Re: IVOPT improvement patch

["H.J. Lu" <hjl.tools@gmail.com>]

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On Fri, Jul 30, 2010 at 10:54 AM, Xinliang David Li <davidxl@google.com> wrote:
> Why is start offset not 1 to begin with? Let's assume it is correct,
> there are a couple of problems in this patch:
>
> 1) when the precision of the HOST_WIDE_INT is the same as the bitsize
> of the address_mode, max_offset = (HOST_WIDE_INT) 1 << width will
> produce a negative number
> 2) last_off should be initialized to 0 to match the original behavior
> 3) The i&& guard will make sure the loop terminates, but the offset
> compuation will be wrong -- i<<1 will first overflows to a negative
> number, then gets truncated to zero,  that means when this happens,
> the last_off will be negative when the loop terminates.
>
> David

I don't know exactly what get_address_cost is supposed to do. Here is
a new patch which avoids overflow and speeds up finding max/min offsets.


H.J.
---
>
> On Fri, Jul 30, 2010 at 10:27 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Fri, Jul 30, 2010 at 9:58 AM, Xinliang David Li <davidxl@google.com> wrote:
>>> There is a problem in this patch -- when i wraps to zero and terminate
>>> the loop, the maxoffset computed will be zero which is wrong.
>>>
>>> My previous patch won't have this problem.
>>
>> Your patch changed the start offset.  Here is the updated patch.
>>
>>
>> H.J.
>>>
>>> David
>>>
>>> On Fri, Jul 30, 2010 at 9:49 AM, Xinliang David Li <davidxl@google.com> wrote:
>>>> This looks fine to me -- Zdenek or other reviewers --- is this one ok?
>>>>
>>>> Thanks,
>>>>
>>>> David
>>>>
>>>> On Fri, Jul 30, 2010 at 8:45 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>> On Thu, Jul 29, 2010 at 6:04 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> It looks strange:
>>>>>>
>>>>>> +      width = (GET_MODE_BITSIZE (address_mode) <  HOST_BITS_PER_WIDE_INT - 1)
>>>>>> +          ? GET_MODE_BITSIZE (address_mode) : HOST_BITS_PER_WIDE_INT - 1;
>>>>>>       addr = gen_rtx_fmt_ee (PLUS, address_mode, reg1, NULL_RTX);
>>>>>> -      for (i = start; i <= 1 << 20; i <<= 1)
>>>>>> +      for (i = 1; i < width; i++)
>>>>>>        {
>>>>>> -         XEXP (addr, 1) = gen_int_mode (i, address_mode);
>>>>>> +          HOST_WIDE_INT offset = (1ll << i);
>>>>>> +         XEXP (addr, 1) = gen_int_mode (offset, address_mode);
>>>>>>          if (!memory_address_addr_space_p (mem_mode, addr, as))
>>>>>>            break;
>>>>>>        }
>>>>>>
>>>>>> HOST_WIDE_INT may be long or long long. "1ll" isn't always correct.
>>>>>> I think width can be >= 31. Depending on HOST_WIDE_INT,
>>>>>>
>>>>>> HOST_WIDE_INT offset = -(1ll << i);
>>>>>>
>>>>>> may have different values. The whole function looks odd to me.
>>>>>>
>>>>>>
>>>>>
>>>>> Here is a different approach to check address overflow.
>>>>>
>>>>>
>>>>> --
>>>>> H.J.
>>>>> --
>>>>> 2010-07-29  H.J. Lu  <hongjiu.lu@intel.com>
>>>>>
>>>>>        PR bootstrap/45119
>>>>>        * tree-ssa-loop-ivopts.c (get_address_cost): Re-apply revision
>>>>>        162652.  Check address overflow.
>>>>>
>>>>
>>>
>>
>>
>>
>> --
>> H.J.
>>
>



-- 
H.J.

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2010-07-29  H.J. Lu  <hongjiu.lu@intel.com>

	PR bootstrap/45119
	* tree-ssa-loop-ivopts.c (get_address_cost): Re-apply revision
	162652.  Avoid address overflow.

diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 1d65b4a..b5ab63e 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -3241,9 +3241,8 @@ get_address_cost (bool symbol_present, bool var_present,
   if (!data)
     {
       HOST_WIDE_INT i;
-      HOST_WIDE_INT start = BIGGEST_ALIGNMENT / BITS_PER_UNIT;
       HOST_WIDE_INT rat, off;
-      int old_cse_not_expected;
+      int old_cse_not_expected, width;
       unsigned sym_p, var_p, off_p, rat_p, add_c;
       rtx seq, addr, base;
       rtx reg0, reg1;
@@ -3252,33 +3251,37 @@ get_address_cost (bool symbol_present, bool var_present,
 
       reg1 = gen_raw_REG (address_mode, LAST_VIRTUAL_REGISTER + 1);
 
+      width = (GET_MODE_BITSIZE (address_mode) < HOST_BITS_PER_WIDE_INT - 1)
+          ? GET_MODE_BITSIZE (address_mode) : HOST_BITS_PER_WIDE_INT - 1;
       addr = gen_rtx_fmt_ee (PLUS, address_mode, reg1, NULL_RTX);
-      for (i = start; i <= 1 << 20; i <<= 1)
+
+      for (i = width; i; i--)
 	{
-	  XEXP (addr, 1) = gen_int_mode (i, address_mode);
-	  if (!memory_address_addr_space_p (mem_mode, addr, as))
+	  off = -((HOST_WIDE_INT) 1 << i);
+	  XEXP (addr, 1) = gen_int_mode (off, address_mode);
+	  if (memory_address_addr_space_p (mem_mode, addr, as))
 	    break;
 	}
-      data->max_offset = i == start ? 0 : i >> 1;
-      off = data->max_offset;
+      data->min_offset = off;
 
-      for (i = start; i <= 1 << 20; i <<= 1)
+      for (i = width; i; i--)
 	{
-	  XEXP (addr, 1) = gen_int_mode (-i, address_mode);
-	  if (!memory_address_addr_space_p (mem_mode, addr, as))
+	  off = ((HOST_WIDE_INT) 1 << i) - 1;
+	  XEXP (addr, 1) = gen_int_mode (off, address_mode);
+	  if (memory_address_addr_space_p (mem_mode, addr, as))
 	    break;
 	}
-      data->min_offset = i == start ? 0 : -(i >> 1);
+      data->max_offset = off;
 
       if (dump_file && (dump_flags & TDF_DETAILS))
 	{
 	  fprintf (dump_file, "get_address_cost:\n");
-	  fprintf (dump_file, "  min offset %s %d\n",
+	  fprintf (dump_file, "  min offset %s " HOST_WIDE_INT_PRINT_DEC "\n",
 		   GET_MODE_NAME (mem_mode),
-		   (int) data->min_offset);
-	  fprintf (dump_file, "  max offset %s %d\n",
+		   data->min_offset);
+	  fprintf (dump_file, "  max offset %s " HOST_WIDE_INT_PRINT_DEC "\n",
 		   GET_MODE_NAME (mem_mode),
-		   (int) data->max_offset);
+		   data->max_offset);
 	}
 
       rat = 1;