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Re: [ARM/FDPIC v5 04/21] [ARM] FDPIC: Add support for FDPIC for arm architecture


On 16/07/2019 13:58, Richard Sandiford wrote:
Christophe Lyon <christophe.lyon@st.com> writes:
The FDPIC register is hard-coded to r9, as defined in the ABI.

We have to disable tailcall optimizations if we don't know if the
target function is in the same module. If not, we have to set r9 to
the value associated with the target module.

When generating a symbol address, we have to take into account whether
it is a pointer to data or to a function, because different
relocations are needed.

2019-XX-XX  Christophe Lyon  <christophe.lyon@st.com>
	Mickaël Guêné <mickael.guene@st.com>

	* config/arm/arm-c.c (__FDPIC__): Define new pre-processor macro
	in FDPIC mode.
	* config/arm/arm-protos.h (arm_load_function_descriptor): Declare
	new function.
	* config/arm/arm.c (arm_option_override): Define pic register to
	FDPIC_REGNUM.
	(arm_function_ok_for_sibcall): Disable sibcall optimization if we
	have no decl or go through PLT.
	(arm_load_pic_register): Handle TARGET_FDPIC.
	(arm_is_segment_info_known): New function.
	(arm_pic_static_addr): Add support for FDPIC.
	(arm_load_function_descriptor): New function.
	(arm_assemble_integer): Add support for FDPIC.
	* config/arm/arm.h (PIC_OFFSET_TABLE_REG_CALL_CLOBBERED):
	Define. (FDPIC_REGNUM): New define.
	* config/arm/arm.md (call): Add support for FDPIC.
	(call_value): Likewise.
	(*restore_pic_register_after_call): New pattern.
	(untyped_call): Disable if FDPIC.
	(untyped_return): Likewise.
	* config/arm/unspecs.md (UNSPEC_PIC_RESTORE): New.

Change-Id: I8fb1a6b85ace672184013568c5d28fbda2f7fda4

diff --git a/gcc/config/arm/arm-c.c b/gcc/config/arm/arm-c.c
index 6e256ee..34695fa 100644
--- a/gcc/config/arm/arm-c.c
+++ b/gcc/config/arm/arm-c.c
@@ -203,6 +203,8 @@ arm_cpu_builtins (struct cpp_reader* pfile)
        builtin_define ("__ARM_EABI__");
      }
+ def_or_undef_macro (pfile, "__FDPIC__", TARGET_FDPIC);
+
    def_or_undef_macro (pfile, "__ARM_ARCH_EXT_IDIV__", TARGET_IDIV);
    def_or_undef_macro (pfile, "__ARM_FEATURE_IDIV", TARGET_IDIV);
diff --git a/gcc/config/arm/arm-protos.h b/gcc/config/arm/arm-protos.h
index 485bc68..272968a 100644
--- a/gcc/config/arm/arm-protos.h
+++ b/gcc/config/arm/arm-protos.h
@@ -139,6 +139,7 @@ extern int arm_max_const_double_inline_cost (void);
  extern int arm_const_double_inline_cost (rtx);
  extern bool arm_const_double_by_parts (rtx);
  extern bool arm_const_double_by_immediates (rtx);
+extern rtx arm_load_function_descriptor (rtx funcdesc);
  extern void arm_emit_call_insn (rtx, rtx, bool);
  bool detect_cmse_nonsecure_call (tree);
  extern const char *output_call (rtx *);
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 45abcd8..d9397b5 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -3485,6 +3485,15 @@ arm_option_override (void)
    if (flag_pic && TARGET_VXWORKS_RTP)
      arm_pic_register = 9;
+ /* If in FDPIC mode then force arm_pic_register to be r9. */
+  if (TARGET_FDPIC)
+    {
+      arm_pic_register = FDPIC_REGNUM;
+      if (! TARGET_ARM && ! TARGET_THUMB2)
+	sorry ("FDPIC mode is supported on architecture versions that "
+	       "support ARM or Thumb-2 only.");
+    }
+
    if (arm_pic_register_string != NULL)
      {
        int pic_register = decode_reg_name (arm_pic_register_string);

Isn't this equivalent to rejecting Thumb-1?  I think that would be
clearer in both the condition and the error message.

