On Tue, Jun 21, 2016 at 2:40 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
> On Mon, Jun 20, 2016 at 12:46 PM, Richard Sandiford
> <rdsandif...@googlemail.com> wrote:
>> Uros Bizjak <ubiz...@gmail.com> writes:
>>> On Mon, Jun 20, 2016 at 9:19 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>> On Mon, Jun 20, 2016 at 12:13 PM, Uros Bizjak <ubiz...@gmail.com> wrote:
>>>>> On Mon, Jun 20, 2016 at 7:05 PM, H.J. Lu <hongjiu...@intel.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> This patch implements the alternate code sequence recommended in
>>>>>>
>>>>>> https://groups.google.com/forum/#!topic/x86-64-abi/de5_KnLHxtI
>>>>>>
>>>>>> to load external function address via GOT slot with
>>>>>>
>>>>>> movq func@GOTPCREL(%rip), %rax
>>>>>>
>>>>>> so that linker won't create an PLT entry for extern function
>>>>>> address.
>>>>>>
>>>>>> Tested on x86-64. OK for trunk?
>>>>>
>>>>>> + else if (ix86_force_load_from_GOT_p (op1))
>>>>>> + {
>>>>>> + /* Load the external function address via the GOT slot to
>>>>>> + avoid PLT. */
>>>>>> + op1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, op1),
>>>>>> + (TARGET_64BIT
>>>>>> + ? UNSPEC_GOTPCREL
>>>>>> + : UNSPEC_GOT));
>>>>>> + op1 = gen_rtx_CONST (Pmode, op1);
>>>>>> + op1 = gen_const_mem (Pmode, op1);
>>>>>> + /* This symbol must be referenced via a load from the Global
>>>>>> + Offset Table. */
>>>>>> + set_mem_alias_set (op1, ix86_GOT_alias_set ());
>>>>>> + op1 = convert_to_mode (mode, op1, 1);
>>>>>> + op1 = force_reg (mode, op1);
>>>>>> + emit_insn (gen_rtx_SET (op0, op1));
>>>>>> + /* Generate a CLOBBER so that there will be no REG_EQUAL note
>>>>>> + on the last insn to prevent cse and fwprop from replacing
>>>>>> + a GOT load with a constant. */
>>>>>> + rtx tmp = gen_reg_rtx (Pmode);
>>>>>> + emit_clobber (tmp);
>>>>>> + return;
>>>>>
>>>>> Jeff, is this the recommended way to prevent CSE, as far as RTL
>>>>> infrastructure is concerned? I didn't find any example of this
>>>>> approach with other targets.
>>>>>
>>>>
>>>> FWIW, the similar approach is used in ix86_expand_vector_move_misalign,
>>>> ix86_expand_convert_uns_didf_sse and ix86_expand_vector_init_general
>>>> as well as other targets:
>>>>
>>>> frv/frv.c: emit_clobber (op0);
>>>> frv/frv.c: emit_clobber (op1);
>>>> im32c/m32c.c: /* emit_clobber (gen_rtx_REG (HImode, R0L_REGNO)); */
>>>> s390/s390.c: emit_clobber (addr);
>>>> s390/s390.md: emit_clobber (reg0);
>>>> s390/s390.md: emit_clobber (reg1);
>>>> s390/s390.md: emit_clobber (reg0);
>>>> s390/s390.md: emit_clobber (reg0);
>>>> s390/s390.md: emit_clobber (reg1);
>>>
>>> These usages mark the whole register as being "clobbered"
>>> (=undefined), before only a part of register is written, e.g.:
>>>
>>> emit_clobber (int_xmm);
>>> emit_move_insn (gen_lowpart (DImode, int_xmm), input);
>>>
>>> They aren't used to prevent unwanted CSE.
>>
>> Since it's being called in the move expander, I thought the normal
>> way of preventing the constant being rematerialised would be to reject
>> it in the move define_insn predicates.
>>
>> FWIW, I agree that using a clobber for this is going to be fragile.
>>
>
> Here is the patch without clobber. Tested on x86-64. OK for
> trunk?
No, your patch has multiple problems:
1. It won't work for e.g. &bar+1, since you have to legitimize the
symbol in two places of ix86_expand_move. Also, why use TARGET_64BIT
in
+ op1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, op1),
+ (TARGET_64BIT
+ ? UNSPEC_GOTPCREL
+ : UNSPEC_GOT));
when ix86_force_load_from_GOT_p rejects non-64bit targets?
2. New check should be added to ix86_legitimate_constant_p, not to
predicates of move insn patterns. Unfortunately, we still have to
change x86_64_immediate_operand in two places.
I have attached my version of the patch. It handles all your
testcases, plus &foo+1 case. Bootstrap is still running.
Does the patch work for you?
Uros.
