potential for controlling memory alignment?

Simon Marlow marlowsd at gmail.com
Wed Sep 18 19:10:45 UTC 2013

On 12/09/13 20:28, Nicolas Frisby wrote:
> On my laptop, a program showed a consistent slowdown with -fdicts-strict
> I didn't find any obvious causes in the Core differences, so I turned to
> Intel's Performance Counter Monitor for measurements. After trying a few
> counters, I eventuall saw that there are about an order of magnitude
> more misaligned memory loads with -fdicts-strict than without, so I
> think that may be a significant part of the slowdown. I'm not sure if
> these are code or data reads.
> Can anyone suggest how to validate this hypothesis about misaligned reads?

Is this 32 bit or 64 bit?  On 32 bits we might get misaligned loads of 
doubles from the heap and stack, but we shouldn't have that problem on 

If you're on Linux you can use perf to track misaligned accesses and 
annotate the assembly to find the hotspots.  Some info on the wiki here: 


> A subsequent commit has changed the behavior I was seeing, so I'm not
> interested in alternatives means to determine if -fdicts-strict is
> somehow at fault — I'm just asking specifically about data/code memory
> alignment in GHC and how to diagnose/experiment with it.
> Thanks.
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