Disable the SRT offset optimisation on MachO platforms

Authored by bgamari on May 22 2018, 8:36 AM.

Description

Disable the SRT offset optimisation on MachO platforms

Unfortunately, this optimisation is infeasible on MachO platforms (e.g.
Darwin) due to an object format limitation. Specifically, linking fails
with errors of the form:

error: unsupported relocation with subtraction expression, symbol
'_integerzmgmp_GHCziIntegerziType_quotInteger_closure' can not be
undefined in a subtraction expression

Apparently MachO does not permit relocations' subtraction expressions to
refer to undefined symbols. As far as I can tell this means that it is
essentially impossible to express an offset between symbols living in
different compilation units. This means that we lively can't use this
optimisation on MachO platforms.

Test Plan: Validate on Darwin

Reviewers: simonmar, erikd

Subscribers: rwbarton, thomie, carter, angerman

GHC Trac Issues: Trac #15169

Differential Revision: https://phabricator.haskell.org/D4715