https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434
Bug ID: 99434
Summary: std::bit_cast generates more instructions than
__builtin_bit_cast and memcpy with -march=native
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: unlvsur at live dot com
Target Milestone: ---
https://godbolt.org/z/5KWM8Y
struct u64x2_t
{
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
std::uint64_t high,low;
#else
std::uint64_t low,high;
#endif
};
u64x2_t umul5(std::uint64_t a,std::uint64_t b) noexcept
{
return std::bit_cast<u64x2_t>(static_cast<__uint128_t>(a)*b);
}
u64x2_t umul_builtin(std::uint64_t a,std::uint64_t b) noexcept
{
return __builtin_bit_cast(u64x2_t,static_cast<__uint128_t>(a)*b);
}
assembly:
umul5(unsigned long, unsigned long):
movq %rdi, %rdx
mulx %rsi, %rdx, %rcx
movq %rdx, %rax
movq %rcx, %rdx
ret
umul_builtin(unsigned long, unsigned long):
movq %rdi, %rdx
mulx %rsi, %rax, %rdx
ret
There is another issue:
std::uint64_t umul128(std::uint64_t a,std::uint64_t b,std::uint64_t& high)
noexcept
{
__uint128_t res{static_cast<__uint128_t>(a)*b};
high=static_cast<std::uint64_t>(res>>64);
return static_cast<std::uint64_t>(res);
}
I cannot do this since this generates more instructions than using memcpy to
pun types.
clang does not have this issue and all cases are dealt with correctly.
