[Haskell-cafe] llvm is generating invalid instructions (problem with autovectorization?)

[Mike Izbicki]

Geoffrey:  After a quick google of binutils, it's not obvious to me what
role it plays in the compilation process or how I would find out if it
supports sarxq?

Carter:  It's on linux (centos); gcc version is 4.4.7.  Why is gcc even
getting involved in this process?  I didn't think ghc used it at all
anymore.


On Wed, May 21, 2014 at 6:57 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> Is this on a Mac that is using gcc4.2?  The older GCC assembler stuff
> doesn't understand simd.
>
>
> On Wednesday, May 21, 2014, Geoffrey Mainland <mainland at apeiron.net>
> wrote:
>
>> Perhaps your installed version of binutils does not support the sarxq
>> instruction?
>>
>> You can ask GHC to pass flags to llc and friends:
>>
>>
>> http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/options-phases.html#forcing-options-through
>>
>> Best,
>> Geoff
>>
>> On 05/21/2014 06:50 PM, Mike Izbicki wrote:
>> > I have some code* that is causing llvm to throw a fit.  In particular,
>> > I get (compiling with GHC 7.8 and llvm 3.4):
>> >
>> >     /tmp/ghc1685_0/ghc1685_5.bc:150324:0:
>> >          Error: no such instruction: `sarxq %rax,%rbx,%r14'
>> >
>> > Similar errors get repeated several thousand times, and it's always a
>> > sarxq instruction.  Now, my code doesn't use any of the simd primops,
>> > so the sarxq must be coming from llvm's autovectorization.  Is it
>> > possible that somehow GHC's output bytecode could be responsible, or
>> > is the problem more likely with llvm proper?  Is there any way I can
>> > force GHC to pass flags to llvm so I can experiment with different
>> > settings?
>> >
>> > Another clue that the cause might be in llvm's autovectorization is
>> > that when I compile with llvm 3.0 (which does not support
>> > autovectorization), there are no error messages.
>> >
>> > * I suspect my code is far too complicated for it to shed any light on
>> > this question, but it's available at:
>> >
>> https://github.com/mikeizbicki/HLearn/tree/dev-7.8/src/examples/hlearn-allknn
>> > If need be, I can create a minimal sample of code that causes the
>> > error, but I don't want to go through the trouble if it's not needed.
>> >
>> >
>>
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