[PATCH, MIPS] Frame header optimization for MIPS O32 ABI

[Moore, Catherine]

> -----Original Message-----
> From: Steve Ellcey [mailto:sellcey@imgtec.com]
> Sent: Friday, September 11, 2015 2:06 PM
> To: Matthew Fortune
> Cc: GCC Patches; Moore, Catherine
> Subject: RE: [PATCH, MIPS] Frame header optimization for MIPS O32 ABI
> 
> On Fri, 2015-09-04 at 01:40 -0700, Matthew Fortune wrote:
> 
> > A few comments below. I found some of the comments a bit hard to parse
> but have
> > not attempted any rewording. I'd like Catherine to comment too as I have
> barely
> > any experience at the gimple level to know if this accounts for any
> necessary
> > subtleties.
> 
> Catherine said she would look at this next week but I have updated the
> patch in the mean time to address your comments and give Catherine a
> more up-to-date patch to look over.
> 

Hi Steve, I'm sorry for the delay in reviewing this patch. 
Some changes have been committed upstream (see revision #227941) that will require updates to this patch.
Please post the update for review.  Other comments are embedded.

> 
> diff --git a/gcc/config.gcc b/gcc/config.gcc
> index 5712547..eea97de 100644
> --- a/gcc/config.gcc
> +++ b/gcc/config.gcc
> @@ -420,6 +420,7 @@ microblaze*-*-*)
>  mips*-*-*)
>  	cpu_type=mips
>  	extra_headers="loongson.h"
> +	extra_objs="frame-header-opt.o"
>  	extra_options="${extra_options} g.opt fused-madd.opt mips/mips-
> tables.opt"
>  	;;
>  nds32*)
> diff --git a/gcc/config/mips/frame-header-opt.c b/gcc/config/mips/frame-
> header-opt.c
> index e69de29..5c4e93c 100644
> --- a/gcc/config/mips/frame-header-opt.c
> +++ b/gcc/config/mips/frame-header-opt.c
> @@ -0,0 +1,221 @@
> +/* Analyze functions to determine if calling functions need to allocate
> +   stack space (a frame header) for its called functions to write out their
> +   arguments on to the stack.  This optimization is only applicable to
> +   TARGET_OLDABI targets because calling functions on TARGET_NEWABI
> targets
> +   do not allocate any stack space for arguments (the called function does it
> +   if needed).
> +

Overall, I agree with Matthew regarding the comments being a little hard to parse.
How about:

/* Analyze functions to determine if callers need to allocate a frame header on the stack.  The frame header is used by callees to save its arguments.
   This optimization is specific to TARGET_OLDABI targets.  For TARGET_NEWABI targets, if a frame header is required, it is allocated by the callee.  */


> +
> +/* Look at all the functions this function calls and return true if none of
> +   them need the argument stack space that this function would normally
> +   allocate.  Return false if one or more functions does need this space
> +   or if we cannot determine that all called functions do not need the
> +   space.  */

/* Returns TRUE if the argument stack space allocated by function FN is used.
     Returns FALSE if the space is needed or if the need for the space cannot be determined.  */
> +
> +static bool
> +}
> +
> +/* This optimization scans all the functions in the compilation unit to find
> +   out which ones do not need the frame header that their caller normally
> +   allocates.  Then it does a second scan of all the functions to determine
> +   which functions can skip the allocation because none of the functions it
> +   calls need the frame header.  */
> +

  /* Scan each function to determine those that need its frame headers.  Perform a second
       scan to determine if the allocation can be skipped because none of its callees require the frame header.  */
> +}
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index c0ec0fd..8152645 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -17940,6 +17941,18 @@ if @var{ra-address} is nonnull.
> 
>  The default is @option{-mno-mcount-ra-address}.
> 
> +@item -mframe-header-opt
> +@itemx -mno-frame-header-opt
> +@opindex mframe-header-opt
> +Enable (disable) frame header optimization in the o32 ABI.  When using
> +the o32 ABI, calling functions allocate 16 bytes on the stack in case
> +the called function needs to write out register arguments to memory so
> +that their address can be taken.  When enabled, this optimization will
> +cause the calling function to not allocate that space if it can determine
> +that none of its called functions use it.
> +
> +This optimization is off by default at all optimization levels.
> +
>  @end table
> 
>  @node MMIX Options

How about this instead:
Enable (disable) frame header optimization in the o32 ABI.  When using the o32
ABI, calling functions will allocate 16 bytes on the stack for the called function
to write out register arguments.  When enabled, this optimization will suppress the
allocation of the frame header if it can be determined that it is unused.

This optimization is off by default at all optimization levels.

Catherine