Thank you for the suggestion! It does seem to be something huge. I can
get output from the specialisation pass for some programs and I see
things like:
Result size of Specialise
= {terms: 200, types: 552, coercions: 95, joins: 0/21}
Result size of Specialise
= {terms: 29,314, types: 316,883, coercions: 73,030, joins: 0/627}
Result size of Specialise
= {terms: 2,362,118,
types: 25,657,037,
coercions: 5,919,604,
joins: 0/49,227}
I'll attempt to use a modified GHC :pray:
On Tue, Aug 4, 2026 at 3:41 PM Simon Peyton Jones
<[email protected]> wrote:
>
> Maybe the specialiser is somehow looping or generating a huge program.
>
> What I would do is to un-comment some of the `pprTrace` lines in
> GHC.Core.Opt.Specialise and try again to ss what's going on inside the
> specialiser. But that requires you to build GHC itself, and I don't know if
> you are set up to do that.
>
> Simon
>
> On Tue, 4 Aug 2026 at 22:08, Justin Bailey via ghc-devs
> <[email protected]> wrote:
>>
>> All,
>>
>> Looking for some guidance on narrowing down an issue I am seeing at
>> work. Some background: large monorepo, many packages; I'm on MacOS
>> Tahoe (26.6).
>>
>> I have a problematic module that causes GHC to allocate a HUGE amount
>> of memory under certain conditions. Compilation never finishes -
>> instead, the process is killed by the OS.
>>
>> * With -O1, GHC consumes a huge amount of memory (> 32GB) before
>> getting killed by the OS.
>> * Without -O1, but with all available -f flags that seemed to be
>> implied by -O1, GHC does NOT blow up and compilation succeeds. (List
>> of flags below.)
>>
>> The last output (with -ddump-phases) in the failing case is always
>> "*** Specialise [aaa.Tests.Service.Client.TestHelpers]:" followed by a
>> newline, then nothing else.
>>
>> I'm working to isolate the code that's causing the problem, to get to
>> a repro case, but in the meantime can anyone suggest why -O1 fails but
>> all the -f flags do not?
>>
>> Thank you!
>>
>> p.s. I'm happy to share logs but don't want to spam the list if those
>> aren't necessary.
>>
>> **********
>> Here are all the flags I used in place of -O1:
>>
>> -fblock-layout-cfg
>> -fcall-arity
>> -fcase-folding
>> -fcase-merge
>> -fcore-constant-folding
>> -fcmm-control-flow
>> -fcmm-elim-common-blocks
>> -fcmm-sink
>> -fcmm-static-pred
>> -fcore-constant-folding
>> -fcpr-anal
>> -fcross-module-specialise
>> -fcse
>> -fenable-rewrite-rules
>> -fexitification
>> -ffloat-in
>> -ffull-laziness
>> -fignore-asserts
>> -finline-generics
>> -floopification
>> -fnum-constant-folding
>> -fsolve-constant-dicts
>> -fspecialise
>> -fstg-cse
>> -fstrictness
>> -funbox-small-strict-fields
>> -fworker-wrapper
>>
>>
>> **********
>>
>> Here is the (possibly) relevant logs for compilation with -O1 that failed:
>>
>> *** Checking old interface for aaa.Tests.Service.Client.TestHelpers
>> (use -ddump-hi-diffs for more details):
>> [62 of 74] Compiling aaa.Tests.Service.Client.TestHelpers (
>> test/aaa/Tests/Service/Client/TestHelpers.hs,
>> /Users/xxx.yyy/Source/health-engine/well-he-aaa/worktrees/master/dist-newstyle/build/aarch64-osx/ghc-9.14.1/aaa-service-client-0.0.0/t/test/noopt/build/test/test-tmp/aaa/Tests/Service/Client/TestHelpers.o,
>> /Users/xxx.yyy/Source/health-engine/well-he-aaa/worktrees/master/dist-newstyle/build/aarch64-osx/ghc-9.14.1/aaa-service-client-0.0.0/t/test/noopt/build/test/test-tmp/aaa/Tests/Service/Client/TestHelpers.dyn_o
>> )
>> *** Parser [aaa.Tests.Service.Client.TestHelpers]:
>> !!! Parser [aaa.Tests.Service.Client.TestHelpers]: finished in 7.28
>> milliseconds, allocated 2.470 megabytes
>> *** Renamer/typechecker [aaa.Tests.Service.Client.TestHelpers]:
>> *** ConsistencyCheck [aaa.Tests.Service.Client.TestHelpers]:
>> !!! ConsistencyCheck [aaa.Tests.Service.Client.TestHelpers]: finished
