939f03eb0e
Records how the landed implementation differs from the plan: a single unified hoist region (EmitArgHoist) for open-array literals, record-call sret buffers and string pin slots, and shape-based protection at unknown-signature sites instead of a TInterfaceTypeDesc const-flag extension.
100 lines
4.8 KiB
Plaintext
100 lines
4.8 KiB
Plaintext
= Porting shape-aware const-string argument ARC to the native backend
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:toc: macro
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:toclevels: 2
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== Status
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IMPLEMENTED 2026-06-11 — the port landed atomically in the native
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backend along the lines below, with one major design improvement: a
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single unified hoist region (EmitArgHoist) carries open-array literal
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blocks, record-call argument buffers AND const-string pin slots, which
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eliminated the sret-buffer/argument-slot interleaving bug as a side
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effect. At unknown-signature sites (interface dispatch) every
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string-typed value argument is protected by shape — pin/consume are
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correct on top of a value-param callee pair too, so no
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TInterfaceTypeDesc const-flag extension was needed (the var flags it
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already carries mark positions to skip). Validated: full suite both
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backends, FIXPOINT_OK, NATIVE_FIXPOINT_OK, natively-compiled
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TestRunner (two pre-existing generic-template round-trip failures
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remain — see bugs.txt).
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The original analysis below is kept for the record.
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The QBE backend gained shape-aware
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const-string argument handling (commits 91f44ec, 17c14b3): literals and
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named consts borrow (no ARC ops), owned returns are consumed (one
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post-call release), plain locals/params borrow behind aliasing
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exclusions, everything else pins. Combined with the const conversion of
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the hot TStringList signatures this removed most caller/callee ARC pairs
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on the compiler's hottest paths (self-compile 0.78 s -> 0.60 s).
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The native backend currently uses the OPPOSITE convention: the callee
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retains const string params at entry and releases at exit
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(blaise.codegen.native.x86_64.pas, the entry/exit passes in
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EmitFunctionDef). That convention was chosen deliberately during the
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native TestRunner bring-up: it protects rc=0 transients at EVERY call
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site with zero per-site machinery, because the protection lives in the
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callee.
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== Why the port must land atomically
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Flipping the native callee to skip the const-string pair makes EVERY
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call path responsible for caller-side protection. Any path that passes
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a const-string argument without protection reintroduces the exact heap
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corruption fixed during the TestRunner bring-up (rc=0 concat transient
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freed mid-call). The native backend has more call-emission paths than
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the QBE backend:
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. The ~10 standard push-loop sites already funnelled through
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BeginCallArgs/PushCallArg/EndCallArgs (open-array hoist machinery) —
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straightforward to extend.
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. EmitCall's two inline strategies (≤6 push/pop and >6 slot-block) —
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extend like the open-array literal hoist.
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. EmitInterfaceCall — args are pushed via EmitMethodArgPush(nil, Arg):
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the TMethodParam is nil, so const-ness is invisible at the call site
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today. The information EXISTS (TInterfaceTypeDesc carries method
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param signatures, incl. var-param flags) but is not wired through.
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Without this wiring, interface method implementors whose params are
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const would be called unprotected.
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. EmitCallIndirect / EmitMethodPtrCall — TProceduralTypeDesc signatures
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available; same wiring needed.
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. EmitIntfSretCall — no call frame (the sret buffer must sit exactly at
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%rsp at call time); needs its own pin-slot region or a loud
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unsupported-shape error.
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== Mechanism (per-site pin slots)
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Extend TOALCallFrame:
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PinDepths: TList<Integer>; { per-arg slot depth in the pre-region; -1 = none }
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PinModes: TList<Integer>; { 1 = pin (AddRef+Release), 2 = consume (Release) }
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BeginCallArgs phase A: after hoisting open-array literal blocks, reserve
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one zeroed slot (pushq $0, Total += 8) per const-string argument whose
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NativeConstArgMode is pin or consume. PushCallArg: after the value is
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pushed, copy it into its slot at (Total - PinDepth + Pushed)(%rsp); for
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pin mode also AddRef it. EndCallArgs: after the callq, before the
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region addq — release every reserved slot (release of a still-zero slot
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is a nil no-op). The release loop must preserve the call's return
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registers: %rax, %rdx (reg-class record returns) and %xmm0/%xmm1
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(SSE-class record returns) around the _StringRelease calls.
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NativeConstArgMode mirrors the QBE classification. For borrowed locals
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the address-taken walker (CollectAddressTaken + the nested-capture
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union) must be shared — it is pure AST logic; move it out of
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TCodeGenQBE into a common unit rather than duplicating it.
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== Validation requirements
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The full QBE-side lesson applies: a codegen-rule change can pass the
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fixpoint while a second-generation binary breaks elsewhere. Required:
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two self-hosted generations, gen-2 `--backend native` emission of the
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full compiler, the native-runner suites (127/134 currently green), the
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e2e suite on both backends, and both fixpoints.
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== Expected gain
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Lower than the QBE side's, and it accrues to native-COMPILED programs,
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not to compiler build times (the compiler runs as a QBE-built binary).
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Worth doing for output-code quality once the wiring above is in place.
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