Adding an explicit export list halves compilation time.

A file only containing a 10000 constructor enumeration type compiles twice as fast with an explicit export list compare to having no export list.

# print "module A where"
print "module A (T ( .. ) ) where"
print "\n"
print "data T = C"
for i in range(0,10000):
    print ("    | C" + str(i))

Without an export list:

[1 of 1] Compiling A                ( A.hs, A.o )

real	0m22.872s
user	0m19.653s
sys	0m0.868s

With the export list:

[1 of 1] Compiling A                ( A.hs, A.o )

real	0m11.249s
user	0m10.428s
sys	0m0.435s

The culprit here is exports_from_avail, in the case that no export list is provided, the things to export are gathered from the GlobalRdrEnv. Each constructor is placed in its own Avail which are then combined by plusAvail which is implemented using the expensive unionLists.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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