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Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

556db2f3

by sheaf at 2026-08-10T21:32:06-04:00

Reduce SpecConstr threshold in GHC.Tc.Solver.Rewrite

As remarked in #27628, this module currently sits on a knife's edge: if
the body of 'simplifyArgsWorker' is made even a tiny bit smaller, then
SpecConstr suddenly kicks in and causes disastrous reboxing of the
LiftingContext argument.

To make this less likely to happen, this commit lowers the SpecConstr
threshold.

1 changed file:

compiler/GHC/Tc/Solver/Rewrite.hs

Changes:

compiler/GHC/Tc/Solver/Rewrite.hs

 +{-# OPTIONS_GHC -fspec-constr-threshold=1200 #-}
 +  -- This threshold (smaller than the default 2000) avoids SpecConstr from firing
 +  -- on the inner loop of GHC.Core.Reduction.simplifyArgsWorker (inlined into
 +  -- this module) based on a 'LiftingContext' argument, as this causes
 +  -- significant reboxing (regressing compile-time allocations in T9872d by ~4%),
 +  -- as per #27628.
 +  --
 +  -- The 1200 threshold was chosen to avoid this issue while still allowing
 +  -- beneficial SpecConstr to fire in the rest of the module.
++
  module GHC.Tc.Solver.Rewrite(
     rewrite, rewriteForErrors, rewriteArgsNom,
     rewriteType