https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85882
Bug ID: 85882
Summary: Value of local variable changes unintentionally if
certain optimization are enabled
Product: gcc
Version: 8.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: stefan.bolus at gmx dot de
Target Milestone: ---
In the method ...
void readMessageBegin() {
int32_t sz;
readI32(sz); // Emulates reading from wire, `sz` passed by ref.
// Here, the value of `sz` is -2147418110.
std::cout << "Value before if-throw: " << sz << std::endl;
int32_t version = sz & ((int32_t) 0xffff0000);
if (version != ((int32_t) 0x80010000)) {
// This seems to have a `throw` for the error to happen.
throw 123;
}
// But here, the value of `sz` suddenly is -2147418112.
std::cout << "Value after if-throw : " << sz << std::endl;
}
... the value of `sz` changes between the `cout` line if `-O2` is turned on. If
any of the optimizations `-ftree-vrp -finline -ftree-fre` is turned off, then
the change does not occur.
A minimal example is attached to the ticket. Additionally, a Makefile which
disables most other optimizations with the error still being there, is also
attached.
$ g++ --version
g++ (GCC) 8.1.1 20180502 (Red Hat 8.1.1-1)
$ uname -a
Linux blackbox 4.16.8-300.fc28.x86_64 #1 SMP Wed May 9 20:23:40 UTC 2018 x86_64
x86_64 x86_64 GNU/Linux
--- Comment #1 from Stefan Bolus <stefan.bolus at gmx dot de> ---
Created attachment 44168
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44168&action=edit
Makefile to disable most optimization options.
