[Haskell-cafe] 2 optimization bugs ?
George Colpitts
george.colpitts at gmail.com
Wed Jul 3 18:14:51 UTC 2024
Thanks Noon. I should have been clearer, I expected 10^6 to be changed to
1000000 at compile time as part of constant folding. Even when done at run
time I can't see how that conversion would take that long or why it would
allocate so many bytes.
On Wed, Jul 3, 2024 at 8:12 AM Noon van der Silk <noonsilk at gmail.com> wrote:
> 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 <george.colpitts at gmail.com>
> 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 at 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 at 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 at 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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