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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/93934] Unnecessary fld of uninitialized float stack
 variable results in ub of valid C++ code
Date: Thu, 14 Oct 2021 18:00:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93934

--- Comment #25 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
(In reply to joseph@codesourcery.com from comment #24)
> This is a fundamentally different test, because it involves (in the=20
> abstract machine) lvalue-to-rvalue conversion of a sNaN representation.=
=20=20
> That means that, unlike the present bug, and the two others I referenced,=
=20
> (a) it's only valid with -fsignaling-nans, (b) it's at most a=20
> quality-of-implementation issue because of the rule that assignment to th=
e=20
> same format may be a convertFormat operation rather a copy operation, and=
=20
> (c) the ABI means the exception can't be avoided when an sNaN is returned=
.=20=20
> Effectively, this test is bug 56831, whereas the present bug is more like=
=20
> bug 58416 and bug 71460.

I would like to point out (in the context of trapping behavior of x87) that=
 I
don't think we should disable FP conditional moves with the patch in Comment
#18 to fix the presented corner case, as this will effect the runtime
performance of the vast majority of applications that don't care about trap=
s.
Looking at the above referred bugs, it is IMO time to throw in the towel and
declare x87 math as kind-of-broken w.r.t trapping. The compiler will genera=
te
code that speculatively loads values at various places (as shown by the
testcase in Comment #20), not only at the FCMOV site.

I'm not a language lawyer, but IMO access to a non-volatile object is not
forbidden *if the value is not used*. Unfortunately, some accesses have
secondary effects that are not considered by the compiler, and the trapping
behavior of FLDS/FLDL is certainly one of them.=