Re: ping^^: [PATCH] rs6000: Enable const_anchor for 'addi'

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi David,

Thanks!

David Edelsohn <dje.gcc@gmail.com> writes:

> This Message Is From an External Sender 
> This message came from outside your organization. 
>  
> On Tue, May 30, 2023 at 11:00 PM Jiufu Guo <guojiufu@linux.ibm.com> wrote:
>
>  Gentle ping...
>
>  Jiufu Guo via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>
>  > Gentle ping...
>  >
>  > Jiufu Guo via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>  >
>  >> Hi,
>  >>
>  >> I'm thinking that we may enable this patch for stage1, so ping it.
>  >> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603530.html
>  >>
>  >> BR,
>  >> Jeff (Jiufu)
>  >>
>  >> Jiufu Guo <guojiufu@linux.ibm.com> writes:
>  >>
>  >>> Hi,
>  >>>
>  >>> There is a functionality as const_anchor in cse.cc.  This const_anchor
>  >>> supports to generate new constants through adding small gap/offsets to
>  >>> existing constant.  For example:
>  >>>
>  >>> void __attribute__ ((noinline)) foo (long long *a)
>  >>> {
>  >>>   *a++ = 0x2351847027482577LL;
>  >>>   *a++ = 0x2351847027482578LL;
>  >>> }
>  >>> The second constant (0x2351847027482578LL) can be compated by adding '1'
>  >>> to the first constant (0x2351847027482577LL).
>  >>> This is profitable if more than one instructions are need to build the
>  >>> second constant.
>  >>>
>  >>> * For rs6000, we can enable this functionality, as the instruction
>  >>> 'addi' is just for this when gap is smaller than 0x8000.
>  >>>
>  >>> * Besides enabling TARGET_CONST_ANCHOR on rs6000, this patch also fixed
>  >>> one issue. The issue is:
>  >>> "gcc_assert (SCALAR_INT_MODE_P (mode))" is an requirement for function
>  >>> "try_const_anchors". 
>  >>>
>  >>> * One potential side effect of this patch:
>  >>> Comparing with
>  >>> "r101=0x2351847027482577LL
>  >>> ...
>  >>> r201=0x2351847027482578LL"
>  >>> The new r201 will be "r201=r101+1", and then r101 will live longer,
>  >>> and would increase pressure when allocating registers.
>  >>> But I feel, this would be acceptable for this const_anchor feature.
>  >>>
>  >>> * With this patch, I checked the performance change on SPEC2017, while,
>  >>> and the performance is not aggressive, since this functionality is not
>  >>> hit on any hot path. There are runtime wavings/noise(e.g. on
>  >>> povray_r/xalancbmk_r/xz_r), that are not caused by the patch.
>  >>>
>  >>> With this patch, I also checked the changes in object files (from
>  >>> GCC bootstrap and SPEC), the significant changes are the improvement
>  >>> that: "addi" vs. "2 or more insns: lis+or.."; it also exposes some
>  >>> other optimizations opportunities: like combine/jump2. While the
>  >>> code to store/load one more register is also occurring in few cases,
>  >>> but it does not impact overall performance.
>  >>>
>  >>> * To refine this patch, some history discussions are referenced:
>  >>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33699
>  >>> https://gcc.gnu.org/pipermail/gcc-patches/2009-April/260421.html
>  >>> https://gcc.gnu.org/pipermail/gcc-patches/2021-March/566744.html
>  >>>
>  >>>
>  >>> Bootstrap and regtest pass on ppc64 and ppc64le for this patch.
>  >>> Is this ok for trunk?
>
> Hi, Jiufu
>
> Thanks for developing this patch and your persistence.
>
> The rs6000.cc part of the patch (TARGET_CONST_ANCHOR) is okay for
> Stage 1.  This is approved. 
>
> I don't have the authority to approve the change to cse_insn.  Is the
> cse_insn change a prerequisite?  Will the rs6000 change break or
> produce wrong code 
> without the cse change?  The second part of the patch should be posted
> separately to the mailing list, with a cc for appropriate maintainers,
> because most maintainers will not be following this specific thread
> to approve the other part of the patch.

I would extract the cse part as a seperate patch.
Yes, cse part is prerequest, the bug could be exposed by rs6000 part
change.

