https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108931
Bug ID: 108931
Summary: conditional operator for variably modified types
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: muecker at gwdg dot de
Target Milestone: ---
Created attachment 54536
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54536&action=edit
patch against trunk
For conditional operators with size expressions in both branches, the behavior
is erratic. While this seems to be undefined behavior (although it shouldn't
be), it is surprising and my cause subtle bugs, e.g. when using such casts or
statement expressions in macros. Also it seems easy to fix by always evaluating
the size of the active branch only and taking this size for the result.
For example, in some cases size expressions from both branches are evaluated:
int foo(void)
{
return sizeof(*(0 ? (char(*)[f()])0
: (char(*)[g()])0));
}
foo:
sub rsp, 8
call g
call f
add rsp, 8
ret
In other cases size from an unevaluated branch is taken as the result:
https://godbolt.org/z/o74e7T16x
