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

[Jakub Jelinek <jakub@redhat.com>]

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.

> -/* 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;

	Jakub