Re: 2 optimization bugs ?

I haven't looked at the second one, but for the first you may be interested
in the source code -
https://hackage.haskell.org/package/ghc-internal-9.1001.0/docs/src/GHC.Internal.Real.html#%5E
- and the comments around [Inlining] and [Powers with small exponent].

Notably, it only covers until x^a until a = 5, and indeed with 10^5 the
results of the two different ways of writing the number are the same.

--
Noon


On Tue, 2 Jul 2024 at 12:35, George Colpitts <[email protected]>
wrote:

> It seems like I have found two optimization bugs. Before filing bugs , I
> wanted to double check if others agreed and can reproduce the bugs.
>
> Compile and run the attached file:
>
> ghc -O2 optBugs.hs
> Loaded package environment from
> /Users/avie/.ghc/aarch64-darwin-9.10.1/environments/default
> [1 of 2] Compiling Main             ( optBugs.hs, optBugs.o )
> optBugs.hs:45:49: warning: [GHC-18042] [-Wtype-defaults]
>     • Defaulting the type variable ‘b0’ to type ‘Integer’ in the following
> constraints
>         (Integral b0) arising from a use of ‘^’ at optBugs.hs:45:49
>         (Num b0) arising from the literal ‘6’ at optBugs.hs:45:50
>     • In the first argument of ‘counts’, namely ‘(10 ^ 6)’
>       In the second argument of ‘(.)’, namely ‘counts (10 ^ 6)’
>       In the second argument of ‘(.)’, namely ‘elems . counts (10 ^ 6)’
>    |
> 45 | solve = product . map (+ 1) . elems . counts (10^6) --  1000000
>    |
>         ^
>
> [2 of 2] Linking optBugs
> avie@Anabela-Air Documents % ./optBugs +RTS -s
> ./optBugs +RTS -s
> 39001250856960000
>    2,932,564,768 bytes allocated in the heap
>      143,405,696 bytes copied during GC
>       76,022,072 bytes maximum residency (4 sample(s))
>       11,034,312 bytes maximum slop
>              164 MiB total memory in use (0 MiB lost due to fragmentation)
>
>
>                                      Tot time (elapsed)  Avg pause  Max
> pause
>   Gen  0       263 colls,     0 par    0.032s   0.036s     0.0001s
>  0.0020s
>   Gen  1         4 colls,     0 par    0.048s   0.060s     0.0150s
>  0.0368s
>
>
>   INIT    time    0.003s  (  0.003s elapsed)
>   MUT     time    3.343s  (  3.333s elapsed)
>   GC      time    0.080s  (  0.096s elapsed)
>   EXIT    time    0.002s  (  0.008s elapsed)
>   Total   time    3.428s  (  3.440s elapsed)
>
>
>   %GC     time       0.0%  (0.0% elapsed)
>
>
>   Alloc rate    877,099,801 bytes per MUT second
>
>
>   Productivity  97.5% of total user, 96.9% of total elapsed
>
>
> Now change the line
>
>     solve = product . map (+ 1) . elems . counts (10^6) --  1000000
>
> to
>
>     solve = product . map (+ 1) . elems . counts 1000000
>
> save the file to disk, compile and rerun:
>
> ghc -O2 optBugs.hs
> Loaded package environment from
> /Users/avie/.ghc/aarch64-darwin-9.10.1/environments/default
> [1 of 2] Compiling Main             ( optBugs.hs, optBugs.o ) [Source file
> changed]
> [2 of 2] Linking optBugs [Objects changed]
> avie@Anabela-Air Documents % ./optBugs +RTS -s
> ./optBugs +RTS -s
> 39001250856960000
>    1,920,053,000 bytes allocated in the heap
>           16,488 bytes copied during GC
>        8,028,224 bytes maximum residency (2 sample(s))
>          393,152 bytes maximum slop
>               22 MiB total memory in use (0 MiB lost due to fragmentation)
>
>
>                                      Tot time (elapsed)  Avg pause  Max
> pause
>   Gen  0       239 colls,     0 par    0.000s   0.001s     0.0000s
>  0.0002s
>   Gen  1         2 colls,     0 par    0.000s   0.001s     0.0007s
>  0.0011s
>
>
>   INIT    time    0.003s  (  0.003s elapsed)
>   MUT     time    2.011s  (  2.015s elapsed)
>   GC      time    0.001s  (  0.002s elapsed)
>   EXIT    time    0.000s  (  0.007s elapsed)
>   Total   time    2.015s  (  2.027s elapsed)
>
>
>   %GC     time       0.0%  (0.0% elapsed)
>
>
>   Alloc rate    954,738,679 bytes per MUT second
>
>
>   Productivity  99.8% of total user, 99.4% of total elapsed
>
>
> Allocation decreased by a billion bytes and runtime has decreased by
> almost a third. This is hard to believe. The obvious explanation is that I
> dropped a zero when I wrote out 10^6 but I checked that I have six zeros.
> Also I get the same answer in both cases.
>
> Next uncomment the following two lines in factor:
>
>       -- let p = sm ! m
>
>     -- sm = smallest maxN
>
> and comment out the line:
>
>    let p = smallest maxN ! m
>
> save the file, compile and rerun:
>
>
> ghc -O2 optBugs.hs
> Loaded package environment from
> /Users/avie/.ghc/aarch64-darwin-9.10.1/environments/default
> [1 of 2] Compiling Main             ( optBugs.hs, optBugs.o ) [Source file
> changed]
> [2 of 2] Linking optBugs [Objects changed]
> avie@Anabela-Air Documents % ./optBugs +RTS -s
> ./optBugs +RTS -s
> 39001250856960000
>       16,051,152 bytes allocated in the heap
>           10,040 bytes copied during GC
>           44,328 bytes maximum residency (1 sample(s))
>           29,400 bytes maximum slop
>               22 MiB total memory in use (0 MiB lost due to fragmentation)
>
>
>                                      Tot time (elapsed)  Avg pause  Max
> pause
>   Gen  0         2 colls,     0 par    0.000s   0.000s     0.0001s
>  0.0002s
>   Gen  1         1 colls,     0 par    0.000s   0.001s     0.0015s
>  0.0015s
>
>   INIT    time    0.003s  (  0.003s elapsed)
>   MUT     time    0.019s  (  0.018s elapsed)
>   GC      time    0.000s  (  0.002s elapsed)
>   EXIT    time    0.001s  (  0.002s elapsed)
>   Total   time    0.023s  (  0.025s elapsed)
>
>   %GC     time       0.0%  (0.0% elapsed)
>
>   Alloc rate    866,786,478 bytes per MUT second
>
>   Productivity  78.9% of total user, 72.5% of total elapsed
>
> Allocation has dropped from 2 billion bytes to 16 million and runtime has
> dropped from 2 seconds to 0.02 seconds. I think the optimizer should move
> the call to smallest outside of the "loop" in go. This seems like a bug to
> me.
>
>
>
>
>
>
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-- 
Noon van der Silk

http://silky.github.io/

"My programming language is kindness."

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