How does this interact with arm_pic_data_is_text_relative?  Are both
values supported?

@@ -7295,6 +7304,21 @@ arm_function_ok_for_sibcall (tree decl, tree exp)
    if (cfun->machine->sibcall_blocked)
      return false;
+ if (TARGET_FDPIC)
+    {
+      /* In FDPIC, never tailcall something for which we have no decl:
+	 the target function could be in a different module, requiring
+	 a different FDPIC register value.  */
+      if (decl == NULL)
+	return false;
+
+      /* Don't tailcall if we go through the PLT since the FDPIC
+	 register is then corrupted and we don't restore it after
+	 static function calls.  */
+      if (!targetm.binds_local_p (decl))
+	return false;
+    }
+
    /* Never tailcall something if we are generating code for Thumb-1.  */
    if (TARGET_THUMB1)
      return false;
@@ -7711,7 +7735,9 @@ arm_load_pic_register (unsigned long saved_regs ATTRIBUTE_UNUSED, rtx pic_reg)
  {
    rtx l1, labelno, pic_tmp, pic_rtx;
- if (crtl->uses_pic_offset_table == 0 || TARGET_SINGLE_PIC_BASE)
+  if (crtl->uses_pic_offset_table == 0
+      || TARGET_SINGLE_PIC_BASE
+      || TARGET_FDPIC)
      return;
gcc_assert (flag_pic);
@@ -7780,28 +7806,142 @@ arm_load_pic_register (unsigned long saved_regs ATTRIBUTE_UNUSED, rtx pic_reg)
    emit_use (pic_reg);
  }
+/* Try to determine whether an object, referenced via ORIG, will be
+   placed in the text or data segment.  This is used in FDPIC mode, to
+   decide which relocations to use when accessing ORIG.  IS_READONLY
+   is set to true if ORIG is a read-only location, false otherwise.
+   Return true if we could determine the location of ORIG, false
+   otherwise.  IS_READONLY is valid only when we return true.  */

Maybe *IS_READONLY in both cases?

+static bool
+arm_is_segment_info_known (rtx orig, bool *is_readonly)
+{
+  bool res = false;
+
+  *is_readonly = false;
+
+  if (GET_CODE (orig) == LABEL_REF)
+    {
+      res = true;
+      *is_readonly = true;
+    }

Think this function would be easier to read with early returns.

+  else if (SYMBOL_REF_P (orig))

...so "if" rather than "else if" here.

+    {
+      if (CONSTANT_POOL_ADDRESS_P (orig))
+	{
+	  res = true;
+	  *is_readonly = true;
+	}
+      else if (SYMBOL_REF_LOCAL_P (orig)
+	       && !SYMBOL_REF_EXTERNAL_P (orig)
+	       && SYMBOL_REF_DECL (orig)
+	       && (!DECL_P (SYMBOL_REF_DECL (orig))
+		   || !DECL_COMMON (SYMBOL_REF_DECL (orig))))
+	{
+	  tree decl = SYMBOL_REF_DECL (orig);
+	  tree init = (TREE_CODE (decl) == VAR_DECL)
+	    ? DECL_INITIAL (decl) : (TREE_CODE (decl) == CONSTRUCTOR)
+	    ? decl : 0;
+	  int reloc = 0;
+	  bool named_section, readonly;
+
+	  if (init && init != error_mark_node)
+	    reloc = compute_reloc_for_constant (init);
+
+	  named_section = TREE_CODE (decl) == VAR_DECL
+	    && lookup_attribute ("section", DECL_ATTRIBUTES (decl));

Here too I think it would be better to return false early.

