[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

--- Comment #9 from Segher Boessenkool  ---
With all three patches together (Peter's, mine, Jakub's), I get a code size
increase of only 0.047%, much more acceptable.  Now looking what that diff
really *is* :-)

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

--- Comment #8 from Segher Boessenkool  ---
This is the same problem as PR88233.

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

Segher Boessenkool  changed:

   What|Removed |Added

 CC||segher at gcc dot gnu.org

--- Comment #7 from Segher Boessenkool  ---
Combine starts with

insn_cost 4 for11: {r122:DI=r126:DI>>0x34;clobber ca:DI;}
  REG_DEAD r126:DI
  REG_UNUSED ca:DI
insn_cost 8 for12: r127:DI=[unspec[`*.LC1',%2:DI] 46]
  REG_EQUAL `j'
insn_cost 4 for13: [r127:DI]=r122:DI
  REG_DEAD r127:DI
insn_cost 4 for16: r128:CC=cmp(r122:DI,0)
  REG_DEAD r122:DI

and we want 11 and 16 combined (later passes put 16 earlier btw).

But combine does not try to combine anything into 16: the first use of r122
as set in insn 11 is insn 13, not insn 16, and 11+13 does not combine.

But 11+13+16 is not tried.  Maybe we should?

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[bje at gcc dot gnu dot org]

--- Comment #6 from bje at gcc dot gnu dot org  2009-05-14 03:04 ---
Still present in GCC (GNU) 4.5.0 20090513 (experimental) [trunk revision
147498].


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[pinskia at gcc dot gnu dot org]

--- Comment #5 from pinskia at gcc dot gnu dot org  2006-04-08 02:20 ---
I should note that there are some PPC cores where most recording are
microcoded.


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pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||pinskia at gcc dot gnu dot
   ||org
  GCC build triplet|powerpc64-linux |
   GCC host triplet|powerpc64-linux |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[bonzini at gnu dot org]

--- Comment #4 from bonzini at gnu dot org  2006-02-06 14:06 ---
More precisely, combine is attempting to do instruction selection, and in the
case of this bug it cannot.

A similar ARM test case is

  int f (int a, int b)
  {
int s = a - b;
int t = a  b ? a : s;
x = s;
y = t;
  }

which should be something like (pseudo-assembly)

  rsbs  s, b, a
  str   [x], s
  movlt s, a
  str   [y], t

it is possible to teach the combiner about this reduced testcase, but it won't
match in the full testcase from which this was extracted, because s is used
between its definition and the definition of t.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[pinskia at gcc dot gnu dot org]

--- Comment #3 from pinskia at gcc dot gnu dot org  2005-11-02 17:16 ---
All P1 enhancements not targeted towards 4.1, moving to P5.


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pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

   Priority|P1  |P5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[nathan at gcc dot gnu dot org]

--- Additional Comments From nathan at gcc dot gnu dot org  2004-11-09 
12:25 ---
The problem here is that combine only combines chains of instructions that have
no other uses of the intermediate results.  In this case, the intermediate
result is stored into a global variable.

One solution would be to add a peephole, but that would only trigger when (a)
the shift and compare were adjacent, and (b) the compare used CR0.  You'd also
need peepholers for all the instructions that can implicitly set CR0.

Really combine, which is a generic peepholer, needs some kind of templatizing on
the target machine.  It currently functions by generating promising patterns and
 then seeing if they exist.  This is wasteful, in that it generates many
patterns  that don't exist on the particular target, and it is blind in that it
doesn't spot any special cases the target might have (like this case).  What's
needed is something like pre-reg-alloc peepholes.  That of course would be a
large amount of work.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798

[Bug target/16798] PowerPC - Opportunity to use recording form instruction.

[pinskia at gcc dot gnu dot org]

-- 
   What|Removed |Added

   Keywords||missed-optimization
   Last reconfirmed|2004-08-01 00:54:58 |2004-10-31 04:58:03
   date||


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16798