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* [Bug tree-optimization/37810] New: Bad store sinking job
@ 2008-10-12 15:14 carlo at gcc dot gnu dot org
2008-10-12 15:21 ` [Bug tree-optimization/37810] " rguenth at gcc dot gnu dot org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: carlo at gcc dot gnu dot org @ 2008-10-12 15:14 UTC (permalink / raw)
To: gcc-bugs
The following code snippet:
void g();
struct A {
int n;
int m;
A& operator++(void)
{
if (__builtin_expect(n == m, false))
g();
else
++n;
return *this;
}
A() : n(0), m(0) { }
friend bool operator!=(A const& a1, A const& a2) { return a1.n != a2.n; }
};
void testfunction(A& iter)
{
A const end;
while (iter != end)
++iter;
}
Results in the following assembly code, using maximum optimization:
movl (%rdi), %eax
jmp .L6
.L4:
cmpl %eax, 4(%rdi) // n == m ?
je .L8 // unlikely jump
addl $1, %eax // ++n
movl %eax, (%rdi) // *** store result to memory ***
.L6:
testl %eax, %eax // iter != end ?
jne .L4 // continue while loop
The storing (back) of %eax to (%rdi) remains inside the inner
loop no matter what I try. It could/should be moved outside
the loop, since nothing inside the L4 loop is accessing (%rdi)
or could possibly be accessing that memory.
This loop has two exits: below the last jne .L4, and the
jump to .L8. The store could be sinked to both exits.
This grows the code, but it seems reasonable to do for
a loop with a very small body, especially if one of the
exits is marked as unlikely :p.
--
Summary: Bad store sinking job
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carlo at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/37810] Bad store sinking job
2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
@ 2008-10-12 15:21 ` rguenth at gcc dot gnu dot org
2008-10-12 15:27 ` rguenth at gcc dot gnu dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2008-10-12 15:21 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from rguenth at gcc dot gnu dot org 2008-10-12 15:20 -------
store-sinking doesn't do its job because it thinks that
Memory reference 0: iter_1(D)->n
Memory reference 1: iter_1(D)->m
...
Querying dependencies of ref 0 in loop 1: dependent
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
Severity|normal |enhancement
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Keywords| |missed-optimization
Last reconfirmed|0000-00-00 00:00:00 |2008-10-12 15:20:19
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/37810] Bad store sinking job
2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
2008-10-12 15:21 ` [Bug tree-optimization/37810] " rguenth at gcc dot gnu dot org
@ 2008-10-12 15:27 ` rguenth at gcc dot gnu dot org
2008-10-12 15:30 ` rguenth at gcc dot gnu dot org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2008-10-12 15:27 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from rguenth at gcc dot gnu dot org 2008-10-12 15:25 -------
The original testcase (from an IRC discussion) reduced to a C testcase is:
struct A {
int n;
int m;
};
void g();
void test (struct A* iter)
{
struct A end = { 0, 0 };
while (iter->n != end.n)
{
iter->n = iter->n + 1;
if (iter->n == iter->m)
g();
}
}
where there is an optimization possibility to sink the store to iter->n to
before the call and apply load-store motion to iter->n for the remaining loop.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |alias
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/37810] Bad store sinking job
2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
2008-10-12 15:21 ` [Bug tree-optimization/37810] " rguenth at gcc dot gnu dot org
2008-10-12 15:27 ` rguenth at gcc dot gnu dot org
@ 2008-10-12 15:30 ` rguenth at gcc dot gnu dot org
2008-10-12 15:34 ` carlo at gcc dot gnu dot org
2009-04-03 12:34 ` rguenth at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2008-10-12 15:30 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from rguenth at gcc dot gnu dot org 2008-10-12 15:29 -------
It looks like the testcase in comment #2 should be fixed by SSUPRE? We have
*p = ...;
if ()
foo();
where foo() is an "implicit" store to *p. Still store sinking should be
applied
to the subloop.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |dberlin at gcc dot gnu dot
| |org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/37810] Bad store sinking job
2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
` (2 preceding siblings ...)
2008-10-12 15:30 ` rguenth at gcc dot gnu dot org
@ 2008-10-12 15:34 ` carlo at gcc dot gnu dot org
2009-04-03 12:34 ` rguenth at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: carlo at gcc dot gnu dot org @ 2008-10-12 15:34 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from carlo at gcc dot gnu dot org 2008-10-12 15:32 -------
Note that the original code was:
A& operator++(void)
{
++n;
if (__builtin_expect(n == m, false))
g();
return *this;
}
but g++ fails to optimize that by decrementing m outside
the loop (so I'm decrementing m myself now and use the
former code). The former code has as advantage, namely,
that the result of the addl $1,%eax can be used for the
conditional jump. However, gcc ALSO doesn't do that: in
the above assembly it follows the add with a redundant
testl %eax,%eax.
Anyway, using the operator++ given in this comment,
the assembly code is:
movl (%rdi), %eax
jmp .L3
.L4:
addl $1, %eax
cmpl 4(%rdi), %eax
movl %eax, (%rdi)
je .L8
.L3:
testl %eax, %eax
jne .L4
which is essentially the same, except now the
testl %eax,%eax is indeed "needed" ...
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/37810] Bad store sinking job
2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
` (3 preceding siblings ...)
2008-10-12 15:34 ` carlo at gcc dot gnu dot org
@ 2009-04-03 12:34 ` rguenth at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2009-04-03 12:34 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from rguenth at gcc dot gnu dot org 2009-04-03 12:34 -------
Re-confirmed.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed|2008-10-12 15:20:19 |2009-04-03 12:34:44
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2009-04-03 12:34 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2008-10-12 15:14 [Bug tree-optimization/37810] New: Bad store sinking job carlo at gcc dot gnu dot org
2008-10-12 15:21 ` [Bug tree-optimization/37810] " rguenth at gcc dot gnu dot org
2008-10-12 15:27 ` rguenth at gcc dot gnu dot org
2008-10-12 15:30 ` rguenth at gcc dot gnu dot org
2008-10-12 15:34 ` carlo at gcc dot gnu dot org
2009-04-03 12:34 ` rguenth at gcc dot gnu dot org
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