How much variation do you support here for named sections?  E.g. can a
linker script really put SECTION_WRITE sections in the text segment?
Seems like there are some cases that could be handled.

(Just asking, not suggesting you should change anything.)

+	  readonly = decl_readonly_section (decl, reloc);
+
+	  /* We don't know where the link script will put a named
+	     section, so return false in such a case.  */
+	  res = !named_section;
+
+	  if (!named_section)
+	    *is_readonly = readonly;
+	}
+      else
+	{
+	  /* We don't know.  */
+	  res = false;
+	}
+    }
+  else
+    gcc_unreachable ();
+
+  return res;
+}
+
  /* Generate code to load the address of a static var when flag_pic is set.  */
  static rtx_insn *
  arm_pic_static_addr (rtx orig, rtx reg)
  {
    rtx l1, labelno, offset_rtx;
+  rtx_insn *insn;
gcc_assert (flag_pic); - /* We use an UNSPEC rather than a LABEL_REF because this label
-     never appears in the code stream.  */
-  labelno = GEN_INT (pic_labelno++);
-  l1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, labelno), UNSPEC_PIC_LABEL);
-  l1 = gen_rtx_CONST (VOIDmode, l1);
+  bool is_readonly = false;
+  bool info_known = false;
- /* On the ARM the PC register contains 'dot + 8' at the time of the
-     addition, on the Thumb it is 'dot + 4'.  */
-  offset_rtx = plus_constant (Pmode, l1, TARGET_ARM ? 8 : 4);
-  offset_rtx = gen_rtx_UNSPEC (Pmode, gen_rtvec (2, orig, offset_rtx),
-                               UNSPEC_SYMBOL_OFFSET);
-  offset_rtx = gen_rtx_CONST (Pmode, offset_rtx);
+  if (TARGET_FDPIC
+      && SYMBOL_REF_P (orig)
+      && !SYMBOL_REF_FUNCTION_P (orig))
+      info_known = arm_is_segment_info_known (orig, &is_readonly);

Excess indendentation.  Feels like it might be slightly simpler
to handle SYMBOL_REF_FUNCTION_P in arm_is_segment_info_known,
but I guess the idea is that it might not then be clear whether
the caller is asking about a descriptor or the function itself.

- return emit_insn (gen_pic_load_addr_unified (reg, offset_rtx, labelno));
+  if (TARGET_FDPIC
+      && SYMBOL_REF_P (orig)
+      && !SYMBOL_REF_FUNCTION_P (orig)
+      && !info_known)
+    {
+      /* We don't know where orig is stored, so we have be
+	 pessimistic and use a GOT relocation.  */
+      rtx pat;
+      rtx mem;
+      rtx pic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+
+      pat = gen_calculate_pic_address (reg, pic_reg, orig);
+
+      /* Make the MEM as close to a constant as possible.  */
+      mem = SET_SRC (pat);
+      gcc_assert (MEM_P (mem) && !MEM_VOLATILE_P (mem));
+      MEM_READONLY_P (mem) = 1;
+      MEM_NOTRAP_P (mem) = 1;
+
+      insn = emit_insn (pat);

Think "pat = ..." onwards should be split out into a helper, since it's
a cut-&-paste of the code in legitimize_pic_address.

