--- Begin Message ---
- From: Geoff Keating <geoffk at geoffk dot org>
- To: Andrew Cagney <cagney at mac dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: 07 Mar 2002 15:48:28 -0800
- Subject: Re: -fstrict-aliasing and naughty code?
- References: <3C87F8DD.2010407@mac.com>
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>
--- End Message ---