#7866: floor (0/0) :: Int is different with -O0 and -O1
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Reporter: alang9 | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.2
Resolution: | Keywords:
Operating System: Linux | Architecture: x86_64
Type of failure: Incorrect result at runtime | (amd64)
Test Case: | Difficulty:
Blocking: | Unknown
| Blocked By:
| Related Tickets:
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Changes (by bgamari):
* difficulty: => Unknown
Comment:
This appears to be due to the (lack of) handling of NaN in
`compiler/nativeGen/X86/CodeGen.hs:coerceFP2Int` which in the SSE case
lowers to a plain `CVTTSD2TIQ` instruction. In the event that the source
is NaN, this places the indeterminate integer value (which appears to be
0x80000000) in the destination register. Oddly enough, your test produces
instead a value of -0x8000000000000000.
Regardless, assuming the produce is NaN handling, it's not entirely clear
how this should be handled without performance loss. If the goal were
perfect "correctness" (whatever this means in the fact of NaN values being
cast to integers), we'd want to ensure we check for an indeterminate
result and return the "appropriate" value (which judging from the -O0
results is apparently 0). This seems like an excessive measure, however,
punishing all users for what is likely a rare occurrence and one that
should be handled explicitly anyways.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7866#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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