3x slowdown on GHC HEAD with file containing lots of overloaded string literals

I just noticed that compiling xmlhtml-0.2.3.5 from Hackage takes an extraordinarily long time on GHC HEAD. The slowdown can be attributed to the Text.XmlHtml.HTML.Meta module. I've extracted it and tweaked it so that it requires no dependencies to build (I now use ByteString instead of Text, but the slowdowns for both types were about the same.)

On GHC 8.0.2, it takes about 3 seconds to compile the attached module:

$ /usr/bin/time /opt/ghc/8.0.2/bin/ghc -fforce-recomp TextXmlHtmlHTMLMeta.hs 
[1 of 1] Compiling Text.XmlHtml.HTML.Meta ( TextXmlHtmlHTMLMeta.hs, TextXmlHtmlHTMLMeta.o )
3.44user 0.07system 0:03.51elapsed 99%CPU (0avgtext+0avgdata 208480maxresident)k
0inputs+21688outputs (0major+43934minor)pagefaults 0swaps

But on GHC HEAD, it takes about 10 seconds:

$ /usr/bin/time ~/Software/ghc2/inplace/bin/ghc-stage2 -fforce-recomp TextXmlHtmlHTMLMeta.hs 
[1 of 1] Compiling Text.XmlHtml.HTML.Meta ( TextXmlHtmlHTMLMeta.hs, TextXmlHtmlHTMLMeta.o )
9.93user 0.11system 0:10.03elapsed 100%CPU (0avgtext+0avgdata 233656maxresident)k
0inputs+21464outputs (0major+54853minor)pagefaults 0swaps

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
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Related
Blocking
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Operating system
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changed milestone to %8.2.1

changed weight to 7

added Tbug Trac import labels

Attached file TextXmlHtmlHTMLMeta.hs (download).

That's bad! But it's so egregiously bad that surely it ought not to be hard to find where the time is going!

Ben, Reid, David: might one of you look? If we can fix this we may fix other less-egregious cases.

Simon

I took a look but on my systems it doesn't compile in anywhere near 3.5 seconds under GHC 8.0.2. It takes about 15 seconds for me and a recent (but not absolutely up-to-date) HEAD is similar. RyanGlScott are you sure about these numbers?

closed

Ugh, it turns out this was caused by me using a GHC HEAD that was built with the quick flavor instead of the perf flavor. I had thought that the only difference between the two was that the latter built Haddocks and other non-essentials—that couldn't have been more wrong!

After rebuilding GHC HEAD with perf, I get this time:

$ /usr/bin/time ~/Software/ghc4/inplace/bin/ghc-stage2 -fforce-recomp TextXmlHtmlHTMLMeta.hs
[1 of 1] Compiling Text.XmlHtml.HTML.Meta ( TextXmlHtmlHTMLMeta.hs, TextXmlHtmlHTMLMeta.o )
3.08user 0.07system 0:03.17elapsed 99%CPU (0avgtext+0avgdata 196820maxresident)k
0inputs+21464outputs (0major+41873minor)pagefaults 0swaps

Which is even //better// than 8.0.2! So this tale had a happy ending after all.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added compiler perf label

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