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Review: fold trivial constant interface boxing
Solid optimization overall — the static itab / static ifacebox globals replace runtime NewItab and AllocU for compile-time constants, and the interequal fix correctly keeps T2I (static itab) and I2I (NewItab) representations of the same iface equal by comparing the (inter, _type) pair with proper nil guards. Verified: the removed LTO/compiler.used template comments are no longer stale, the staticItab hash matches the concrete type descriptor, and boxed constants are safely immutable under Go semantics (assertions copy out; no addressable path reaches the read-only box).
Two correctness concerns worth addressing before merge, plus one minor doc nit — inline.
| case *ssa.ChangeType: | ||
| chain[t] = true | ||
| v = t.X | ||
| case *ssa.Convert: |
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[P1] Convert in fold chain reinterprets the raw constant under the wrong type
analyzeTrivialIfaceBox accepts *ssa.Convert in the chain and returns concrete = mi.X.Type() (the type after the conversion), while foldConstantMakeValue then applies the callee argument's raw constant via b.Const(c.Value, concrete) at cl/constiface.go:50 — skipping the conversion arithmetic.
This is correct only for representation-preserving conversions. For a value-changing Convert it is unsound. Example: func box(x int32) any { return any(string(x)) } called as box(65). Here concrete is string but c.Value is the integer 65; b.Const dispatches on the target kind (ssa/expr.go), hits the types.String branch, and calls constant.StringVal on an integer constant — producing a wrong result or a panic, where real Go yields any("A"). Numeric narrowing/widening and int↔float Convert have the same hazard.
*ssa.Convert in go/ssa is exactly the set of value-changing conversions, so including it here is unsafe. Suggest dropping *ssa.Convert from the recognized chain (keeping only ChangeType/ChangeInterface, which are representation-identical), or skip folding whenever a Convert is present.
| if g.impl.IsNil() { | ||
| return llvm.Value{}, false | ||
| } | ||
| g.impl.SetGlobalConstant(true) |
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[P1] staticIfaceBox mutates the shared zero-sized sentinel for zero-sized types
For a zero-sized value type (e.g. struct{}, [0]T, or a named type over them — none are directIfaceType, so MakeInterface routes them here), NewVarEx → doNewVarEx does not create a dedicated global. It returns the shared module zero-sized sentinel (__llgo.moduleZeroSizedAlloc$) as an isZeroSizedAlias Global (ssa/decl.go:209-219).
g.Init(x) is a no-op for that alias, but the following lines are not guarded and run on the shared sentinel:
g.impl.SetGlobalConstant(true)
g.impl.SetUnnamedAddr(true)
b.Pkg.setODRLinkage(g.impl, llvm.WeakODRLinkage)Boxing any zero-sized constant thus flips the shared sentinel to GlobalConstant and rewrites its linkage (from LinkOnceODRLinkage, or re-COMDATs it on Windows) — a cross-cutting side effect on the address handed out for all zero-sized allocations in the module. It also pollutes the _llgo_ifacebox$ dedup cache with an entry pointing at the sentinel.
Suggest bailing out (return false) before the constant/linkage mutations when the resulting Global is a zero-sized alias — the isZeroSizedAlias field already exists on aGlobal, or guard on TypeAllocSize(storageType)==0. Zero-sized boxes don't benefit from this optimization anyway.
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Known concrete-to-interface conversions emit a read-only _llgo_itab$
global (Inter, Type, Hash, Fun) with weak_odr linkage, matching
cmd/compile's go:itab symbols.
Ordinary builds use that global as the runtime vtable. The T2I site is
the only reference, so --gc-sections/-dead_strip can drop itabs that
belong to dead functions. LTO and deadcode-drop keep calling NewItab so
unused interface methods can be dropped; Fun[] would pin them. LTO still
emits the static template for de-virt and erases unused templates after
the plugin runs.
Non-direct interface values (integers, bools, small aggregates) still
use IfaceIndir, matching Go: data is a pointer to a copy. Compile-time
constants go in a WeakODR _llgo_ifacebox$ global instead of AllocU,
like cmd/compile's static temps.
Box names hash the constant's bits, not LLVM's printed form, so
same-length strings in different packages do not collide under
Windows COMDAT. Repeated boxing of the same LLVM value reuses a
per-package pointer cache. Zero-sized constants stay AllocU so boxing
does not mutate the shared sentinel.
Together, a constant T2I is {itab, box} with no runtime call or heap
allocation. Hash is copied from the type descriptor when present.
interequal compares the (inter, _type) pair so a static itab and a
dynamically allocated itab for the same conversion compare equal.
A call whose SSA body is MakeInterface of a Convert/ChangeType of a
constant parameter is lowered at the call site to MakeInterface of
that concrete value. No function-name matching: constant.MakeInt64
and user helpers such as boxMyInt(x int64) any { return myInt(x) }
use the same path.
Depends on static itabs and iface boxes so the folded value is a
compile-time {itab, box} pair.
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Summary
A call whose SSA body is
MakeInterfaceof aConvert/ChangeTypeof a constant parameter is lowered at the call site toMakeInterfaceof that concrete value.The callee is recognized by that SSA shape (single block, no
//go:noinline), not by package path or function name.constant.MakeInt64/MakeBoolmatch because they arereturn int64Val(x)/return boolVal(b). User helpers such asfunc boxMyInt(x int64) any { return myInt(x) }use the same path. Functions with extra control flow (MakeString) are left as real calls.Depends on: #2704 (static itabs + iface boxes). Without those, this fold would emit
AllocU+NewItabper call. Merge after #2704. This branch contains #2704 plus this commit; only the latest commit is in scope.Test plan
cl/_testrt/constiface(MakeInt64,MakeBool, and a localboxMyIntall become{itab, box})