Re: Turn on -fomit-frame-pointer by default for 32bit x86?

[Jeff Law <law at redhat dot com>]

On 07/16/10 13:35, Andi Kleen wrote:
> "H.J. Lu"<hjl.tools@gmail.com>  writes:
>
>    
> > [hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3
> > [hjl@gnu-6 tmp]$ cat x.s
> > 	.file	"x.c"
> > 	.text
> > .globl foo
> > 	.type	foo, @function
> > foo:
> > 	pushl	%ebp
> > 	movl	%esp, %ebp
> > 	popl	%ebp
> > 	ret
> >
> > Can we turn on -fomit-frame-pointer by default for 32bit?
> >      
> I think it's a good idea, but in general one side effect one should
> be aware of is that it breaks many profilers with calltrace per sample
> support. Those tend to not be able to read unwind tables.
>    
Speaking of, I sent you a message on fentry about a month ago, but 
didn't get a reply:

I was ready to commit, except that we get a few new testsuite failures 
which I think touch on the issue of implicit setting of 
-fomit-frame-pointer issue.

 See:
/* Set the default values for switches whose default depends on 
TARGET_64BIT
     in case they weren't overwritten by command line options.  */
  if (TARGET_64BIT)
    {
      /* Mach-O doesn't support omitting the frame pointer for now.  */
      if (flag_omit_frame_pointer == 2)
        flag_omit_frame_pointer = (TARGET_MACHO ? 0 : 1);
      if (flag_asynchronous_unwind_tables == 2)
        flag_asynchronous_unwind_tables = 1;
      if (flag_pcc_struct_return == 2)
        flag_pcc_struct_return = 0;
    }
  else
    {
      if (flag_omit_frame_pointer == 2)
        flag_omit_frame_pointer = 0;
      if (flag_asynchronous_unwind_tables == 2)
        flag_asynchronous_unwind_tables = 0;
      if (flag_pcc_struct_return == 2)
        flag_pcc_struct_return = DEFAULT_PCC_STRUCT_RETURN;
    }



For x86_64, -fomit-frame-pointer is implicitly set; therefore with your 
patch someone compiling with -O2 -pg will get a diagnostic informing 
them those options are incompatible except with -mfentry is added.  I 
don't think you intended to get a diagnostic in this case.  It's 
certainly a behaviour change.

Jeff