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* [Bug c++/110492] New: Attempted optimization of switch statement pessimizes it instead
@ 2023-06-30 2:52 qufanat at gmail dot com
2023-06-30 3:27 ` [Bug c++/110492] " pinskia at gcc dot gnu.org
2023-06-30 7:16 ` [Bug middle-end/110492] " rguenth at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: qufanat at gmail dot com @ 2023-06-30 2:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110492
Bug ID: 110492
Summary: Attempted optimization of switch statement pessimizes
it instead
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: qufanat at gmail dot com
Target Milestone: ---
This happens on my local GCC 11.3, but you can also see it on 13.1 at this
godbolt link https://godbolt.org/z/G3qecWxPr
I'm creating a switch statement of hashed strings, which compiles to a binary
search on the hashes, all well and good. However, with -O3 specified, GCC
peels back the last multiplication of the hash for some of the comparison
branches, but is unable to do it for others, resulting in longer assembly with
twice as many comparisons as is necessary. Here is lines 15..19 in
get_choice_1()
(end of the hash loop)
imul esi, eax, 16777619
test dl, dl
jne .L3
(start of the switch)
cmp eax, 1954414351
je .L8
cmp esi, 1901626525
ja .L4
eax is the hash without the last imul, and esi is the final hash. If we
prevent inlining of the hash function, the compiler can't make this
"optimization" and gives the assembly I expect. Here is lines 90..93 in
get_choice_2()
call hash32_noinline(char const*)
cmp eax, 1901626525
je .L33
jbe .L46
Now it only does one comparison per entry and uses it for both the == and <=
branches.
This isn't that important to my program but I thought you'd like to know.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/110492] Attempted optimization of switch statement pessimizes it instead
2023-06-30 2:52 [Bug c++/110492] New: Attempted optimization of switch statement pessimizes it instead qufanat at gmail dot com
@ 2023-06-30 3:27 ` pinskia at gcc dot gnu.org
2023-06-30 7:16 ` [Bug middle-end/110492] " rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-06-30 3:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110492
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 55431
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55431&action=edit
testcase
Next time please attach or paste inline the code instead of posting just a link
to godbolt.org .
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/110492] Attempted optimization of switch statement pessimizes it instead
2023-06-30 2:52 [Bug c++/110492] New: Attempted optimization of switch statement pessimizes it instead qufanat at gmail dot com
2023-06-30 3:27 ` [Bug c++/110492] " pinskia at gcc dot gnu.org
@ 2023-06-30 7:16 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-06-30 7:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110492
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-06-30
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
On GIMPLE there are exactly the same number of compare-and-branch stmts
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-06-30 7:16 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2023-06-30 2:52 [Bug c++/110492] New: Attempted optimization of switch statement pessimizes it instead qufanat at gmail dot com
2023-06-30 3:27 ` [Bug c++/110492] " pinskia at gcc dot gnu.org
2023-06-30 7:16 ` [Bug middle-end/110492] " rguenth at gcc dot gnu.org
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