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From: "njs at pobox dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc
 trunk in cddce2 pass
Date: Thu, 09 Feb 2023 07:44:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108737

            Bug ID: 108737
           Summary: Apparent miscompile of infinite loop on gcc trunk in
                    cddce2 pass
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: njs at pobox dot com
  Target Milestone: ---

There's a meme going around about strange compilation outputs caused
infinite-loop-related UB, and while discussing it a friend stumbled on this
weird behavior:

https://godbolt.org/z/qfj1jabMW

C++ code is:

extern int foo();

void blah() {
    int x =3D foo();
    while (1) {
        if(x) foo();
    }
}


When compiled with whatever "gcc trunk" means on compiler explorer, and with
-O3 (but not -O2), as C++ (but not C), then gcc reduces this to a single ca=
ll
to `foo()` followed by an empty infinite loop:

blah():
        sub     rsp, 8
        call    foo()
.L2:
        jmp     .L2

As far as I can tell, there's no UB here -- 'foo' being extern means it mig=
ht
do anything, and infinite loops with side-effects are well-defined in C++. =
So
this seems like a straight-up optimizer bug?

Poking through the pass analysis in CE, it looks like the output of "unroll=
jam
(tree)" is still correct, but then the output of the next pass "cddce2 (tre=
e)"
has deleted everything. Somehow it concludes that the hoisted `if` can be
eliminated? idgi.=