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

[Martin Sebor]

On 03/14/2015 08:34 AM, Segher Boessenkool wrote:
> 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.

Thanks for the review and the clarification. FWIW, I mentioned
-pg because the reporter had noted that in prior versions of
GCC specifying -pg in addition to -mprofile-kernel wasn't
necessary to get the expected effect.

>
>
>> 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.

Sorry, I must have mangled the ChangeLog sopmehow while copying
it from one terminal to another. I fixed it in the new patch
(attached) along with the other issues you pointed out.

I tested the changes in powerpc64*-linux-* native builds and on
an x86_64 host in a build for the powerpc-unknown-linux-gnu and
powerpc64-apple-darwin targets. Of these, the -mprofile-kernel
option is only accepted for powerpc64*-linux-* (which was also
confirmed by inspecting the sources) so I adjusted the test
target accordingly and kept the body of
rs6000_keep_leaf_when_profiled you suggested.

Martin

>
>> +/* -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
>

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