mips-elf-gcc -fno-delayed-branch problem

[Toshi Morita]

Maybe GAS could recognize -fno-delayed-branch to selectively disable branch slot filling?

Is there a list of optimizations performed by MIPS GAS listed somewhere?
It would be nice if these could be selectively enabled.

Toshi

--- On Thu, 5/19/11, Richard Sandiford <rdsandiford@googlemail.com> wrote:

> From: Richard Sandiford <rdsandiford@googlemail.com>
> Subject: Re: mips-elf-gcc -fno-delayed-branch problem
> To: "Toshi Morita" <tm314159@yahoo.com>
> Cc: gcc@gcc.gnu.org
> Date: Thursday, May 19, 2011, 1:03 AM
> Toshi Morita <tm314159@yahoo.com>
> writes:
> > There's a problem where compiling code with
> -fno-delayed-branch still
> > fills branch delay slots.
> > [...]
> > The problem appears to be caused with GNU AS. It now
> has the
> > capability to reorder instructions, and there is no
> ".set noreorder"
> > emitted by the compiler, so the assembler fills the
> branch delay slot.
> 
> FWIW, I think this was a deliberate historical
> choice.  But...
> 
> > So when the -fno-delayed-branch switch is passed to
> mips-elf-gcc, a
> > ".set noreorder" directive should be emitted into the
> assembly file.
> 
> The problem is that ".set noreorder" has other side-effects
> too,
> such as stopping the assembler from inserting nops to avoid
> data
> hazards, or to work around errata.  There are various
> situations
> in which we can't rely on GCC to insert these nops, and in
> which
> GCC must therefore treat the function as ".set reorder".
> 
> One of those situations is when the function has inline
> asm, as in
> your example.  GCC doesn't know what the inline asm
> does, so it also
> doesn't know what nops would be needed between the asm and
> the
> GCC-generated code, or within the asm code itself. 
> Another situation
> is when compiling with -mno-explicit-relocs or with various
> -mfix-*
> options.  While those are somewhat niche options, it'd
> be inconsistent
> to "guarantee" that -fno-delayed-branch emits everything as
> ".set noreorder"
> except when those options are used.
> 
> At the moment, the way to force no delayed branches is to
> use:
> 
>     -fno-delayed-branch -Wa,-O0
> 
> Maybe it would be worth breaking with tradition and making
> -fno-delayed-branch imply -Wa,-O0 though.  Back in the
> day,
> the assembler's version of delayed-branch filling was
> applied
> to pretty much every function, so the separation was
> probably
> more obvious.  However, CCC now tries to emit most
> functions as
> ".set noreorder"/ ".set nomacro", so -fno-delayed-branch
> does
> actually stop the assembler filling branches in most
> situations.
> I can see that it'd be confusing to suddenly have the
> assembler
> kick in when you add something like an inline asm.
> 
> So yeah, I'll try to get around to that this weekend...
> 
> Richard
>