[Haskell-cafe] rounding and undefined behavior

Neil Mayhew neil_mayhew at users.sourceforge.net
Thu Feb 28 01:06:05 UTC 2019

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 

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.

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