True divergence: FP precision differences #37335

Activity

1. aartbik

   added

   area-vmUse area-vm for VM related issues, including code coverage, and the AOT and JIT backends.Use area-vm for VM related issues, including code coverage, and the AOT and JIT backends.

   dartfuzzFound with Dart fuzzing (DartFuzz, libFuzzer, etc.)Found with Dart fuzzing (DartFuzz, libFuzzer, etc.)

   on Jun 21, 2019
2. aartbik

   self-assigned this

   on Jun 21, 2019
3. aartbik commented on Jul 1, 2019

   

   aartbik

   on Jul 1, 2019

   Last edited by aartbik

   ContributorAuthor

   More actions

   Indeed due to 32-bit vs. 64-bit implementation of the pow() method (which ultimately is done by native code). If Dart does not have the same rigid precision requirements as e.g. Java, perhaps it makes sense to avoid testing 32-bit vs. 64-bit divergences as different execution modes.

   2.9065489552295786e-304 ^ 0.61017368125593 is 6.165122919860247e-186 (64-bit)
   2.9065489552295786e-304 ^ 0.61017368125593 is 6.165122919860246e-186 (32-bit)
4. aartbik commented on Jul 2, 2019

   

   aartbik

   on Jul 2, 2019

   ContributorAuthor

   More actions

   Lasse gives some background.

   The language specification just says that Dart double values are IEEE-754 64-bit floating point values, but nothing more The operations are then defined by the platform libraries, which do not promise any specific implementation.

   So, the libraries do not promise anything, and the VM is free to implement them as inconsistently as JavaScript implementations. Consistency is great, but we prefer to not overpromise :)

   Given this, I think we should deal with this in DartFuzz by either

   • no longer compare 32-bit vs 64-bit execution modes, but only pair same bit size archs
   • no longer generate FP operations when 32-bit vs 64-bit modes are run
5. aartbik commented on Jul 18, 2019

   

   aartbik

   on Jul 18, 2019

   ContributorAuthor

   More actions

   @feli-citas
6. feli-citas commented on Jul 18, 2019

   

   feli-citas

   on Jul 18, 2019

   Contributor

   More actions

   JIT-DEPOPTEVERY-ReleaseX64 - JIT-DebugIA32: !DIVERGENCE! 1.13:483434976 (output=true)

   < 0.00021296142396241748
   ---
   > 0.00021296142396241743
   

   JIT-DebugSIMARM - JIT-ReleaseX64: !DIVERGENCE! 1.13:1962568294 (output=true)

   < 0.509659585465119_
   ---
   > 0.5096595854651191
   
7. dart-bot

   added a commit that references this issue on Jul 22, 2019

   [dart/fuzzer] Add flag to control FP operations

   ...

   c2af59f
8. aartbik

   closed this as completedon Jul 22, 2019