https://bugs.llvm.org/show_bug.cgi?id=51759
Bug ID: 51759
Summary: x86-64: __builtin_bit_cast(anything, long double)
wrongly errors with "indeterminate value"
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
On a plain old x86-64 Linux or OSX system, where `long double` is 128-bit and
IEC559 standard as far as I know:
constexpr int f() {
long double ld = 1.0;
__uint128_t u = __builtin_bit_cast(__uint128_t, ld);
return 4;
}
int a[f()];
GCC is happy with this program. Clang complains:
<source>:1:15: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr int f() {
^
<source>:3:21: note: indeterminate value can only initialize an object of type
'unsigned char' or 'std::byte'; 'unsigned __int128' is invalid
__uint128_t u = __builtin_bit_cast(__uint128_t, ld);
^
And it's not only when converting to __uint128_t; converting to a trivial
16-byte struct type gives the same diagnostic. It seems that Clang flatly
doesn't support __builtin_bit_cast from a `long double` source operand.
https://godbolt.org/z/EYraxf9aY
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