On Sun, Feb 2, 2025 at 4:20 PM Richard Biener
<richard.guent...@gmail.com> wrote:
>
>
>
> > Am 02.02.2025 um 08:59 schrieb H.J. Lu <hjl.to...@gmail.com>:
> >
> > On Sun, Feb 2, 2025 at 3:33 PM Richard Biener
> > <richard.guent...@gmail.com> wrote:
> >>
> >>
> >>
> >>>> Am 02.02.2025 um 08:00 schrieb H.J. Lu <hjl.to...@gmail.com>:
> >>>
> >>> Don't increase callee-saved register cost by 1000x, which leads to that
> >>> callee-saved registers aren't used to preserve local variable values
> >>> across calls, by capping the scale to 300.
> >>
> >>> PR rtl-optimization/111673
> >>> PR rtl-optimization/115932
> >>> PR rtl-optimization/116028
> >>> PR rtl-optimization/117081
> >>> PR rtl-optimization/118497
> >>> * ira-color.cc (assign_hard_reg): Cap callee-saved register cost
> >>> scale to 300.
> >>>
> >>> Signed-off-by: H.J. Lu <hjl.to...@gmail.com>
> >>> ---
> >>> gcc/ira-color.cc | 16 ++++++++++++++--
> >>> 1 file changed, 14 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/gcc/ira-color.cc b/gcc/ira-color.cc
> >>> index 0699b349a1a..707ff188250 100644
> >>> --- a/gcc/ira-color.cc
> >>> +++ b/gcc/ira-color.cc
> >>> @@ -2175,13 +2175,25 @@ assign_hard_reg (ira_allocno_t a, bool retry_p)
> >>> /* We need to save/restore the hard register in
> >>> epilogue/prologue. Therefore we increase the cost. */
> >>> {
> >>> + int scale;
> >>> + if (optimize_size)
> >>> + scale = 1;
> >>> + else
> >>> + {
> >>> + scale = REG_FREQ_FROM_BB (ENTRY_BLOCK_PTR_FOR_FN (cfun));
> >>> + /* Don't increase callee-saved register cost by 1000x,
> >>> + which leads to that callee-saved registers aren't
> >>> + used to preserve local variable values across calls,
> >>> + by capping the scale to 300. */
> >>> + if (REG_FREQ_MAX == 1000 && scale == REG_FREQ_MAX)
> >>> + scale = 300;
> >>
> >> That leads to 300 for 1000 but 999 for 999 which is odd. I’d have
> >> expected to scale this down to [0, 300] or is MAX a magic value?
> >
> > There are
> >
> > * The weights for each insn varies from 0 to REG_FREQ_BASE.
> > This constant does not need to be high, as in infrequently executed
> > regions we want to count instructions equivalently to optimize for
> > size instead of speed. */
> > #define REG_FREQ_MAX 1000
> >
> > /* Compute register frequency from the BB frequency. When optimizing for
> > size,
> > or profile driven feedback is available and the function is never
> > executed,
> > frequency is always equivalent. Otherwise rescale the basic block
> > frequency. */
> > #define REG_FREQ_FROM_BB(bb) ((optimize_function_for_size_p (cfun)
> > \
> > || !cfun->cfg->count_max.initialized_p ())
> > \
> > ? REG_FREQ_MAX
> > \
> > : ((bb)->count.to_frequency (cfun)
> > \
> > * REG_FREQ_MAX / BB_FREQ_MAX)
> > \
> > ? ((bb)->count.to_frequency (cfun)
> > \
> > * REG_FREQ_MAX / BB_FREQ_MAX)
> > \
> > : 1)
> >
> > 1000 is the default. If it isn't 1000, it isn't the default. I only want
> > to get a more reasonable default scale, instead of 1000. Lower
> > scale will fail the PR rtl-optimization/111673 test on powerpc64.
>
> I see. Why not adjust the above macro then? That would be a bit more
> obvious. Like use MAX/2 or so?
commit 3b9b8d6cfdf59337f4b7ce10ce92a98044b2657b
Author: Surya Kumari Jangala <jskum...@linux.ibm.com>
Date: Tue Jun 25 08:37:49 2024 -0500
ira: Scale save/restore costs of callee save registers with block frequency
uses REG_FREQ_FROM_BB as the cost scale. I don't know if it is a misuse.
I don't want to change REG_FREQ_FROM_BB since it is used in other places,
not as a cost scale. Maybe the above commit should be reverted and we add
a target hook for callee-saved register cost scale. Each target can choose
a proper cost scale, install of increasing the cost by 1000x for everyone.
> >
> >
> >>> + }
> >>> rclass = REGNO_REG_CLASS (hard_regno);
> >>> add_cost = ((ira_memory_move_cost[mode][rclass][0]
> >>> + ira_memory_move_cost[mode][rclass][1])
> >>> * saved_nregs / hard_regno_nregs (hard_regno,
> >>> mode) - 1)
> >>> - * (optimize_size ? 1 :
> >>> - REG_FREQ_FROM_BB (ENTRY_BLOCK_PTR_FOR_FN (cfun)));
> >>> + * scale;
> >>> cost += add_cost;
> >>> full_cost += add_cost;
> >>> }
> >>> --
> >>> 2.48.1
> >>>
> >
> >
> >
> > --
> > H.J.
--
H.J.
