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Re: -fstrict-aliasing and naughty code?


Andrew Cagney <cagney@mac.com> writes:

> Hello,
> 
> I'm trying to understand how to write ``bad'' (host dependant) code
> that doesn't get screwed by strict aliasing.
> 
> For instance, the code snipit:
> 
> >  unsigned i;
> >       unsigned64 tmp_reg, tmp_reg1;
> >       for (i = 0; i < 4; i++)
> > 	*( (i < 2 ? (unsigned32 *) &tmp_reg
> > 	          : (unsigned32 *) &tmp_reg1)
> > 	   + (1 - i % 2) ) = ...;
> >       cpu->registers[...] = tmp_reg;
> >
> 
> I'm told, is bad because:
> 
> > apparently, when -fstrict-aliasing is in effect, gcc is
> > allowed to assume that the expression inside the for loop
> > has no effect on the value of tmp_reg and tmp_reg1, since
> > the assignment is to an object of dissimilar type.
> 
> Provided I make (wild?) assumptions about the host and compiler, can I
> instead write the above to use something like:
> 
> 	union {
> 	  unsigned64 u64;
> 	  unsigned32 u32[2];
> 	} tmp_reg, tmp_reg1;
> 
>         for (i = 0; i < 4; i++)
> 	  if (i < 2)
> 	    tmp_reg.u32[1 - i % 2] = ...
> 	  else
> 	    tmp_reg1.u32[1 - i %2] = ...;
> 	cpu->registers[...] = tmp_reg.u64;

Yes, this is documented to work:

     The practice of reading from a different union member than the one
     most recently written to (called "type-punning") is common.  Even
     with `-fstrict-aliasing', type-punning is allowed, provided the
     memory is accessed through the union type.

However, it will be no more efficient than the more portable

  unsigned32 tmp_reg[2], tmp_reg1[2];

  for (i = 0; i < 4; i++)
  if (i < 2)
    tmp_reg[1 - i % 2] = ...
  else
    tmp_reg1[1 - i %2] = ...;
cpu->registers[...] = (unsigned64)tmp_reg[0] << 32 | tmp_reg[1];

in fact it will usually be less efficient because GCC will allocate
registers better for the second example.

-- 
- Geoffrey Keating <geoffk@geoffk.org> <geoffk@redhat.com>


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