Re: split_edge, reg-stack and function.c fix

[Jeffrey A Law <law at cygnus dot com>]

  In message <19990705082403.22270@atrey.karlin.mff.cuni.cz>you write:
  > 
  > Hi
  > While looking at the asm outputs, I've found following code:
  > 	jne .L11
  >         jmp .L9
  > .L15:
  > 	fxch %st(2)
  >         .p2align 4,,7
  > .L9:
  >         <loop body>
  >         cmpb $1,%ah
  >         je .L15
  > .L16:
  > .L17:
Unfortunately, you did not provide any testcode, only the results of compiling
testcodes.  So we can't track the problem you were trying to solve with this
patch.


  > (BTW also note the redundant label L17. Why this happends?
It's normal and nothing to get particularly concerned about.  Most commonly
it happens when there is a NOTE of some kind between two CODE_LABELs.


  >  There are
  > few notes between in the .stack dump, but jump IMO ought to remove it or
  > not?),
It depends on a number of factors.  We actually want to keep those separate
labels until fairly late in the compilation if there's a LOOP note between
them.  

Similarly I think we need to keep them if there's a block scope note between
them.  Other notes I'm less sure about.


  > The problem I am shooting for is that the alignment pseudo is *inside* the
  > loop, resulting code is much slower, because it has to eat all those nops
  > every iteration.
  > 
  > I it is caused by reg-stack by by coexistence of two problems:
  > 1) The LOOP_BEG function is placed incorrectly before the basic block for
  >    the jmp L9 instruction.
I believe this was fixed by a change I made to split_edge a while back.


  > 2) Shorten branches is done before reg-stack so alginment of most branches
  >    touched by reg-stack is completely wrong.
This sounds like something we should fix.  The only thing that should happen
after branch shortening is peepholes and assembly output.

[ ... ]

  > I've also attached patch to i386.c in new_ia32_branch to make length
  > function happy about this code.
I fixed this a while back too.

  > 
  > Honza
  > 
  > Sun Jul  4 06:01:01 CEST 1999  Jan Hubicka  <hubicka@freesoft.cz>
  > 	* flow.c (loops_starting_here): New function.
  > 	(loops_ending_here): New function.
  > 	(end_of_loop): New function.
  > 	(move_loop_beg_notes): New function.
  > 	(split_edge): 
This code is no longer needed.

  > 	* toplev.c (rest_of_compilation): Do shorted_branches after regstack.
I've installed this change.

  > 	* function.c (thread_prologue_and_epilogue_insns): Mess up flow info
  > 	in more split_edge friendly way, call commit_edge_insertions before
  > 	this mess is done.
  > 	* i386.c (ix86_attr_length_default): Handle FXCH nicely.
These should no longer be desirable or necessary either.

jeff