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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101701] GCC optimization and code generation
 for if-else chains vs ternary chains vs a switch
Date: Fri, 30 Jul 2021 23:48:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101701

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-30
             Status|UNCONFIRMED                 |NEW
           Severity|normal                      |enhancement

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So the problem here is iftoswitch should ignore if it is profitable to conv=
ert
the ifs to a switch statement (unless there are only two ifs) and then allow
the switchlower pass to lower the switch again.

I thought there was a bug about this already too.

What is interesting is that the cost for ifs are so high on arm target
(compared to a switch), we end up with the best code already:
foo(int):
        subs    r3, r0, #1
        cmp     r3, #3
        it      hi
        movhi   r0, #-1
        bx      lr
bar(int):
        subs    r3, r0, #1
        cmp     r3, #3
        it      hi
        movhi   r0, #-1
        bx      lr
baz(int):
        subs    r3, r0, #1
        cmp     r3, #4
        it      cs
        movcs   r0, #-1
        bx      lr

:)

So x86 we get:
;; Canonical GIMPLE case clusters: 1 2 3 4=20
For foo
and then for bar:
;; Canonical GIMPLE case clusters: 1 2 3 4=20
;; BT can be built: BT(values:4 comparisons:8 range:4 density: 200.00%):1-4=
=20
/app/example.cpp:10:21: optimized: Condition chain with 4 BBs transformed i=
nto
a switch statement.=