Stack unwinding (#18163)
This MR enables GHC to provide backtraces/stacktraces by using the Info Table Provenance Entry Table (IPE table). For this neither DWARF nor profiling support is used.
The basic idea is:
- The IPE table provides a map from Info Table pointers (addresses) to a descriptive source locations
- GHC emits an IPE for every return frame at compile time
- To get a stacktrace:
- Clone the stack (to avoid concurrent modification issues)
- Traverse the cloned stack and lookup the IPE for every return frame's info table (pointer)
The best entry to this MR is the user facing API: libraries/base/GHC/Stack/CloneStack.hs
A Quick Summary of the API
libraries/base/GHC/Stack/CloneStack.hs reduced to the most important type signatures:
-- | A frozen snapshot of the state of an execution stack.
--
-- @since 2.16.0.0
data StackSnapshot = StackSnapshot !StackSnapshot#
foreign import prim "stg_decodeStackzh" decodeStack# :: StackSnapshot# -> State# RealWorld -> (# State# RealWorld, Array# (Ptr InfoProvEnt) #)
foreign import prim "stg_cloneMyStackzh" cloneMyStack# :: State# RealWorld -> (# State# RealWorld, StackSnapshot# #)
foreign import prim "stg_sendCloneStackMessagezh" sendCloneStackMessage# :: ThreadId# -> StablePtr# PrimMVar -> State# RealWorld -> (# State# RealWorld, (# #) #)
-- | Clone the stack of the executing thread
--
-- @since 2.16.0.0
cloneMyStack :: IO StackSnapshot
-- | Clone the stack of a thread identified by its 'ThreadId'
--
-- @since 2.16.0.0
cloneThreadStack :: ThreadId -> IO StackSnapshot
-- | Represetation for the source location where a return frame was pushed on the stack.
-- This happens every time when a @case ... of@ scrutinee is evaluated.
data StackEntry = StackEntry
{ functionName :: String,
moduleName :: String,
srcLoc :: String,
closureType :: Word
}
deriving (Show, Eq)
-- | Decode a 'StackSnapshot' to a stacktrace (a list of 'StackEntry').
-- The stacktrace is created from return frames with according 'InfoProv'
-- entries. To generate them, use the GHC flag @-finfo-table-map@. If there are
-- no 'InfoProv' entries, an empty list is returned.
--
-- @since 2.16.0.0
decode :: StackSnapshot -> IO [StackEntry]Source Notes
The approach is documented in several source notes. The reviewer might want to read them after understanding the API described before.
- Stack Cloning
- Stack Decoding
- Stacktraces from Info Table Provenance Entries (IPE based stack unwinding)
Tests
The tests provide working examples of the API usage. Most significant is testsuite/tests/rts/decodeMyStack.hs.
The initial design was described by @bgamari in this comment: #18741 (comment 342188)
To get meaningful source locations from GHC's libraries, you need to use -finfo-table-map for them in Hadrian.
For source locations in user code, it's sufficient to compile it with -finfo-table-map (no special build of GHC needed).
ToDo
-
cloneStack#should be an ordinary C function (no primop) -
Update Notes to describe the C interface -
Decide if cloning and decoding should be two separate operations or not -
Use Array#instead ofMutableArray#to return decoding results -
Return an array of Ptr InfoProvEnton decoding (dolookupIPEdirectly in the RTS while decoding)
Merge request reports
Activity
68 72 freeStablePtr ptr 69 73 takeMVar resultVar 70 74 75 -- TODO: Cannot use `import GHC.Exts.Heap (StgInfoTable(..))` -> hidden package 76 type InfoTable = Word changed this line in version 3 of the diff
@bgamari ,
I went into another direction and declared this as
data InfoTable(without any constructors)
This is because it's not always a
StgInfoTable*, this is only true if the build is non-"Tables Next to Code". Otherwise ("Tables Next to Code"), it's a pointer toStgInfoTable->code.If someone would try to deserialize the latter case (e.g. with
ghc-heap), the result would befunnyunpredictable.
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- e78bed6d - Generate InfoTable provenances after the Cmm Pipeline ran
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56a7ee48...0b398d55 - 88 commits from branch
master - 1d2f31c9 - deriveConstants: Add hie.yaml
- 5ea2cf72 - base: Generalize newStablePtrPrimMVar
- fe0ab407 - Introduce snapshotting of thread's own stack
- 23e6e2aa - Introduce cloning of other threads' stacks
- 664e65ca - Add another test for stack cloning
- 1b10762c - Assure that cloned RET_BIG closures are valid (#18741 (closed))
- 7ba6ac9f - Provide StgInfoTable* as list of pointers
- 5a73b7e1 - Generate InfoTable provenances after the Cmm Pipeline ran
- 1de4cb8c - Cleanup
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56a7ee48...0b398d55 - 88 commits from branch
added 1 commit
- cafd1d3a - Emit meaningful provenances for return frame info tables
assigned to @supersven
unassigned @supersven
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- 60b7715a - Ensure that no provenance entries are emitted when -finfo-table-map is turned off
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60b7715a...d2399a46 - 31 commits from branch
master - 0a2face3 - deriveConstants: Add hie.yaml
- 19eeeda8 - base: Generalize newStablePtrPrimMVar
- 89cfdf15 - Introduce snapshotting of thread's own stack
- 372389fd - Introduce cloning of other threads' stacks
- a9d5504a - Add another test for stack cloning
- 475ca2e2 - Assure that cloned RET_BIG closures are valid (#18741 (closed))
- cd73930f - Provide StgInfoTable* as list of pointers
- 39b17fec - Generate InfoTable provenances after the Cmm Pipeline ran
- d062fb94 - Cleanup
- d10dcc95 - Emit meaningful provenances for return frame info tables
- 65b13bbd - Cleanup
- a84689b1 - Extract GHC.Driver.GenerateCgIPEStub
- d9a0e202 - Filter for CmmInfoTables with stack representation
- b2d0ea86 - Format tests
- f11024fe - Implement decode
- cb5aa8a1 - Ensure that no provenance entries are emitted when -finfo-table-map is turned off
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60b7715a...d2399a46 - 31 commits from branch
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- 09127151 - Fix: Emit info table provenances not only for stack replicated info tables
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- 034b7f1d - Only lookup tickishes for stack rep info tables, only emit IPEs when...