[Git][ghc/ghc][wip/T21972] nativeGen: Disable asm-shortcutting on Darwin

[Ben Gamari (@bgamari)]

Ben Gamari pushed to branch wip/T21972 at Glasgow Haskell Compiler / GHC


Commits:
62eaa982 by Ben Gamari at 2023-01-22T08:29:08-05:00
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.

- - - - -


2 changed files:

- compiler/GHC/CmmToAsm.hs
- docs/users_guide/using-optimisation.rst


Changes:

=====================================
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


=====================================
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.



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/62eaa9824e655ade942f0f8152bd715ae433898a

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/62eaa9824e655ade942f0f8152bd715ae433898a
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