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* [Bug tree-optimization/103857] New: implement ternary without jump (and comparison)
@ 2021-12-29 12:16 drepper.fsp+rhbz at gmail dot com
2021-12-29 13:03 ` [Bug tree-optimization/103857] " jakub at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: drepper.fsp+rhbz at gmail dot com @ 2021-12-29 12:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103857
Bug ID: 103857
Summary: implement ternary without jump (and comparison)
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
This test case is derived from actual code and I expect it to be not uncommon
in general. Maybe not exactly in this form but perhaps the matcher can catch a
few more cases.
Take this code:
extern void g(int);
void f1(int* a, int b, int c)
{
for (unsigned i = 0; i < 100; ++i)
g(a[i] == b ? c : b);
}
void f2(int* a)
{
for (unsigned i = 0; i < 100; ++i)
g(a[i] == 42 ? 10 : 42);
}
The function 'g' is called in each loop with one of two values which is the
opposite from the one that is match in the condition of the ternary operation.
This allows the respective loops to be rewritten as
for (unsigned i = 0; i < 100; ++i)
g(a[i] ^ c ^ b);
and
for (unsigned i = 0; i < 100; ++i)
g(a[i] ^ 10 ^ 42);
In the former case the c ^ b operation can be hoisted.
This should be faster and smaller in pretty much all situations and on all
platforms.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103857] implement ternary without jump (and comparison)
2021-12-29 12:16 [Bug tree-optimization/103857] New: implement ternary without jump (and comparison) drepper.fsp+rhbz at gmail dot com
@ 2021-12-29 13:03 ` jakub at gcc dot gnu.org
2021-12-29 13:24 ` drepper.fsp+rhbz at gmail dot com
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-12-29 13:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103857
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I don't think that's equivalent. For a[i] equal to b it will be and similarly
it will be for a[i] equal to c, but not for all the other values.
E.g. 13 == 42 ? 24 : 42 is 42, but 13 ^ (24 ^ 42) is 63.
So, we could do this only if something assures us that a[i] only has 2 possible
values, either equal to b or to c. So that would rule out non-constant b or c,
perhaps it could be done if value range is two adjacent constants equal to b
and c, or perhaps if the non-zero bits has a single bit and one of b or c is 0
and the other one is that non-zero bitmask of the other comparison operand.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103857] implement ternary without jump (and comparison)
2021-12-29 12:16 [Bug tree-optimization/103857] New: implement ternary without jump (and comparison) drepper.fsp+rhbz at gmail dot com
2021-12-29 13:03 ` [Bug tree-optimization/103857] " jakub at gcc dot gnu.org
@ 2021-12-29 13:24 ` drepper.fsp+rhbz at gmail dot com
2021-12-29 20:23 ` pinskia at gcc dot gnu.org
2021-12-29 20:26 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: drepper.fsp+rhbz at gmail dot com @ 2021-12-29 13:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103857
--- Comment #2 from Ulrich Drepper <drepper.fsp+rhbz at gmail dot com> ---
(In reply to Jakub Jelinek from comment #1)
> I don't think that's equivalent.
You're right, I tried to generalize the code and failed. I my actual case this
was a single variable the compiler saw the assignments of. Adding
if (a[i] != 42 && a[i] != 10)
__builtin_unreachable();
etc could simulate this.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103857] implement ternary without jump (and comparison)
2021-12-29 12:16 [Bug tree-optimization/103857] New: implement ternary without jump (and comparison) drepper.fsp+rhbz at gmail dot com
2021-12-29 13:03 ` [Bug tree-optimization/103857] " jakub at gcc dot gnu.org
2021-12-29 13:24 ` drepper.fsp+rhbz at gmail dot com
@ 2021-12-29 20:23 ` pinskia at gcc dot gnu.org
2021-12-29 20:26 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-29 20:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103857
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Keywords| |missed-optimization
Ever confirmed|0 |1
Severity|normal |enhancement
Last reconfirmed| |2021-12-29
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
int f(int a, int b, int c)
{
if (a != b && a != c) __builtin_unreachable();
return a == b ? b : c;
}
This should just translate into:
int f1(int a, int b, int c)
{
if (a != b && a != c) __builtin_unreachable();
return a;
}
The ^ part is not needed really if it is just selecting between two values.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103857] implement ternary without jump (and comparison)
2021-12-29 12:16 [Bug tree-optimization/103857] New: implement ternary without jump (and comparison) drepper.fsp+rhbz at gmail dot com
` (2 preceding siblings ...)
2021-12-29 20:23 ` pinskia at gcc dot gnu.org
@ 2021-12-29 20:26 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-29 20:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103857
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> Reduced testcase:
> int f(int a, int b, int c)
> {
> if (a != b && a != c) __builtin_unreachable();
> return a == b ? b : c;
> }
>
> This should just translate into:
> int f1(int a, int b, int c)
> {
> if (a != b && a != c) __builtin_unreachable();
> return a;
> }
>
> The ^ part is not needed really if it is just selecting between two values.
Oh it was swapping around the two values:
int f(int a, int b, int c)
{
if (a != b && a != c) __builtin_unreachable();
return a == b ? c : b;
}
int f1(int a, int b, int c)
{
if (a != b && a != c) __builtin_unreachable();
return a ^ b ^ c;
}
And then the ^ are needed.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-12-29 20:26 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-12-29 12:16 [Bug tree-optimization/103857] New: implement ternary without jump (and comparison) drepper.fsp+rhbz at gmail dot com
2021-12-29 13:03 ` [Bug tree-optimization/103857] " jakub at gcc dot gnu.org
2021-12-29 13:24 ` drepper.fsp+rhbz at gmail dot com
2021-12-29 20:23 ` pinskia at gcc dot gnu.org
2021-12-29 20:26 ` pinskia at gcc dot gnu.org
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