>> in 1763.61 milliseconds, allocated 750.592 megabytes
>> !!! Renamer/typechecker [aaa.Tests.Service.Client.TestHelpers]:
>> finished in 2449.36 milliseconds, allocated 962.845 megabytes
>> *** Desugar [aaa.Tests.Service.Client.TestHelpers]:
>> Result size of Desugar (before optimization)
>> = {terms: 713, types: 33,167, coercions: 15,521, joins: 0/171}
>> Result size of Desugar (after optimization)
>> = {terms: 551, types: 27,957, coercions: 15,828, joins: 0/93}
>> !!! Desugar [aaa.Tests.Service.Client.TestHelpers]: finished in 5.46
>> milliseconds, allocated 2.524 megabytes
>> *** Simplifier [aaa.Tests.Service.Client.TestHelpers]:
>> Result size of Simplifier iteration=1
>> = {terms: 2,677, types: 74,181, coercions: 37,908, joins: 0/155}
>> Result size of Simplifier iteration=2
>> = {terms: 1,737, types: 52,195, coercions: 27,615, joins: 0/88}
>> Result size of Simplifier iteration=3
>> = {terms: 1,673, types: 51,005, coercions: 19,466, joins: 0/80}
>> Result size of Simplifier iteration=4
>> = {terms: 1,630, types: 50,531, coercions: 19,333, joins: 0/78}
>> Result size of Simplifier
>> = {terms: 1,630, types: 50,531, coercions: 19,333, joins: 0/78}
>> !!! Simplifier [aaa.Tests.Service.Client.TestHelpers]: finished in
>> 217.38 milliseconds, allocated 120.723 megabytes
>> *** Specialise [aaa.Tests.Service.Client.TestHelpers]:
>>
>> **********
>>
>> Here are those same logs (up to specialisation) for a compilation that
>> succeeded (no -O1, only the -f flags):
>>
>> *** Checking old interface for aaa.Tests.Service.Client.TestHelpers
>> (use -ddump-hi-diffs for more details):
>> [62 of 74] Compiling aaa.Tests.Service.Client.TestHelpers (
>> test/aaa/Tests/Service/Client/TestHelpers.hs,
>> /Users/xxx.yyy/Source/health-engine/well-he-aaa/worktrees/master/dist-newstyle/build/aarch64-osx/ghc-9.14.1/aaa-service-client-0.0.0/t/test/noopt/build/test/test-tmp/aaa/Tests/Service/Client/TestHelpers.o,
>> /Users/xxx.yyy/Source/health-engine/well-he-aaa/worktrees/master/dist-newstyle/build/aarch64-osx/ghc-9.14.1/aaa-service-client-0.0.0/t/test/noopt/build/test/test-tmp/aaa/Tests/Service/Client/TestHelpers.dyn_o
>> )
>> *** Parser [aaa.Tests.Service.Client.TestHelpers]:
>> !!! Parser [aaa.Tests.Service.Client.TestHelpers]: finished in 5.70
>> milliseconds, allocated 2.470 megabytes
>> *** Renamer/typechecker [aaa.Tests.Service.Client.TestHelpers]:
>> *** ConsistencyCheck [aaa.Tests.Service.Client.TestHelpers]:
>> !!! ConsistencyCheck [aaa.Tests.Service.Client.TestHelpers]: finished
>> in 1707.88 milliseconds, allocated 749.094 megabytes
>> !!! Renamer/typechecker [aaa.Tests.Service.Client.TestHelpers]:
>> finished in 2305.51 milliseconds, allocated 959.358 megabytes
>> *** Desugar [aaa.Tests.Service.Client.TestHelpers]:
>> Result size of Desugar (before optimization)
>> = {terms: 713, types: 33,167, coercions: 15,521, joins: 0/171}
>> Result size of Desugar (after optimization)
>> = {terms: 551, types: 27,957, coercions: 15,828, joins: 0/93}
>> !!! Desugar [aaa.Tests.Service.Client.TestHelpers]: finished in 4.66
>> milliseconds, allocated 2.526 megabytes
>> *** Simplifier [aaa.Tests.Service.Client.TestHelpers]:
>> Result size of Simplifier iteration=1
>> = {terms: 555, types: 27,953, coercions: 15,828, joins: 0/93}
>> Result size of Simplifier
>> = {terms: 555, types: 27,953, coercions: 15,828, joins: 0/93}
>> !!! Simplifier [aaa.Tests.Service.Client.TestHelpers]: finished in
>> 8.60 milliseconds, allocated 5.818 megabytes
>> *** Specialise [aaa.Tests.Service.Client.TestHelpers]:
>> Result size of Specialise
>> = {terms: 555, types: 27,953, coercions: 15,828, joins: 0/10}
>> !!! Specialise [aaa.Tests.Service.Client.TestHelpers]: finished in
>> 3.98 milliseconds, allocated 2.227 megabytes
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