+    }
+  else if (TARGET_FDPIC
+	   && SYMBOL_REF_P (orig)
+	   && (SYMBOL_REF_FUNCTION_P (orig)
+	       || (info_known && !is_readonly)))
+    {
+      /* We use the GOTOFF relocation.  */
+      rtx pic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+
+      rtx l1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, orig), UNSPEC_PIC_SYM);
+      emit_insn (gen_movsi (reg, l1));
+      insn = emit_insn (gen_addsi3 (reg, reg, pic_reg));
+    }
+  else
+    {
+      /* Not FDPIC, not SYMBOL_REF_P or readonly: we can use
+	 PC-relative access.  */
+      /* We use an UNSPEC rather than a LABEL_REF because this label
+	 never appears in the code stream.  */
+      labelno = GEN_INT (pic_labelno++);
+      l1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, labelno), UNSPEC_PIC_LABEL);
+      l1 = gen_rtx_CONST (VOIDmode, l1);
+
+      /* On the ARM the PC register contains 'dot + 8' at the time of the
+	 addition, on the Thumb it is 'dot + 4'.  */
+      offset_rtx = plus_constant (Pmode, l1, TARGET_ARM ? 8 : 4);
+      offset_rtx = gen_rtx_UNSPEC (Pmode, gen_rtvec (2, orig, offset_rtx),
+				   UNSPEC_SYMBOL_OFFSET);
+      offset_rtx = gen_rtx_CONST (Pmode, offset_rtx);
+
+      insn = emit_insn (gen_pic_load_addr_unified (reg, offset_rtx,
+						   labelno));
+    }
+
+  return insn;
  }
/* Return nonzero if X is valid as an ARM state addressing register. */
@@ -16112,9 +16252,36 @@ get_jump_table_size (rtx_jump_table_data *insn)
    return 0;
  }
+/* Emit insns to load the function address from FUNCDESC (an FDPIC
+   function descriptor) into a register and the GOT address into the
+   FDPIC register, returning an rtx for the register holding the
+   function address.  */
+
+rtx
+arm_load_function_descriptor (rtx funcdesc)
+{
+  rtx fnaddr_reg = gen_reg_rtx (Pmode);
+  rtx pic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+  rtx fnaddr = gen_rtx_MEM (Pmode, funcdesc);
+  rtx gotaddr = gen_rtx_MEM (Pmode, plus_constant (Pmode, funcdesc, 4));
+  rtx par = gen_rtx_PARALLEL (VOIDmode, rtvec_alloc (3));
+
+  emit_move_insn (fnaddr_reg, fnaddr);
+  /* The ABI requires the entry point address to be loaded first, so
+     prevent the load from being moved after that of the GOT
+     address.  */

Do you mean that the move insn above has to come before the
pattern below?  If so, I think that should be enforced by making this...

+  XVECEXP (par, 0, 0) = gen_rtx_UNSPEC (VOIDmode,
+					gen_rtvec (2, pic_reg, gotaddr),
+					UNSPEC_PIC_RESTORE);
+  XVECEXP (par, 0, 1) = gen_rtx_USE (VOIDmode, gotaddr);
+  XVECEXP (par, 0, 2) = gen_rtx_CLOBBER (VOIDmode, pic_reg);
+  emit_insn (par);
+
+  return fnaddr_reg;
+}
+

...use fnaddr_reg.

Does the instruction actually use pic_reg?  We only get here for
non-symbolic addresses after all.

It seems simpler to make *restore_pic_register_after_call a named pattern
and use gen_restore_pic_register_after_call instead.

  /* Return the maximum amount of padding that will be inserted before
     label LABEL.  */
-
  static HOST_WIDE_INT
  get_label_padding (rtx label)
  {
@@ -23069,9 +23236,37 @@ arm_assemble_integer (rtx x, unsigned int size, int aligned_p)
  		  && (!SYMBOL_REF_LOCAL_P (x)
  		      || (SYMBOL_REF_DECL (x)
  			  ? DECL_WEAK (SYMBOL_REF_DECL (x)) : 0))))
-	    fputs ("(GOT)", asm_out_file);
+	    {
+	      if (TARGET_FDPIC && SYMBOL_REF_FUNCTION_P (x))
+		fputs ("(GOTFUNCDESC)", asm_out_file);
+	      else
+		fputs ("(GOT)", asm_out_file);
+	    }
  	  else
-	    fputs ("(GOTOFF)", asm_out_file);
+	    {
+	      if (TARGET_FDPIC && SYMBOL_REF_FUNCTION_P (x))
+		fputs ("(GOTOFFFUNCDESC)", asm_out_file);
+	      else
+		{
+		  bool is_readonly;
+
+		  if (arm_is_segment_info_known (x, &is_readonly))
+		    fputs ("(GOTOFF)", asm_out_file);
+		  else
+		    fputs ("(GOT)", asm_out_file);
+		}
+	    }
+	}
+
+      /* For FDPIC we also have to mark symbol for .data section.  */
+      if (TARGET_FDPIC
+	  && NEED_GOT_RELOC
+	  && flag_pic
+	  && !making_const_table
+	  && SYMBOL_REF_P (x))
+	{
+	  if (SYMBOL_REF_FUNCTION_P (x))
+	    fputs ("(FUNCDESC)", asm_out_file);
  	}
        fputc ('\n', asm_out_file);
        return true;

