Re: [PATCH] pr 63354 - gcc -pg -mprofile-kernel creates unused stack frames on leaf functions on ppc64le

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Fri, Mar 13, 2015 at 03:54:57PM -0600, Martin Sebor wrote:
> Attached is a patch that eliminates the unused stack frame
> allocated by gcc 5 with -pg -mprofile-kernel on powepc64le
> and brings the code into parity with previous gcc versions.
> 
> The patch doesn't do anything to change the emitted code
> when -mprofile-kernel is used without -pg. Since the former
> option isn't fully documented (as noted in pr 65372) it's
> unclear what effect it should be expected to have without
> -pg.

-mprofile-kernel does nothing without profiling enabled.  Maybe it
should just have been called -pk or something horrid like that.

The effect it should have is to do what the only user of the option
(the 64-bit PowerPC Linux kernel) wants.  The effect it does have
is to make the 64-bit ABI more like the 32-bit ABI for mcount.


> 2015-03-13  Anton Blanchard  <anton@samba.org>
> 
> 	PR target/63354
> 	* gcc/config/rs6000/linux64.h (ARGET_KEEP_LEAF_WHEN_PROFILED): Define.
                                       ^ typo

> 	* cc/config/rs6000/rs6000.c (rs6000_keep_leaf_when_profiled). New
          ^ typo                                                    ^ typo

It shouldn't have "gcc/" in the path names at all, actually.

> +/* -mprofile-kernel code calls mcount before the function prolog,

"prologue".

> +   so a profiled leaf function should stay a leaf function.  */
> +
> +static bool
> +rs6000_keep_leaf_when_profiled (void)
> +{
> +  return TARGET_PROFILE_KERNEL;
> +}

Something like

  switch (DEFAULT_ABI)
    {
      case ABI_AIX:
      case ABI_ELFv2:
	return TARGET_PROFILE_KERNEL;

      default:
	return true;
    }

although I'm not sure about Darwin here.  More conservative is to
return false for anything untested, of course.


> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/pr63354.c
> @@ -0,0 +1,10 @@
> +/* { dg-do compile { target { powerpc*-*-* } } } */
> +/* { dg-options "-O2 -pg -mprofile-kernel" } */
> +
> +int foo (void)
> +{
> +  return 1;
> +}
> +
> +/* { dg-final { scan-assembler "bl _mcount" } } */
> +/* { dg-final { scan-assembler-not "\(addi|stdu\) 1," } } */

Either you should run this only on AIX/ELFv2 ABIs, or you want to
test for "stwu" as well.  Bare "1" does not work for all assemblers
(only Darwin again?)


Segher