[Git][ghc/ghc][wip/local-symbols-2] 2 commits: codeGen: Produce local symbols for module-internal functions
Ben Gamari
gitlab at gitlab.haskell.org
Mon Oct 26 21:22:05 UTC 2020
Ben Gamari pushed to branch wip/local-symbols-2 at Glasgow Haskell Compiler / GHC
Commits:
49c69308 by Ben Gamari at 2020-10-26T17:21:57-04:00
codeGen: Produce local symbols for module-internal functions
It turns out that some important native debugging/profiling tools (e.g.
perf) rely only on symbol tables for function name resolution (as
opposed to using DWARF DIEs). However, previously GHC would emit
temporary symbols (e.g. `.La42b`) to identify module-internal
entities. Such symbols are dropped during linking and therefore not
visible to runtime tools (in addition to having rather un-helpful unique
names). For instance, `perf report` would often end up attributing all
cost to the libc `frame_dummy` symbol since Haskell code was no covered
by any proper symbol (see #17605).
We now rather follow the model of C compilers and emit
descriptively-named local symbols for module internal things. Since this
will increase object file size this behavior can be disabled with the
`-fno-expose-all-symbols` flag.
With this `perf record` can finally be used against Haskell executables.
Even more, with `-g3` `perf annotate` provides inline source code.
- - - - -
6a977ea5 by Ben Gamari at 2020-10-26T17:21:57-04:00
Enable -fexpose-all-symbols when debug level is set
- - - - -
9 changed files:
- compiler/GHC/Cmm/CLabel.hs
- compiler/GHC/CmmToAsm.hs
- compiler/GHC/CmmToAsm/Config.hs
- compiler/GHC/CmmToAsm/Monad.hs
- compiler/GHC/CmmToAsm/X86/Ppr.hs
- compiler/GHC/Driver/Flags.hs
- compiler/GHC/Driver/Session.hs
- docs/users_guide/debug-info.rst
- docs/users_guide/phases.rst
Changes:
=====================================
compiler/GHC/Cmm/CLabel.hs
=====================================
@@ -118,6 +118,7 @@ module GHC.Cmm.CLabel (
LabelStyle (..),
pprDebugCLabel,
pprCLabel,
+ ppInternalProcLabel,
-- * Others
dynamicLinkerLabelInfo,
@@ -1360,6 +1361,39 @@ pprCLabel platform sty lbl =
CmmLabel _ _ fs CmmRet -> maybe_underscore $ ftext fs <> text "_ret"
CmmLabel _ _ fs CmmClosure -> maybe_underscore $ ftext fs <> text "_closure"
+-- Note [Internal proc labels]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+--
+-- Some tools (e.g. the `perf` utility on Linux) rely on the symbol table
+-- for resolution of function names. To help these tools we provide the
+-- (enabled by default) -fexpose-all-symbols flag which causes GHC to produce
+-- symbols even for symbols with are internal to a module (although such
+-- symbols will have only local linkage).
+--
+-- Note that these labels are *not* referred to by code. They are strictly for
+-- diagnostics purposes.
+--
+-- To avoid confusion, it is desireable to add a module-qualifier to the
+-- symbol name. However, the Name type's Internal constructor doesn't carry
+-- knowledge of the current Module. Consequently, we have to pass this around
+-- explicitly.
+
+-- | Generate a label for a procedure internal to a module (if
+-- 'Opt_ExposeAllSymbols' is enabled).
+-- See Note [Internal proc labels].
