On Tue, 16 Jul 2019 at 14:42, Kyrill Tkachov
<kyrylo.tkac...@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.l...@st.com>
> > > Mickaël Guêné <mickael.gu...@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
> > >