Do you expect to reach here for LABEL_REFs with TARGET_FDPIC?  The second
block of code tests for SYMBOL_REF_P but the first tests
SYMBOL_REF_FUNCTION_P without checking SYMBOL_REF_P first.

Can NEED_GOT_RELOC or flag_pic be false for TARGET_FDPIC?
Is !flag_pic TARGET_FDPIC supported?

diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index 0aecd03..9036255 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -8127,6 +8127,23 @@
      rtx callee, pat;
      tree addr = MEM_EXPR (operands[0]);
+ /* Force FDPIC register (r9) before call. */
+    if (TARGET_FDPIC)
+      {
+	/* No need to update r9 if calling a static function.
+	   In other words: set r9 for indirect or non-local calls.  */
+	callee = XEXP (operands[0], 0);
+	if (!SYMBOL_REF_P (callee)
+	    || !SYMBOL_REF_LOCAL_P (callee)
+	    || arm_is_long_call_p (SYMBOL_REF_DECL (callee)))

IMO it would be better to calculate this once rather than repeat
it below.

+	  {
+	    emit_insn (gen_blockage ());

Why's the blockage needed?  Seems worth a comment.

+	    rtx pic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+	    emit_move_insn (pic_reg, get_hard_reg_initial_val (Pmode, FDPIC_REGNUM));
+	    emit_insn (gen_rtx_USE (VOIDmode, pic_reg));

Is this use keeping the register live for the call?  If so,
I think it'd be better to attach it to the CALL_INSN_FUNCTION_USAGE
instead.

+	 }
+      }
+
      /* In an untyped call, we can get NULL for operand 2.  */
      if (operands[2] == NULL_RTX)
        operands[2] = const0_rtx;
@@ -8140,6 +8157,13 @@
  	: !REG_P (callee))
        XEXP (operands[0], 0) = force_reg (Pmode, callee);
+ if (TARGET_FDPIC && !SYMBOL_REF_P (XEXP (operands[0], 0)))
+      {
+	/* Indirect call: set r9 with FDPIC value of callee.  */
+	XEXP (operands[0], 0)
+	  = arm_load_function_descriptor (XEXP (operands[0], 0));
+      }
+
      if (detect_cmse_nonsecure_call (addr))
        {
  	pat = gen_nonsecure_call_internal (operands[0], operands[1],

Redundant braces.

@@ -8151,10 +8175,38 @@
  	pat = gen_call_internal (operands[0], operands[1], operands[2]);
  	arm_emit_call_insn (pat, XEXP (operands[0], 0), false);
        }
+
+    /* Restore FDPIC register (r9) after call.  */
+    if (TARGET_FDPIC)
+      {
+	/* No need to update r9 if calling a static function.  */
+	if (!SYMBOL_REF_P (callee)
+	    || !SYMBOL_REF_LOCAL_P (callee)
+	    || arm_is_long_call_p (SYMBOL_REF_DECL (callee)))
+	  {
+	    rtx pic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+	    emit_move_insn (pic_reg, get_hard_reg_initial_val (Pmode, FDPIC_REGNUM));
+	    emit_insn (gen_rtx_USE (VOIDmode, pic_reg));
+	    emit_insn (gen_blockage ());
+	  }
+      }
      DONE;
    }"
  )

What's the general assumption about the validity of r9?  Seems odd that
we need to load this value both before and after the call.

