From 955aa684e5ffb0eeca10d1e51befa9378b511222 Mon Sep 17 00:00:00 2001
From: Ben Gamari <ben@smart-cactus.org>
Date: Sun, 22 Jan 2023 08:11:02 -0500
Subject: [PATCH] nativeGen: Disable asm-shortcutting on Darwin
Asm-shortcutting may produce relative references to symbols defined in
other compilation units. This is not something that MachO relocations
support (see #21972). For this reason we disable the optimisation on
Darwin. We do so without a warning since this flag is enabled by `-O2`.
Another way to address this issue would be to rather implement a
PLT-relocatable jump-table strategy. However, this would only benefit
Darwin and does not seem worth the effort.
Closes #21972.
---
compiler/GHC/CmmToAsm.hs | 15 +++++++++++++++
docs/users_guide/using-optimisation.rst | 6 ++++--
2 files changed, 19 insertions(+), 2 deletions(-)
diff --git a/compiler/GHC/CmmToAsm.hs b/compiler/GHC/CmmToAsm.hs
index c0ac96fa799b..94bdaa648b0e 100644
--- a/compiler/GHC/CmmToAsm.hs
+++ b/compiler/GHC/CmmToAsm.hs
@@ -812,6 +812,19 @@ generateJumpTables ncgImpl xs = concatMap f xs
-- -----------------------------------------------------------------------------
-- Shortcut branches
+-- Note [No asm-shortcutting on Darwin]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- Asm-shortcutting may produce relative references to symbols defined in
+-- other compilation units. This is not something that MachO relocations
+-- support (see #21972). For this reason we disable the optimisation on Darwin.
+-- We do so in the backend without a warning since this flag is enabled by
+-- `-O2`.
+--
+-- Another way to address this issue would be to rather implement a
+-- PLT-relocatable jump-table strategy. However, this would only benefit Darwin
+-- and does not seem worth the effort as this optimisation generally doesn't
+-- offer terribly great benefits.
+
shortcutBranches
:: forall statics instr jumpDest. (Outputable jumpDest)
=> NCGConfig
@@ -822,6 +835,8 @@ shortcutBranches
shortcutBranches config ncgImpl tops weights
| ncgEnableShortcutting config
+ -- See Note [No asm-shortcutting on Darwin]
+ , not $ osMachOTarget $ platformOS $ ncgPlatform config
= ( map (apply_mapping ncgImpl mapping) tops'
, shortcutWeightMap mappingBid <$!> weights )
| otherwise
diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst
index d3ca68a1df46..df87fdb6a3da 100644
--- a/docs/users_guide/using-optimisation.rst
+++ b/docs/users_guide/using-optimisation.rst
@@ -262,8 +262,10 @@ by saying ``-fno-wombat``.
of a unconditionally jump, we replace all jumps to A by jumps to the successor
of A.
- This is mostly done during Cmm passes. However this can miss corner cases. So at -O2
- we run the pass again at the asm stage to catch these.
+ This is mostly done during Cmm passes. However this can miss corner cases.
+ So at ``-O2`` this flag runs the pass again at the assembly stage to catch
+ these. Note that due to platform limitations (:ghc-ticket:`21972`) this flag
+ does nothing on macOS.
.. ghc-flag:: -fblock-layout-cfg
:shortdesc: Use the new cfg based block layout algorithm.
--
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