[Bug target/47764] The constant load instruction should be hoisted out of loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47764
Ramana Radhakrishnan ramana at gcc dot gnu.org changed:
What|Removed |Added
CC||ramana at gcc dot gnu.org
--- Comment #4 from Ramana Radhakrishnan ramana at gcc dot gnu.org 2011-08-10
22:17:23 UTC ---
(In reply to comment #3)
Any ideas of how this improvement could be implemented, Carrot?
The root cause of this problem is that arm/thumb store instruction can't
directly store a immediate number to memory, but gcc doesn't realize this
early
enough. In most part of the rtl phase, the following form is kept.
(insn 41 38 42 3 (set (mem:HI (plus:SI (reg/f:SI 169)
(const_int 60 [0x3c])) [2 MEM[(struct deflate_state
*)D.2085
_3 + 60B]+0 S2 A16])
(const_int 0 [0])) src/trees.c:45 696 {*thumb2_movhi_insn}
(expr_list:REG_DEAD (reg/f:SI 169)
(nil)))
Until register allocation it finds the restriction of the store instruction
and
split it into two instructions, load 0 into register and store register to
memory. But it's too late to do a loop optimization.
Eh, how is splitting this early going to help with hoisting this out of a loop
?
Ramana
[Bug target/47764] The constant load instruction should be hoisted out of loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47764
--- Comment #3 from Carrot carrot at google dot com 2011-02-21 03:15:45 UTC
---
Any ideas of how this improvement could be implemented, Carrot?
The root cause of this problem is that arm/thumb store instruction can't
directly store a immediate number to memory, but gcc doesn't realize this early
enough. In most part of the rtl phase, the following form is kept.
(insn 41 38 42 3 (set (mem:HI (plus:SI (reg/f:SI 169)
(const_int 60 [0x3c])) [2 MEM[(struct deflate_state *)D.2085
_3 + 60B]+0 S2 A16])
(const_int 0 [0])) src/trees.c:45 696 {*thumb2_movhi_insn}
(expr_list:REG_DEAD (reg/f:SI 169)
(nil)))
Until register allocation it finds the restriction of the store instruction and
split it into two instructions, load 0 into register and store register to
memory. But it's too late to do a loop optimization.
One possible method is to split this insn earlier than loop optimization (maybe
directly in expand pass), and let loop and cse optimizations do the rest. It
may increase register pressure in part of the program, we should rematerialize
it in such cases.
[Bug target/47764] The constant load instruction should be hoisted out of loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47764 --- Comment #2 from Andrew Pinski pinskia at gcc dot gnu.org 2011-02-19 03:23:51 UTC --- This is most likely a cost issue.
[Bug target/47764] The constant load instruction should be hoisted out of loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47764 Ian Bolton ibolton at gcc dot gnu.org changed: What|Removed |Added Keywords||missed-optimization Status|UNCONFIRMED |NEW Last reconfirmed||2011.02.18 11:36:27 CC||ibolton at gcc dot gnu.org Ever Confirmed|0 |1 Known to fail||4.6.0 --- Comment #1 from Ian Bolton ibolton at gcc dot gnu.org 2011-02-18 11:36:27 UTC --- I have confirmed this for r170052 of trunk. Any ideas of how this improvement could be implemented, Carrot?
