Hi Matt, On 2019-02-27 4:31 PM, Matt Peddie wrote:
For me [gcc] returns 32767 if I enable optimizations and 0 if I don't.
Yes, I get 0 without optimizations. Looking at the generated assembly (with gcc -S) I can see that the non-optimized code is converting the double to a 32-bit integer using a hardware instruction (cvttsd2si) but in the optimized case it's using a precomputed value. It's strange that the precomputed value doesn't match, but as Ian said, all bets are off anyway. According to one authority https://www.felixcloutier.com/x86/cvttsd2si, the conversion produces the 32-bit value 0x80000000. When truncated to int16_t this is 0.
Clang returns 0 independent of any flag changes I tried.
Clang's code uses a precomputed value even without optimization. Looking at GHC's generated core, it converts the Double to an Int and then narrows down to Int16 after that. —Neil