BR,
Jeff (Jiufu Guo)

>
> Thanks, David
>  
>  >>>
>  >>>
>  >>> BR,
>  >>> Jeff (Jiufu)
>  >>>
>  >>> gcc/ChangeLog:
>  >>>
>  >>>     * config/rs6000/rs6000.cc (TARGET_CONST_ANCHOR): New define.
>  >>>     * cse.cc (cse_insn): Add guard condition.
>  >>>
>  >>> gcc/testsuite/ChangeLog:
>  >>>
>  >>>     * gcc.target/powerpc/const_anchors.c: New test.
>  >>>     * gcc.target/powerpc/try_const_anchors_ice.c: New test.
>  >>>
>  >>> ---
>  >>>  gcc/config/rs6000/rs6000.cc                   |  4 ++++
>  >>>  gcc/cse.cc                                    |  3 ++-
>  >>>  .../gcc.target/powerpc/const_anchors.c        | 20 +++++++++++++++++++
>  >>>  .../powerpc/try_const_anchors_ice.c           | 16 +++++++++++++++
>  >>>  4 files changed, 42 insertions(+), 1 deletion(-)
>  >>>  create mode 100644 gcc/testsuite/gcc.target/powerpc/const_anchors.c
>  >>>  create mode 100644 gcc/testsuite/gcc.target/powerpc/try_const_anchors_ice.c
>  >>>
>  >>> diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
>  >>> index d2743f7bce6..80cded6dec1 100644
>  >>> --- a/gcc/config/rs6000/rs6000.cc
>  >>> +++ b/gcc/config/rs6000/rs6000.cc
>  >>> @@ -1760,6 +1760,10 @@ static const struct attribute_spec rs6000_attribute_table[] =
>  >>>  
>  >>>  #undef TARGET_UPDATE_IPA_FN_TARGET_INFO
>  >>>  #define TARGET_UPDATE_IPA_FN_TARGET_INFO rs6000_update_ipa_fn_target_info
>  >>> +
>  >>> +#undef TARGET_CONST_ANCHOR
>  >>> +#define TARGET_CONST_ANCHOR 0x8000
>  >>> +
>  >>>  
>  >>>  
>  >>>  /* Processor table.  */
>  >>> diff --git a/gcc/cse.cc b/gcc/cse.cc
>  >>> index b13afd4ba72..56542b91c1e 100644
>  >>> --- a/gcc/cse.cc
>  >>> +++ b/gcc/cse.cc
>  >>> @@ -5005,7 +5005,8 @@ cse_insn (rtx_insn *insn)
>  >>>        if (targetm.const_anchor
>  >>>       && !src_related
>  >>>       && src_const
>  >>> -     && GET_CODE (src_const) == CONST_INT)
>  >>> +     && GET_CODE (src_const) == CONST_INT
>  >>> +     && SCALAR_INT_MODE_P (mode))
>  >>>     {
>  >>>       src_related = try_const_anchors (src_const, mode);
>  >>>       src_related_is_const_anchor = src_related != NULL_RTX;
>  >>> diff --git a/gcc/testsuite/gcc.target/powerpc/const_anchors.c b/gcc/testsuite/gcc.target/powerpc/const_anchors.c
>  >>> new file mode 100644
>  >>> index 00000000000..39958ff9765
>  >>> --- /dev/null
>  >>> +++ b/gcc/testsuite/gcc.target/powerpc/const_anchors.c
>  >>> @@ -0,0 +1,20 @@
>  >>> +/* { dg-do compile { target has_arch_ppc64 } } */
>  >>> +/* { dg-options "-O2" } */
>  >>> +
>  >>> +#define C1 0x2351847027482577ULL
>  >>> +#define C2 0x2351847027482578ULL
>  >>> +
>  >>> +void __attribute__ ((noinline)) foo (long long *a)
>  >>> +{
>  >>> +  *a++ = C1;
>  >>> +  *a++ = C2;
>  >>> +}
>  >>> +
>  >>> +void __attribute__ ((noinline)) foo1 (long long *a, long long b)
>  >>> +{
>  >>> +  *a++ = C1;
>  >>> +  if (b)
>  >>> +    *a++ = C2;
>  >>> +}
>  >>> +
>  >>> +/* { dg-final { scan-assembler-times {\maddi\M} 2 } } */
>  >>> diff --git a/gcc/testsuite/gcc.target/powerpc/try_const_anchors_ice.c b/gcc/testsuite/gcc.target/powerpc/try_const_anchors_ice.c
>  >>> new file mode 100644
>  >>> index 00000000000..4c8a892e803
>  >>> --- /dev/null
>  >>> +++ b/gcc/testsuite/gcc.target/powerpc/try_const_anchors_ice.c
>  >>> @@ -0,0 +1,16 @@
>  >>> +/* { dg-do compile } */
>  >>> +/* { dg-options "-O2" } */
>  >>> +
>  >>> +/* __builtin_stack_restore could generates {[%1:DI]=0;} in BLK mode,
>  >>> +   it could case ICE in try_const_anchors which only supports SCALAR_INT.  */
>  >>> +
>  >>> +long
>  >>> +foo (const int val)
>  >>> +{
>  >>> +  if (val == (0))
>  >>> +    return 0;
>  >>> +  void *p = __builtin_stack_save ();
>  >>> +  char c = val;
>  >>> +  __builtin_stack_restore (p);
>  >>> +  return c;
>  >>> +}