Re: [PATCH] tree-object-size: Make unknown a computation

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 10/19/21 12:28, Jakub Jelinek wrote:
> On Tue, Oct 19, 2021 at 09:47:45AM +0530, Siddhesh Poyarekar wrote:
>> Compute the unknown size value as a function of the min/max bit of
>> object_size_type.  This transforms into a neat little branchless
>> sequence on x86_64:
>>
>> 	movl	%edi, %eax
>> 	sarl	%eax
>> 	xorl	$1, %eax
>> 	negl	%eax
>> 	cltq
>>
>> which should be faster than loading the value from memory.  A quick
>> unscientific test using
>>
>> `time make check-gcc RUNTESTFLAGS="dg.exp=builtin*"`
> 
> But if you use some other higher bit of object_size_type for the mode
> (normal vs. dynamic) you'd need to mask it away, so it will become longer.
> Anyway, I guess that part is ok.

Yes, my WIP patchset has:

((unsigned HOST_WIDE_INT) -(((object_size_type & MIN_BIT) >> 1) ^ 1));

which results in:

xorl	%eax, %eax
andl	$2, %edi
sete	%al
negq	%rax

>> -/* Compute object_sizes for PTR, defined to an unknown value.  */
>> -
>> -static void
>> -unknown_object_size (struct object_size_info *osi, tree ptr)
>> -{
>> -  int object_size_type = osi->object_size_type;
>> -  unsigned int varno = SSA_NAME_VERSION (ptr);
>> -  unsigned HOST_WIDE_INT bytes;
>> -
>> -  gcc_assert (object_sizes[object_size_type][varno]
>> -	      != unknown[object_size_type]);
>> -  gcc_assert (osi->pass == 0);
>> -
>> -  bytes = unknown[object_size_type];
>> -
>> -  if ((object_size_type & 2) == 0)
>> -    {
>> -      if (object_sizes[object_size_type][varno] < bytes)
>> -	object_sizes[object_size_type][varno] = bytes;
>> -    }
>> -  else
>> -    {
>> -      if (object_sizes[object_size_type][varno] > bytes)
>> -	object_sizes[object_size_type][varno] = bytes;
>> -    }
>> -}
> 
> But I don't think removing this function is desirable.
> Can it be greatly simplified?  Yes, certainly.
> The assert verifies it is not unknown before, and then for mode 0 or 1
> uses maximum which will always be unknown and for mode 2 or 3
> minimum which will always be unknown as well.
> But I'd keep the asserts in there.  So it can become
>    int object_size_type = osi->object_size_type;
>    unsigned int varno = SSA_NAME_VERSION (ptr);
>    unsigned HOST_WIDE_INT bytes = unknown (object_size_type);
> 
>    gcc_checking_assert (object_sizes[object_size_type][varno] != bytes);
>    gcc_checking_assert (osi->pass == 0);
>    object_sizes[object_size_type][varno] = bytes;

OK, I'll send an updated patch.

Thanks,
Siddhesh