https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92641
Bug ID: 92641
Summary: Function called from dead branch
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: sagebar at web dot de
Target Milestone: ---
Created attachment 47339
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47339&action=edit
Listing of problematic code, and similar code that works correctly
Given something like this:```
extern unsigned int get_vla_size(void);
```
This is ok:```
if (0) {
(void)(int(*)[get_vla_size()])0;
} else {
(void)get_vla_size();
}
```
asm:```
call _Z12get_vla_sizev
```
However, re-writing this code to use the ?-operator causes `get_vla_size()` to
be called twice:```
0 ? (
(void)(int(*)[get_vla_size()])0
) : (
(void)get_vla_size()
);
```
asm:```
call _Z12get_vla_sizev
call _Z12get_vla_sizev
```
Note that the first call to `get_vla_size()` happens from within a dead branch,
meaning that the call should have been fully removed, or at the very least have
been skipped by an unconditional jump.
When the first branch is wrapped in statement-expressions, code once again
function as expected:```
0 ? ({
(void)(int(*)[get_vla_size()])0;
}) : (
(void)get_vla_size()
);
```
asm:```
call _Z12get_vla_sizev
```
This problem consistently appears when compiling for c++, regardless of
optimization level, and if I had to guess, it's probably related to the scope
in which some given type is declared, where `type-expr` in `{ int x; 0 ?
(type-expr)expr : expr; }` is declared in the same scope as `x`, and thereby
unconditionally initialized.
The problem can be reproduced using:```
g++ -S attached-file.cc && cat attached-file.s
```
and inspecting the produced output
Note: I hope that `middle-end` was the correct place to report this, and I'm
sorry if it isn't.
