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Re: Help w/ CFG/loop optimization problem?
- From: Dale Johannesen <dalej at apple dot com>
- To: law at redhat dot com
- Cc: Dale Johannesen <dalej at apple dot com>, gcc at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Wed, 11 Sep 2002 13:15:46 -0700
- Subject: Re: Help w/ CFG/loop optimization problem?
On Wednesday, September 11, 2002, at 12:57 PM, Jeff Law wrote:
In message <A21CD16A-C4F8-11D6-93B1-003065C86F94@apple.com>, Dale
Johannesen wr
ites:
I've run into an optimization problem with
if (count > 10)
for (i = 0; i < count; ++i)
pcb += i;
else...
It'd really help to have a compilable example.
Here you go. On ppc/darwin, the second loop is recognized
as a doloop, the first is not. Sorry to have oversimplified.
(I'm looking at this myself along the lines suggested by
Jan Hubicka, BTW.)
void foo (int count, char* pca, char* pcb) {
int i;
if (count > 10)
for (i = 0; i < count; ++i)
pcb += i;
else
for (i = 0; i < count; ++i)
pca += i;
*pca = *pcb;
}
RTL before cfg:
(note 27 25 28 NOTE_INSN_LOOP_CONT)
(code_label 28 27 74 3 5 "" [0 uses])
(note 74 28 29 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(insn 29 74 17 3 0x0 (set (reg/v:SI 121)
(plus:SI (reg/v:SI 121)
(const_int 1 [0x1]))) -1 (nil)
(nil))
(code_label 17 29 75 4 3 "" [1 uses])
(note 75 17 18 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 18 75 19 4 0x0 (set (reg:CC 123)
(compare:CC (reg/v:SI 121)
(reg/v:SI 118))) -1 (nil)
(nil))
(jump_insn 19 18 76 4 0x0 (set (pc)
(if_then_else (lt (reg:CC 123)
(const_int 0 [0x0]))
(label_ref 22)
(pc))) -1 (nil)
(nil))
(note 76 19 20 5 [bb 5] NOTE_INSN_BASIC_BLOCK)
(jump_insn 20 76 21 5 0x0 (set (pc) ... this branch exits the loop
(label_ref 36)) -1 (nil)
(nil))
(barrier 21 20 22)
(code_label 22 21 77 6 6 "" [1 uses])
(note 77 22 33 6 [bb 6] NOTE_INSN_BASIC_BLOCK)
(jump_insn 33 77 34 6 0x0 (set (pc)
(label_ref 30)) -1 (nil)
(nil))
(barrier 34 33 35)
(note 35 34 36 NOTE_INSN_LOOP_END)
(code_label 36 35 78 7 4 "" [1 uses])
(note 78 36 37 7 [bb 7] NOTE_INSN_BASIC_BLOCK)
(jump_insn 37 78 38 7 0x0 (set (pc) ... and is changed to point to
63 instead of 36
(label_ref 63)) -1 (nil)
(nil))
If insn 20 were not changed, it would branch to the next instruction
and would be removed, leaving the loop in a clean form.
?!? It wouldn't branch to the next instruction because of the jump at
insn #33. Unless that's optimized away.
From looking at the post-cfg rtl, we thread the jump at insn 19, that
in turn makes the jump at 33 dead. So indeed we could have just
zapped insn 20 had it not been mucked up.
Exactly, and that's what happens in the 2nd loop in the example.