Re: [rfa?] Add frame_align(); Was: ARM stack alignment on hand called functions

[Andreas Schwab <schwab@suse.de>]

Richard Earnshaw <rearnsha@arm.com> writes:

|> > Richard Earnshaw <rearnsha@arm.com> writes:
|> > 
|> > |> > +/* Ensure that the ARM's stack pointer has the correct alignment for a
|> > |> > +   new frame.  */
|> > |> > +static CORE_ADDR
|> > |> > +arm_frame_align (struct gdbarch *gdbarch, CORE_ADDR addr)
|> > |> > +{
|> > |> > +  return (addr & -16);
|> > |> > +}
|> > |> 
|> > |> Yuck, two's complement assumption.
|> > 
|> > No, -16 is implicitly cast to bfd_vma, which is unsigned, and this
|> > operation is completely defined independent of the representation of
|> > signed integers.
|> 
|> Hmm, strictly speaking you are correct.  I don't have to like it though, 
|> and it means that the code is heavily dependent on the non-obvious fact 
|> that addr is an unsigned type to get the correct behaviour (if bfd_vma 
|> were to be changed to a signed type then this code would quietly break).
|> 
|> So I'll change my comment to:
|> 
|> Yuck, implicit cast of negative number to unsigned.

Note that ~0xf is even worse.  If sizeof(int) < sizeof(bfd_vma) then you
get the wrong value, because ~0xf is _zero_ extended to bfd_vma.

Andreas.

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Andreas Schwab, SuSE Labs, schwab@suse.de
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