Re: Strongly Specify Alignment for FFI Allocation
On Sep 25, 2009, at 07:54 , Duncan Coutts wrote: pessimistic. We could do better on machines that are tolerant of misaligned memory accesses such as x86. We'd need to use cpp to switch Hm. I thought x86 could be tolerant (depending on a cpu configuration bit) but the result was so slow that it wasn't worth it? -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Strongly Specify Alignment for FFI Allocation
On Thu, 2009-09-24 at 23:13 +0100, Don Stewart wrote: > > It would be beneficial if this wording was applied to all allocation > > routines - such as mallocForeignPtrBytes, mallocForeignPtrArray, etc. > > For the curious, this proposal was born from the real-world issue of > > pulling Word32's from a ByteString in an efficient but portable manner > > (binary is portable but inefficient, a straight forward > > unsafePerformIO/peek is efficient but need alignment). > > As a side issue, the get/put primitives on Data.Binary should be > efficient (though they're about twice as fast when specialized to a > strict bytestring... stay tuned for a package in this area). They are efficient within the constraint of doing byte reads and reconstructing a multi-byte word using bit twiddling. eg: getWord16be :: Get Word16 getWord16be = do s <- readN 2 id return $! (fromIntegral (s `B.index` 0) `shiftl_w16` 8) .|. (fromIntegral (s `B.index` 1)) Where as reading an aligned word directly is rather faster. The problem is that the binary API cannot guarantee alignment so we have to be pessimistic. We could do better on machines that are tolerant of misaligned memory accesses such as x86. We'd need to use cpp to switch between two implementations depending on if the arch supports misaligned memory access and if it's big or little endian. #ifdef ARCH_ALLOWS_MISALIGNED_MEMORY_ACCESS #ifdef ARCH_LITTLE_ENDIAN getWord32le = getWord32host #else getWord32le = ... #endif etc Note also that currently the host order binary ops are not documented as requiring alignment, but they do. They will fail eg on sparc or ppc for misaligned access. Duncan ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
