id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
830	"Compiler performance bug: large ""do"" expression"	simonmar		"From Neil Mitchell:

Using GHC 6.4.2 on Windows and 6.4.1 on Linux, I get really poor
compile time performance for large do blocks - for example:

{{{
main = do
  return ()
  return ()
  ....
  return ()
}}}

The following are the compile times in seconds for a given number of
return () lines, GHC 6.4.1 on Linux, with a very beefy processor (my
department server) and loads of RAM.

{{{
100    1.444
200    4.054
300    7.977
400    13.967
500    21.265
600    30.71
700    43.997
}}}

For comparison on my desktop machine I get out of heap space with
268Mb of heap, in a few minutes for 700. Compared to Hugs and Yhc,
which both deal with all of these files instantly.

By splitting the 700 line do block into 7 separate do blocks, each 100
lines, I get a compile time of 8.530.

The particular reason I was running into this issue was with an auto
generated file of tests, about 360 lines of reasonably lines in a do
block, which exhausted the memory in GHCi. I have modified my test
generator to split the files into 50 line blocks.

Thanks

Neil"	bug	closed	normal	6.6.1	Compiler	6.4.2	fixed		ndmitchell@…	Unknown/Multiple	Unknown/Multiple		Unknown