+ppInternalProcLabel :: Module -- ^ the current module
+ -> CLabel
+ -> Maybe SDoc -- ^ the internal proc label
+ppInternalProcLabel this_mod (IdLabel nm _ flavour)
+ | isInternalName nm
+ = Just
+ $ text "_" <> ppr this_mod
+ <> char '_'
+ <> ztext (zEncodeFS (occNameFS (occName nm)))
+ <> char '_'
+ <> pprUniqueAlways (getUnique nm)
+ <> ppIdFlavor flavour
+ppInternalProcLabel _ _ = Nothing
ppIdFlavor :: IdLabelInfo -> SDoc
ppIdFlavor x = pp_cSEP <> case x of
=====================================
compiler/GHC/CmmToAsm.hs
=====================================
@@ -1192,5 +1192,6 @@ initNCGConfig dflags this_mod = NCGConfig
, ncgDwarfEnabled = debugLevel dflags > 0
, ncgDwarfUnwindings = debugLevel dflags >= 1
, ncgDwarfStripBlockInfo = debugLevel dflags < 2 -- We strip out block information when running with -g0 or -g1.
+ , ncgExposeInternalSymbols = gopt Opt_ExposeInternalSymbols dflags
}
=====================================
compiler/GHC/CmmToAsm/Config.hs
=====================================
@@ -18,8 +18,7 @@ import GHC.Utils.Outputable
data NCGConfig = NCGConfig
{ ncgPlatform :: !Platform -- ^ Target platform
, ncgAsmContext :: !SDocContext -- ^ Context for ASM code generation
- , ncgThisModule :: !Module -- ^ The name of the module we are currently compiling (for generating debug information)
- -- See Note [Internal proc labels] in CLabel.
+ , ncgThisModule :: !Module -- ^ The name of the module we are currently compiling
, ncgProcAlignment :: !(Maybe Int) -- ^ Mandatory proc alignment
, ncgExternalDynamicRefs :: !Bool -- ^ Generate code to link against dynamic libraries
, ncgPIC :: !Bool -- ^ Enable Position-Independent Code
@@ -40,6 +39,7 @@ data NCGConfig = NCGConfig
, ncgDwarfEnabled :: !Bool -- ^ Enable Dwarf generation
, ncgDwarfUnwindings :: !Bool -- ^ Enable unwindings
, ncgDwarfStripBlockInfo :: !Bool -- ^ Strip out block information from generated Dwarf
+ , ncgExposeInternalSymbols :: !Bool -- ^ Expose symbol table entries for internal symbols
}
-- | Return Word size
=====================================
compiler/GHC/CmmToAsm/Monad.hs
=====================================
@@ -80,6 +80,8 @@ data NcgImpl statics instr jumpDest = NcgImpl {
canShortcut :: instr -> Maybe jumpDest,
shortcutStatics :: (BlockId -> Maybe jumpDest) -> statics -> statics,
shortcutJump :: (BlockId -> Maybe jumpDest) -> instr -> instr,
+ -- | 'Module' is only for printing internal labels. See Note [Internal proc
+ -- labels] in CLabel.
pprNatCmmDecl :: NatCmmDecl statics instr -> SDoc,
maxSpillSlots :: Int,
allocatableRegs :: [RealReg],
=====================================
compiler/GHC/CmmToAsm/X86/Ppr.hs
=====================================
@@ -90,6 +90,7 @@ pprNatCmmDecl config proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
-- special case for code without info table:
pprSectionAlign config (Section Text lbl) $$
pprProcAlignment config $$
+ pprProcLabel config lbl $$
pprLabel platform lbl $$ -- blocks guaranteed not null, so label needed
vcat (map (pprBasicBlock config top_info) blocks) $$
(if ncgDwarfEnabled config
@@ -99,6 +100,7 @@ pprNatCmmDecl config proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
Just (CmmStaticsRaw info_lbl _) ->
pprSectionAlign config (Section Text info_lbl) $$
pprProcAlignment config $$
+ pprProcLabel config lbl $$
(if platformHasSubsectionsViaSymbols platform
then pdoc platform (mkDeadStripPreventer info_lbl) <> char ':'
else empty) $$
@@ -114,6 +116,15 @@ pprNatCmmDecl config proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
else empty) $$
pprSizeDecl platform info_lbl
+-- | Output an internal proc label. See Note [Internal proc labels] in CLabel.
