py, none complained).

>
>> +      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?
>
I'll check that,

Thanks

> 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
>>
>