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* [Bug middle-end/107443] New: Switch conversion removing code
@ 2022-10-27 18:52 amacleod at redhat dot com
2022-10-27 23:34 ` [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: amacleod at redhat dot com @ 2022-10-27 18:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Bug ID: 107443
Summary: Switch conversion removing code
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: amacleod at redhat dot com
Target Milestone: ---
Compile the following with -O2 -fdump-tree-all -fdisable-tree-evrp
void dead (unsigned n);
void foo (unsigned n);
void func (unsigned n)
{
if (n == 0)
__builtin_unreachable();
if (n == 1)
__builtin_unreachable();
if (n == 2)
__builtin_unreachable();
if (n == 3)
__builtin_unreachable();
if (n == 4)
__builtin_unreachable();
if (n == 5)
__builtin_unreachable();
if (n == 6)
__builtin_unreachable();
if (n == 7)
__builtin_unreachable();
foo (n);
if (n < 8)
dead (n);
}
iftoswitch converts the series of if's into a switch:
<bb 2> :
switch (n_2(D)) <default: <L26> [INV], case 0: <L18> [INV], case 1: <L19>
[INV], case 2: <L20> [INV], case 3: <L21> [INV], case 4: <L22> [INV], case 5:
<L23> [INV], case 6: <L24> [INV], case 7: <L25> [INV]>
<bb 3> :
<L18>:
__builtin_unreachable ();
<bb 4> :
<L19>:
__builtin_unreachable ();
<bb 5> :
<L20>:
__builtin_unreachable ();
<bb 6> :
<L21>:
__builtin_unreachable ();
<bb 7> :
<L22>:
__builtin_unreachable ();
<bb 8> :
<L23>:
__builtin_unreachable ();
<bb 9> :
<L24>:
__builtin_unreachable ();
<bb 10> :
<L25>:
__builtin_unreachable ();
<bb 11> :
<L26>:
foo (n_2(D));
if (n_2(D) <= 7)
goto <bb 12>; [INV]
else
goto <bb 13>; [INV]
<bb 12> :
dead (n_2(D));
switch conversion then runs, eliminates all the blocks, then bails on the
conversion, leaving the IL without any of the __builtin_unreachable calls:
Beginning to process the following SWITCH statement ((null):0) : -------
switch (n_2(D)) <default: <L26> [INV], case 0: <L18> [INV], case 1: <L19>
[INV], case 2: <L20> [INV], case 3: <L21> [INV], case 4: <L22> [INV], case 5:
<L23> [INV], case 6: <L24> [INV], case 7: <L25> [INV]>
Removing basic block 3
Removing basic block 4
Removing basic block 5
Removing basic block 6
Removing basic block 7
Removing basic block 8
Removing basic block 9
Removing basic block 10
Bailing out - switch is a degenerate case
--------------------------------
Merging blocks 2 and 11
void func (unsigned int n)
{
<bb 2> :
foo (n_2(D));
if (n_2(D) <= 7)
goto <bb 3>; [INV]
else
goto <bb 4>; [INV]
<bb 3> :
dead (n_2(D));
<bb 4> :
return;
}
And now the optimizers cannot remove the call to dead() because we lose all the
range information inferred from the __builtin_unreachable calls.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations
2022-10-27 18:52 [Bug middle-end/107443] New: Switch conversion removing code amacleod at redhat dot com
@ 2022-10-27 23:34 ` pinskia at gcc dot gnu.org
2022-10-28 11:57 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-27 23:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |10.5
Last reconfirmed| |2022-10-27
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Known to fail| |8.1.0
Summary|Switch conversion removing |[10/11/12/13 Regression]
|code |Removing switch with
| |__builtin_unreachable
| |causes missed optimizations
Known to work| |7.1.0
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Testcase which shows this is a regression and switch conversion just exposed
it:
void dead (unsigned n);
void foo (unsigned n);
void func (unsigned n)
{
switch(n) {
case 0:
__builtin_unreachable();
case 1:
__builtin_unreachable();
case 2:
__builtin_unreachable();
case 3:
__builtin_unreachable();
case 4:
__builtin_unreachable();
case 5:
__builtin_unreachable();
case 6:
__builtin_unreachable();
case 7:
__builtin_unreachable();
default:
;
}
foo (n);
if (n < 8)
dead (n);
}
after building the CFG we get with the above testcase:
Removing basic block 10
Removing basic block 9
Removing basic block 8
Removing basic block 7
Removing basic block 6
Removing basic block 5
Removing basic block 4
Removing basic block 3
Merging blocks 2 and 11
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations
2022-10-27 18:52 [Bug middle-end/107443] New: Switch conversion removing code amacleod at redhat dot com
2022-10-27 23:34 ` [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations pinskia at gcc dot gnu.org
@ 2022-10-28 11:57 ` rguenth at gcc dot gnu.org
2023-03-24 8:42 ` rguenth at gcc dot gnu.org
2023-07-07 10:44 ` [Bug middle-end/107443] [11/12/13/14 " rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-10-28 11:57 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |marxin at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think we simply "optimize" the switch instead of, like for the if, preserving
the range asserting side-effect until some point in the compilation.
Ideally switch conversion would turn the switch side-effect into a single if.
But I also think the testcase is somewhat artificial so I wonder if we
should worry here at all ...
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations
2022-10-27 18:52 [Bug middle-end/107443] New: Switch conversion removing code amacleod at redhat dot com
2022-10-27 23:34 ` [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations pinskia at gcc dot gnu.org
2022-10-28 11:57 ` rguenth at gcc dot gnu.org
@ 2023-03-24 8:42 ` rguenth at gcc dot gnu.org
2023-07-07 10:44 ` [Bug middle-end/107443] [11/12/13/14 " rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-03-24 8:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P2
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/107443] [11/12/13/14 Regression] Removing switch with __builtin_unreachable causes missed optimizations
2022-10-27 18:52 [Bug middle-end/107443] New: Switch conversion removing code amacleod at redhat dot com
` (2 preceding siblings ...)
2023-03-24 8:42 ` rguenth at gcc dot gnu.org
@ 2023-07-07 10:44 ` rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-07-07 10:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|10.5 |11.5
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-07-07 10:44 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2022-10-27 18:52 [Bug middle-end/107443] New: Switch conversion removing code amacleod at redhat dot com
2022-10-27 23:34 ` [Bug middle-end/107443] [10/11/12/13 Regression] Removing switch with __builtin_unreachable causes missed optimizations pinskia at gcc dot gnu.org
2022-10-28 11:57 ` rguenth at gcc dot gnu.org
2023-03-24 8:42 ` rguenth at gcc dot gnu.org
2023-07-07 10:44 ` [Bug middle-end/107443] [11/12/13/14 " rguenth at gcc dot gnu.org
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