Index: i386-protos.h
===================================================================
--- i386-protos.h (revision 237653)
+++ i386-protos.h (working copy)
@@ -70,6 +70,7 @@ extern bool ix86_expand_set_or_movmem (rtx, rtx, r
extern bool constant_address_p (rtx);
extern bool legitimate_pic_operand_p (rtx);
extern bool legitimate_pic_address_disp_p (rtx);
+extern bool ix86_force_load_from_GOT_p (rtx);
extern void print_reg (rtx, int, FILE*);
extern void ix86_print_operand (FILE *, rtx, int);
Index: i386.c
===================================================================
--- i386.c (revision 237653)
+++ i386.c (working copy)
@@ -15120,6 +15120,19 @@ darwin_local_data_pic (rtx disp)
&& XINT (disp, 1) == UNSPEC_MACHOPIC_OFFSET);
}
+/* True if operand X should be loaded from GOT. */
+
+bool
+ix86_force_load_from_GOT_p (rtx x)
+{
+ return (TARGET_64BIT && !TARGET_PECOFF && !TARGET_MACHO
+ && !flag_plt && !flag_pic
+ && ix86_cmodel != CM_LARGE
+ && GET_CODE (x) == SYMBOL_REF
+ && SYMBOL_REF_FUNCTION_P (x)
+ && !SYMBOL_REF_LOCAL_P (x));
+}
+
/* Determine if a given RTX is a valid constant. We already know this
satisfies CONSTANT_P. */
@@ -15188,6 +15201,12 @@ ix86_legitimate_constant_p (machine_mode mode, rtx
if (MACHO_DYNAMIC_NO_PIC_P)
return machopic_symbol_defined_p (x);
#endif
+
+ /* External function address should be loaded
+ via the GOT slot to avoid PLT. */
+ if (ix86_force_load_from_GOT_p (x))
+ return false;
+
break;
CASE_CONST_SCALAR_INT:
@@ -15596,6 +15615,9 @@ ix86_legitimate_address_p (machine_mode, rtx addr,
return false;
case UNSPEC_GOTPCREL:
+ if (ix86_force_load_from_GOT_p (XVECEXP (XEXP (disp, 0), 0, 0)))
+ goto is_legitimate_pic;
+ /* FALLTHRU */
case UNSPEC_PCREL:
gcc_assert (flag_pic);
goto is_legitimate_pic;
@@ -18164,6 +18186,12 @@ ix86_print_operand_address_as (FILE *file, rtx add
fputs ("ds:", file);
fprintf (file, HOST_WIDE_INT_PRINT_DEC, INTVAL (disp));
}
+ /* Load the external function address via the GOT slot to avoid PLT. */
+ else if (GET_CODE (disp) == CONST
+ && GET_CODE (XEXP (disp, 0)) == UNSPEC
+ && XINT (XEXP (disp, 0), 1) == UNSPEC_GOTPCREL
+ && ix86_force_load_from_GOT_p (XVECEXP (XEXP (disp, 0), 0, 0)))
+ output_pic_addr_const (file, disp, 0);
else if (flag_pic)
output_pic_addr_const (file, disp, 0);
else
@@ -19406,6 +19434,15 @@ ix86_expand_move (machine_mode mode, rtx operands[
return;
op1 = convert_to_mode (mode, op1, 1);
}
+ else if (ix86_force_load_from_GOT_p (op1))
+ {
+ /* Load the external function address via GOT slot to avoid PLT. */
+ op1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, op1),
+ UNSPEC_GOTPCREL);
+ op1 = gen_rtx_CONST (Pmode, op1);
+ op1 = gen_const_mem (Pmode, op1);
+ set_mem_alias_set (op1, ix86_GOT_alias_set ());
+ }
else if ((tmp = legitimize_pe_coff_symbol (op1, false)) != NULL_RTX)
op1 = tmp;
}
@@ -19420,6 +19457,15 @@ ix86_expand_move (machine_mode mode, rtx operands[
model = SYMBOL_REF_TLS_MODEL (symbol);
if (model)
tmp = legitimize_tls_address (symbol, model, true);
+ else if (ix86_force_load_from_GOT_p (symbol))
+ {
+ /* Load the external function address via GOT slot to avoid PLT. */
+ tmp = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, symbol),
+ UNSPEC_GOTPCREL);
+ tmp = gen_rtx_CONST (Pmode, tmp);
+ tmp = gen_const_mem (Pmode, tmp);
+ set_mem_alias_set (tmp, ix86_GOT_alias_set ());
+ }
else
tmp = legitimize_pe_coff_symbol (symbol, true);
Index: predicates.md
===================================================================
--- predicates.md (revision 237653)
+++ predicates.md (working copy)
@@ -160,13 +160,18 @@
return trunc_int_for_mode (val, SImode) == val;
}
case SYMBOL_REF:
+ /* TLS symbols are not constant. */
+ if (SYMBOL_REF_TLS_MODEL (op))
+ return false;
+
+ /* Load the external function address via the GOT slot. */
+ if (ix86_force_load_from_GOT_p (op))
+ return false;
+
/* For certain code models, the symbolic references are known to fit.
in CM_SMALL_PIC model we know it fits if it is local to the shared
library. Don't count TLS SYMBOL_REFs here, since they should fit
only if inside of UNSPEC handled below. */
- /* TLS symbols are not constant. */
- if (SYMBOL_REF_TLS_MODEL (op))
- return false;
return (ix86_cmodel == CM_SMALL || ix86_cmodel == CM_KERNEL
|| (ix86_cmodel == CM_MEDIUM && !SYMBOL_REF_FAR_ADDR_P (op)));
@@ -207,6 +212,11 @@
/* TLS symbols are not constant. */
if (SYMBOL_REF_TLS_MODEL (op1))
return false;
+
+ /* Load the external function address via the GOT slot. */
+ if (ix86_force_load_from_GOT_p (op))
+ return false;
+
/* For CM_SMALL assume that latest object is 16MB before
end of 31bits boundary. We may also accept pretty
large negative constants knowing that all objects are
@@ -273,10 +283,11 @@
return !(INTVAL (op) & ~(HOST_WIDE_INT) 0xffffffff);
case SYMBOL_REF:
- /* For certain code models, the symbolic references are known to fit. */
/* TLS symbols are not constant. */
if (SYMBOL_REF_TLS_MODEL (op))
return false;
+
+ /* For certain code models, the symbolic references are known to fit. */
return (ix86_cmodel == CM_SMALL
|| (ix86_cmodel == CM_MEDIUM
&& !SYMBOL_REF_FAR_ADDR_P (op)));