+(define_insn "*restore_pic_register_after_call"
+  [(parallel [(unspec [(match_operand:SI 0 "s_register_operand" "=r,r")
+		       (match_operand:SI 1 "nonimmediate_operand" "r,m")]
+	       UNSPEC_PIC_RESTORE)
+	      (use (match_dup 1))
+	      (clobber (match_dup 0))])
+  ]
+  ""
+  "@
+  mov\t%0, %1
+  ldr\t%0, %1"
+)
+
  (define_expand "call_internal"
    [(parallel [(call (match_operand 0 "memory_operand" "")
  	            (match_operand 1 "general_operand" ""))

Since operand 0 is significant after the instruction, I think this
should be:

(define_insn "*restore_pic_register_after_call"
   [(set (match_operand:SI 0 "s_register_operand" "+r,r")
	(unspec:SI [(match_dup 0)
		    (match_operand:SI 1 "nonimmediate_operand" "r,m")]
		   UNSPEC_PIC_RESTORE))]
   ...

The (use (match_dup 1)) looks redundant, since the unspec itself
uses operand 1.


When I try that, I have cases where the restore instruction is discarded, when the call happens just before function return. Since r9 is caller-saved, it should be restored but after dse2 the dumps say:
(insn (set (reg:SI 9 r9)
   (unspec:SI [
       (reg:SI 9 r9)
       (reg:SI 4 r4 [121])
     ] UNSPEC_PIC_RESTORE))
(expr_list:REG_UNUSED (reg:SI 9 r9) (nil))))

and this is later removed by cprop_hardreg (which says the exit block uses r4, sp, and lr: should I make it use r9?)

Thanks,

Christophe


@@ -8215,6 +8267,30 @@
      rtx pat, callee;
      tree addr = MEM_EXPR (operands[1]);
+ /* Force FDPIC register (r9) before call. */
+    if (TARGET_FDPIC)
+      {
+	/* No need to update the FDPIC register (r9) if calling a static function.
+	   In other words: set r9 for indirect or non-local calls.  */
+	callee = XEXP (operands[1], 0);
+	if (!SYMBOL_REF_P (callee)
+	    || !SYMBOL_REF_LOCAL_P (callee)
+	    || arm_is_long_call_p (SYMBOL_REF_DECL (callee)))
+	  {
+	    rtx par = gen_rtx_PARALLEL (VOIDmode, rtvec_alloc (3));
+	    rtx fdpic_reg = gen_rtx_REG (Pmode, FDPIC_REGNUM);
+	    rtx initial_fdpic_reg =
+		get_hard_reg_initial_val (Pmode, FDPIC_REGNUM);
+
+	    XVECEXP (par, 0, 0) = gen_rtx_UNSPEC (VOIDmode,
+		gen_rtvec (2, fdpic_reg, initial_fdpic_reg),
+		UNSPEC_PIC_RESTORE);
+	    XVECEXP (par, 0, 1) = gen_rtx_USE (VOIDmode, initial_fdpic_reg);
+	    XVECEXP (par, 0, 2) = gen_rtx_CLOBBER (VOIDmode, fdpic_reg);
+	    emit_insn (par);
+	  }
+      }
+

It's not obvious why this code is different from the call-without-value
case above, which doesn't use UNSPEC_PIC_RESTORE.  I think it should be
split out into a helper function that's used for both call and call_value.

I think it would also be good to have more comments about what
conditions the UNSPEC_PIC_RESTORE pattern is enforcing.

Thanks,
Richard
.



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