RE: Understand BLKmode and returning structure in register.

["Bingfeng Mei" <bmei at broadcom dot com>]

Thanks for the reply. There should be more opportunties for strictly aligned
machines. In my example, the structure is a local variable allocated on stack.
I don't see why it is marked as BLKmode. Compiler has full freedom to make it
aligned and use DImode instead.

Bingfeng

> -----Original Message-----
> From: Richard Sandiford [mailto:rdsandiford@googlemail.com] 
> Sent: 16 March 2009 22:14
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org; Adrian Ashley
> Subject: Re: Understand BLKmode and returning structure in register.
> 
> "Bingfeng Mei" <bmei@broadcom.com> writes:
> > In foo function, compute_record_mode function will set the mode for
> > struct COMPLEX as BLKmode partly because STRICT_ALIGNMENT is 1 on my
> > target. In TARGET_RETURN_IN_MEMORY hook, I return 1 for BLKmode type
> > and 0 otherwise for small size (<8) (like MIPS).  Thus, 
> this structure
> > is still returned through memory, which is not very efficient. More
> > importantly, ABI is NOT FIXED under such situation. If an assembly
> > code programmer writes a function returning a structure. How does he
> > know the structure will be treated as BLKmode or otherwise? So he
> > doesn't know whether to pass result through memory or register. Do I
> > understand correctly?
> 
> Yes.  I think having TARGET_RETURN_IN_MEMORY depend on 
> internal details
> like the RTL mode is often seen as an historical mistake.  As you say,
> the ABI should be defined directly by the type instead.
> 
> Unfortunately, once you start using a mode, it's difficult to stop
> using a mode without breaking compatibility.  So one of the 
> main reasons
> the MIPS port still uses the mode is because no-one dares touch it.
> 
> Likewise, it's now difficult to change the mode attached to a 
> structure
> (which could potentially make structure accesses more 
> efficient) without
> accidentally breaking someone's ABI.
> 
> > On the other hand, if I return 0 only according to struct 
> type's size
> > regardless BLKmode or not, GCC will produces very inefficient
> > code. For example, stack setup code in foo is still 
> generated even it
> > is totally unnecessary.
> 
> Yeah, there's definitely room for improvement here.  And as you say,
> it's already a problem for MIPS.  I think it's just one of 
> those things
> that doesn't occur often enough in critical code for anyone to have
> spent time optimising it.
> 
> Richard
> 
>