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Lag sample in biographical profile is very wrong

In a recent heap profile I took the output of the biographical profiler is useless because of an incorrect LAG sample.

BEGIN_SAMPLE 2.819970                                                           
VOID  2062560                                                                   
LAG 18446744073709084864                                                        
USE 0                                                                           
INHERENT_USE  0                                                                 
DRAG  1152                                                                      
END_SAMPLE 2.819970

This looks like an overflow error perhaps?

The way to reproduce it is

  1. Build ghc with prof flavour
  2. Run biographical profiler on test for #9630 (closed) eventlog-prof/stage1/bin/ghc T9630.hs -fforce-recomp -O2 +RTS -l -hb -hdId -hdIdInfo -hdGRefl

The value is so far out that the problem can probably be identified using gdb or a few traces quite easily.

Edited by Matthew Pickering
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