AArch64 DWARF unwind + macOS os_signpost integration#176
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AArch64 DWARF unwind + macOS os_signpost integration#176angerman wants to merge 2 commits intostable-ghc-9.14from
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The AArch64 native code generator was silently discarding CmmUnwind nodes (`return nilOL`), making DWARF-based profiling and debugging impossible on Apple Silicon and ARM64 Linux. This commit adds full DWARF unwind support, mirroring the existing X86 implementation: - Add UNWIND pseudo-instruction to AArch64.Instr - Convert CmmUnwind nodes to UNWIND instructions in CodeGen - Emit UNWIND after DELTA via addSpUnwindings for SP tracking - Wire extractUnwindPoints into the AArch64 NcgImpl record - Pretty-print UNWIND as a label + comment in Ppr With this change, `ghc -g` on AArch64 produces .debug_frame entries, enabling `lldb` backtraces, `dwarfdump --debug-frame`, and sampler- based profilers (Instruments, Samply) to unwind through Haskell code.
GHC-compiled programs are invisible to Apple Instruments because the
RTS emits no os_signpost events. This makes it hard to correlate GC
pauses and thread scheduling with system-level activity on macOS.
Add a new Signpost.c/Signpost.h module that bridges RTS events to the
os_signpost API, using OS_LOG_CATEGORY_POINTS_OF_INTEREST so events
appear in Instruments by default without a custom .instrpkg:
- GC intervals: begin/end pairs tracked per-capability with unique
signpost IDs, emitting generation, bytes copied, and slop
- Thread lifecycle: create/run/stop as point events with cap and tid
- User events: traceEvent#/traceMarker# forwarded as signposts
All functions gate on os_signpost_enabled() so the overhead when
Instruments is not attached is near zero (a single branch on the
log handle's signpost-enabled flag).
On non-Darwin platforms, all functions compile to empty macros.
Integration points:
- Stats.c: GC begin/end with full statistics
- Trace.h/Trace.c: thread and user event forwarding
- RtsStartup.c: init after initScheduler, free before endTracing
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Summary
Two independent, complementary features that make GHC-compiled Haskell code visible to standard profiling tools on macOS:
Part 1: AArch64 DWARF Unwind Support (GHC #19913)
The AArch64 NCG was silently discarding
CmmUnwindnodes (return nilOL), producing no DWARF unwind information. This madelldb bt, Instruments, and Samply unable to unwind through Haskell frames on Apple Silicon.UNWINDpseudo-instruction to AArch64Instrdata type (mirrors X86)CmmUnwind→UNWINDconversion instmtToInstrsaddSpUnwindingsto emitUNWINDafterDELTA(tracks SP changes)extractUnwindPointsand wire intoNcgImpl(wasconst [])Part 2: macOS os_signpost Integration
The RTS had no
os_signpostsupport, making GC pauses, thread events, and user events invisible in Apple Instruments.rts/Signpost.{h,c}with os_signpost API wrapperstraceEvent#/traceMarker#)os_signpost_enabled()gate)Files Changed
Compiler (AArch64 DWARF):
compiler/GHC/CmmToAsm/AArch64/Instr.hs— UNWIND constructor + pattern matchescompiler/GHC/CmmToAsm/AArch64/CodeGen.hs— CmmUnwind handler, addSpUnwindings, extractUnwindPointscompiler/GHC/CmmToAsm/AArch64/Ppr.hs— UNWIND pretty-printingcompiler/GHC/CmmToAsm/AArch64.hs— Wire extractUnwindPoints into NcgImplRTS (os_signpost):
rts/Signpost.h— Header with Darwin functions / non-Darwin empty macrosrts/Signpost.c— os_signpost implementationrts/RtsStartup.c— initSignposts/freeSignposts lifecyclerts/Stats.c— GC begin/end signpost callsrts/Trace.h— Thread event signpost callsrts/Trace.c— User event signpost callsrts/rts.cabal— Add Signpost.c to buildTest plan
-gand verifydwarfdump --debug-frameshows FDE entrieslldband verifybtshows Haskell framestraceEvent/traceMarkerevents appear as signpost events