https://bugs.llvm.org/show_bug.cgi?id=37778
Bug ID: 37778
Summary: __builtin_nan incorrectly has "const" attribute
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: gfal...@google.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
In Builtins.def, __builtin_nan and friends have the "c" (const) attribute, even
though these functions access the pointed-to memory. This leads the optimizer
into generating incorrect code.
The two functions below return different answers in -O1 and above:
double a() {
char buf[4];
buf[0] = buf[1] = buf[2] = '9';
buf[3] = '\0';
return __builtin_nan(buf);
}
double b() { return __builtin_nan("999"); }
In a(), the optimizer thinks that nothing reads the contents of `buf`, and
eliminates the assignments. See https://godbolt.org/g/sdn5Mh.
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