It's just incredible that there's still no way to force denormals to be rounded silently to zero on x86. You have to go out of your way to check for them, or face horrific performance penalties for a feature you didn't even want or need.
The only instructions you can’t set to flush are the legacy x87 opcodes, which you shouldn’t be using in a performance-sensitive context anyway.
#define CSR_FLUSH_TO_ZERO (1 << 15)
unsigned csr = __builtin_ia32_stmxcsr();
csr |= CSR_FLUSH_TO_ZERO;
__builtin_ia32_ldmxcsr(csr);