+pprProcLabel :: NCGConfig -> CLabel -> SDoc
+pprProcLabel config lbl
+ | ncgExposeInternalSymbols config
+ , Just lbl' <- ppInternalProcLabel (ncgThisModule config) lbl
+ = lbl' <> char ':'
+ | otherwise
+ = empty
+
-- | Output the ELF .size directive.
pprSizeDecl :: Platform -> CLabel -> SDoc
pprSizeDecl platform lbl
=====================================
compiler/GHC/Driver/Flags.hs
=====================================
@@ -274,6 +274,7 @@ data GeneralFlag
-- forwards all -L flags to the collect2 command without using a
-- response file and as such breaking apart.
| Opt_SingleLibFolder
+ | Opt_ExposeInternalSymbols
| Opt_KeepCAFs
| Opt_KeepGoing
| Opt_ByteCode
=====================================
compiler/GHC/Driver/Session.hs
=====================================
@@ -376,7 +376,6 @@ import qualified GHC.LanguageExtensions as LangExt
-- -----------------------------------------------------------------------------
-- DynFlags
-
-- | Used to differentiate the scope an include needs to apply to.
-- We have to split the include paths to avoid accidentally forcing recursive
-- includes since -I overrides the system search paths. See #14312.
@@ -3404,6 +3403,7 @@ fFlagsDeps = [
flagSpec "error-spans" Opt_ErrorSpans,
flagSpec "excess-precision" Opt_ExcessPrecision,
flagSpec "expose-all-unfoldings" Opt_ExposeAllUnfoldings,
+ flagSpec "expose-internal-symbols" Opt_ExposeInternalSymbols,
flagSpec "external-dynamic-refs" Opt_ExternalDynamicRefs,
flagSpec "external-interpreter" Opt_ExternalInterpreter,
flagSpec "flat-cache" Opt_FlatCache,
@@ -4392,7 +4392,13 @@ setVerbosity :: Maybe Int -> DynP ()
setVerbosity mb_n = upd (\dfs -> dfs{ verbosity = mb_n `orElse` 3 })
setDebugLevel :: Maybe Int -> DynP ()
-setDebugLevel mb_n = upd (\dfs -> dfs{ debugLevel = mb_n `orElse` 2 })
+setDebugLevel mb_n =
+ upd (\dfs -> exposeSyms $ dfs{ debugLevel = n })
+ where
+ n = mb_n `orElse` 2
+ exposeSyms
+ | n > 0 = setGeneralFlag' Opt_ExposeInternalSymbols
+ | otherwise = id
data PkgDbRef
= GlobalPkgDb
=====================================
docs/users_guide/debug-info.rst
=====================================
@@ -14,6 +14,7 @@ useable by most UNIX debugging tools.
:category: debugging
:since: 7.10, numeric levels since 8.0
+ :implies: :ghc-flag:`-fexpose-all-symbols`
Emit debug information in object code. Currently only DWARF debug
information is supported on x86-64 and i386. Currently debug levels 0
=====================================
docs/users_guide/phases.rst
=====================================
@@ -720,6 +720,20 @@ Options affecting code generation
all target platforms. See the :ghc-flag:`--print-object-splitting-supported`
flag to check whether your GHC supports object splitting.
+.. ghc-flag:: -fexpose-internal-symbols
+ :shortdesc: Produce symbols for all functions, including internal functions.
+ :type: dynamic
+ :category: codegen
+
+ Request that GHC emits verbose symbol tables which include local symbols
+ for module-internal functions. These can be useful for tools like
+ :ref:`perf <https://perf.wiki.kernel.org/>` but increase object file sizes.
+ This is implied by :ghc-flag:`-g2` and above.
+
+ :ghc-flag:`-fno-expose-internal-symbols` suppresses all non-global symbol
+ table entries, resulting in smaller object file sizes at the expense of
+ debuggability.
+
.. _options-linker:
Options affecting linking
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/566ebbd9ddd7695ce4cb1939d917747a769bc32b...6a977ea5f9ac560568a9a3e5bfa90c0936b25cb9
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/566ebbd9ddd7695ce4cb1939d917747a769bc32b...6a977ea5f9ac560568a9a3e5bfa90c0936b25cb9
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