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Test failure when compiled with -O0 #26
Description
If I compile with -O0, I get the following test failure:
*** Failed! Falsifiable (after 1 test):
0.0
0.0
(used seed -8074436915145524066)
t_shortest: [Failed]
*** Failed! Falsifiable (after 1 test):
0.0
0.0
(used seed 5824167789149766384)
Regressions:
toShortest_overflow: [OK]
Properties Test Cases Total
Passed 0 1 1
Failed 2 0 2
Total 2 1 3
The test code in question is https://github.com/bos/double-conversion/blob/e4b7956d51fbb79ada95412519bd2a6a680def23/tests/Properties.hs#L9-L13
where a = 0.0, b = 0.0.
The problem seems to be the use of realToFrac (0.0 / 0.0). Note that 0.0/0.0 is NaN.
- With
-O,realToFrac NaNreturnsNaN - With
-O0,realToFrac NaNreturns-Infinity
You would expect realToFrac :: Double -> Double to always be id, but apparently that is not the case in GHC right now. It is so only if compile with optimisations.
Activity
nh2 commentedon May 19, 2019
This is due to what's explained on:
As mentioned in the latter issue by Simon Marlow 9 years ago:
I think this means that the code in https://github.com/bos/double-conversion/blob/e4b7956d51fbb79ada95412519bd2a6a680def23/Data/Double/Conversion/ByteString.hs#L68-L72 is wrong, as it uses
realToFracto convert between floating point types (Double -> CDouble).Of course it's somewhat unreasonable to blame
double-conversionfor this. The fact that Haskell does not provide a function to reasonably convertDouble -> CDoubleis insane (I posted here).Kleidukos commentedon Sep 25, 2023
Can still reproduce with GHC 9.6