gcc 옵션 중에-fno-strict-aliasing 란 게 뭔가요?

man gcc
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-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being
compiled. For C (and C++), this activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the same address as an object
of a different type, unless the types are almost the same. For example, an "unsigned int" can
alias an "int", but not a "void*" or a "double". A character type may alias any other type.

Pay special attention to code like this:

union a_union {
int i;
double d;
};

int f() {
a_union t;
t.d = 3.0;
return t.i;
}

The practice of reading from a different union member than the one most recently written to
(called ‘‘type-punning’’) is common. Even with -fstrict-aliasing, type-punning is allowed,
provided the memory is accessed through the union type. So, the code above will work as
expected. However, this code might not:

int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}

Every language that wishes to perform language-specific alias analysis should define a function
that computes, given an "tree" node, an alias set for the node. Nodes in different alias sets
are not allowed to alias. For an example, see the C front-end function "c_get_alias_set".

Enabled at levels -O2, -O3, -Os.
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