https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90029
Bug ID: 90029
Summary: optimizing local exceptions, or are they an observable
side effect
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: federico.kircheis at gmail dot com
Target Milestone: ---
Hello,
this is partly a question, and partly a feature request
Consider following functions
----
int bar_ex_noexcept(int i) noexcept {
struct foo{};
try {
if(i<0){
throw foo{};
}
return i;
}catch(foo){
return -1;
}
}
int bar_ex(int i) {
struct foo{};
try {
if(i<0){
throw foo{};
}
return i;
}catch(foo){
return -1;
}
}
int bar_ret(int i) noexcept {
if(i<0){
return -1;
}
return i;
}
int bar_goto(int i) noexcept {
if(i<0){
goto ex;
}
return i;
ex: return -1;
}
----
All this functions, unless I overlooked something, do exactly the same with
different control structure (goto, exception, early return): return the given
value i positive or 0, -1 if less than 0.
gcc is smart, and all functions, except those that use as implementation detail
an exception, generate the same assembly with (testing with `-O3`).
`bar_ex_noexcept` also has a call to `std::terminate`, even if it will never
get executed (example here: https://godbolt.org/z/XVtgXG).
Since `foo` is defined locally to `bar_ex`/`bar_ex_noexcept`, I expected that
gcc would have been able to optimize the throw and catch clause completely
(calls to `__cxa_allocate_exception` and `__cxa_throw`), but it's not the case.
Also the conditional call to `std::terminate` in `bar_ex_noexcept` surprised
me, since the only thrown exception is always caught and ignored.
Therefore my question: are exceptions an observable behavior, and thus the
compiler can't optimize them completely away, or is simply gcc "not smart
enough" to do those optimization.
If they are not observable, the feature request would be to
* optimize local exceptions
* thus remove typeinfo information for types that can never escape the local
scope (since if the exceptions are optimized, those information are not used by
anyone)
* remove dead calls to `std::terminate`
