#1603: a miscompilation of (`rem` 0x40000000)::Int with -O -fvia-C
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Reporter: Isaac Dupree | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8 branch
Component: Compiler | Version: 6.8.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: numeric/1603 | Architecture: Multiple
Os: Multiple |
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Changes (by simonmar):
* testcase: => numeric/1603
* version: 6.6.1 => 6.8.1
* os: Linux => Multiple
* architecture: x86 => Multiple
* milestone: 6.8.2 => 6.8 branch
Comment:
gcc is being too clever. It is assuming that because the first argument
to % is the result of converting an unsigned to a signed integer, that its
value is therefore non-negative, and uses this knowledge to optimise the
code it generates for 'x % 0x40000000'. I believe gcc is entitled to make
this optimisation, because arithmetic in C is only defined within the
bounds of the types, and that includes type conversions. Unfortunately
the `-fwrapv` option doesn't apply to type conversions, apparently.
I don't see an easy fix (but suggestions are welcome). We use unsigned
types everywhere in our back end, only converting to signed types in order
to perform signed operations. This strategy apparently relies on
undefined C behaviour.
I've added a test (will be pushed shortly).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1603#comment:2>
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