Recompilation avoidance doesn't work for -fno-code/-fwrite-interface

With the latest -fwrite-interface enhancements, a build directory can contain interface files independently of object files. In cases like this, we would like to have recompilation avoidance avoid typechecking files whose interface files are up-to-date.

However, this does not currently work: we always re-typecheck:

t-edyang@cam-05-unx:/5playpen/t-edyang/sandbox/q$ /5playpen/t-edyang/ghc-backpack/inplace/bin/ghc-stage2 --make A.hs -fno-code -fwrite-interface
[1 of 1] Compiling A                ( A.hs, nothing )
t-edyang@cam-05-unx:/5playpen/t-edyang/sandbox/q$ /5playpen/t-edyang/ghc-backpack/inplace/bin/ghc-stage2 --make A.hs -fno-code -fwrite-interface
[1 of 1] Compiling A                ( A.hs, nothing )

The reason for this is that recompilation avoidance logic in compileOne (in DriverPipeline.hs) seems to rely exclusively on "linkables" in order to figure out if source has been modified or not. We never generate an object file with -fwrite-interface, so the compiler always concludes that we need to retypecheck.

However, recompilation avoidance for hs-boot does work. The reason for this is because we create a dummy o-boot linkable. We can't use this strategy for -fno-code, because the dummy object file would imply that we actually compiled the file (which we didn't).

The upshot is that to fix this problem, it looks like we might have to rejigger all of the logic in compileOne, which why I gave up on this for now.

Related to https://github.com/haskell/cabal/issues/1179

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

added backpack label

changed weight to 7

assigned to @ezyang

Trac metadata

Trac field	Value
Priority	normal → high
Differential revisions	- → D596

changed milestone to %7.10.1

changed title from Recompilation avoidance works imperfectly with -fno-code/-fwrite-interface to Recompilation avoidance doesn't work for -fno-code/-fwrite-interface

mentioned in commit 2223e196

added 1 deleted label

Oh, to be clear to whoever merges: you probably need the parent commit https://git.haskell.org/ghc.git/commit/af4d99803ea7676f88f250ad56a8c31c1c8cd5bc

closed

Merged into ghc-7.10.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in commit bc604bdb

added compiler perf label

added Phigh label