[experiment] add alternative wasm sqlite3 implementation available via build-tag (#2863)

This allows for building GoToSocial with [SQLite transpiled to WASM](https://github.com/ncruces/go-sqlite3) and accessed through [Wazero](https://wazero.io/).

1 files changed, 594 insertions, 0 deletions

diff --git a/vendor/github.com/tetratelabs/wazero/internal/engine/wazevo/frontend/frontend.go b/vendor/github.com/tetratelabs/wazero/internal/engine/wazevo/frontend/frontend.go
new file mode 100644
index 000000000..873a35a55
--- /dev/null
+++ b/vendor/github.com/tetratelabs/wazero/internal/engine/wazevo/frontend/frontend.go
@@ -0,0 +1,594 @@
+// Package frontend implements the translation of WebAssembly to SSA IR using the ssa package.
+package frontend
+
+import (
+	"bytes"
+	"math"
+
+	"github.com/tetratelabs/wazero/internal/engine/wazevo/ssa"
+	"github.com/tetratelabs/wazero/internal/engine/wazevo/wazevoapi"
+	"github.com/tetratelabs/wazero/internal/wasm"
+)
+
+// Compiler is in charge of lowering Wasm to SSA IR, and does the optimization
+// on top of it in architecture-independent way.
+type Compiler struct {
+	// Per-module data that is used across all functions.
+
+	m      *wasm.Module
+	offset *wazevoapi.ModuleContextOffsetData
+	// ssaBuilder is a ssa.Builder used by this frontend.
+	ssaBuilder             ssa.Builder
+	signatures             map[*wasm.FunctionType]*ssa.Signature
+	listenerSignatures     map[*wasm.FunctionType][2]*ssa.Signature
+	memoryGrowSig          ssa.Signature
+	memoryWait32Sig        ssa.Signature
+	memoryWait64Sig        ssa.Signature
+	memoryNotifySig        ssa.Signature
+	checkModuleExitCodeSig ssa.Signature
+	tableGrowSig           ssa.Signature
+	refFuncSig             ssa.Signature
+	memmoveSig             ssa.Signature
+	ensureTermination      bool
+
+	// Followings are reset by per function.
+
+	// wasmLocalToVariable maps the index (considered as wasm.Index of locals)
+	// to the corresponding ssa.Variable.
+	wasmLocalToVariable                   [] /* local index to */ ssa.Variable
+	wasmLocalFunctionIndex                wasm.Index
+	wasmFunctionTypeIndex                 wasm.Index
+	wasmFunctionTyp                       *wasm.FunctionType
+	wasmFunctionLocalTypes                []wasm.ValueType
+	wasmFunctionBody                      []byte
+	wasmFunctionBodyOffsetInCodeSection   uint64
+	memoryBaseVariable, memoryLenVariable ssa.Variable
+	needMemory                            bool
+	memoryShared                          bool
+	globalVariables                       []ssa.Variable
+	globalVariablesTypes                  []ssa.Type
+	mutableGlobalVariablesIndexes         []wasm.Index // index to ^.
+	needListener                          bool
+	needSourceOffsetInfo                  bool
+	// br is reused during lowering.
+	br            *bytes.Reader
+	loweringState loweringState
+
+	knownSafeBounds    [] /* ssa.ValueID to */ knownSafeBound
+	knownSafeBoundsSet []ssa.ValueID
+
+	knownSafeBoundsAtTheEndOfBlocks   [] /* ssa.BlockID to */ knownSafeBoundsAtTheEndOfBlock
+	varLengthKnownSafeBoundWithIDPool wazevoapi.VarLengthPool[knownSafeBoundWithID]
+
+	execCtxPtrValue, moduleCtxPtrValue ssa.Value
+
+	// Following are reused for the known safe bounds analysis.
+
+	pointers []int
+	bounds   [][]knownSafeBoundWithID
+}
+
+type (
+	// knownSafeBound represents a known safe bound for a value.
+	knownSafeBound struct {
+		// bound is a constant upper bound for the value.
+		bound uint64
+		// absoluteAddr is the absolute address of the value.
+		absoluteAddr ssa.Value
+	}
+	// knownSafeBoundWithID is a knownSafeBound with the ID of the value.
+	knownSafeBoundWithID struct {
+		knownSafeBound
+		id ssa.ValueID
+	}
+	knownSafeBoundsAtTheEndOfBlock = wazevoapi.VarLength[knownSafeBoundWithID]
+)
+
+var knownSafeBoundsAtTheEndOfBlockNil = wazevoapi.NewNilVarLength[knownSafeBoundWithID]()
+
+// NewFrontendCompiler returns a frontend Compiler.
+func NewFrontendCompiler(m *wasm.Module, ssaBuilder ssa.Builder, offset *wazevoapi.ModuleContextOffsetData, ensureTermination bool, listenerOn bool, sourceInfo bool) *Compiler {
+	c := &Compiler{
+		m:                                 m,
+		ssaBuilder:                        ssaBuilder,
+		br:                                bytes.NewReader(nil),
+		offset:                            offset,
+		ensureTermination:                 ensureTermination,
+		needSourceOffsetInfo:              sourceInfo,
+		varLengthKnownSafeBoundWithIDPool: wazevoapi.NewVarLengthPool[knownSafeBoundWithID](),
+	}
+	c.declareSignatures(listenerOn)
+	return c
+}
+
+func (c *Compiler) declareSignatures(listenerOn bool) {
+	m := c.m
+	c.signatures = make(map[*wasm.FunctionType]*ssa.Signature, len(m.TypeSection)+2)
+	if listenerOn {
+		c.listenerSignatures = make(map[*wasm.FunctionType][2]*ssa.Signature, len(m.TypeSection))
+	}
+	for i := range m.TypeSection {
+		wasmSig := &m.TypeSection[i]
+		sig := SignatureForWasmFunctionType(wasmSig)
+		sig.ID = ssa.SignatureID(i)
+		c.signatures[wasmSig] = &sig
+		c.ssaBuilder.DeclareSignature(&sig)
+
+		if listenerOn {
+			beforeSig, afterSig := SignatureForListener(wasmSig)
+			beforeSig.ID = ssa.SignatureID(i) + ssa.SignatureID(len(m.TypeSection))
+			afterSig.ID = ssa.SignatureID(i) + ssa.SignatureID(len(m.TypeSection))*2
+			c.listenerSignatures[wasmSig] = [2]*ssa.Signature{beforeSig, afterSig}
+			c.ssaBuilder.DeclareSignature(beforeSig)
+			c.ssaBuilder.DeclareSignature(afterSig)
+		}
+	}
+
+	begin := ssa.SignatureID(len(m.TypeSection))
+	if listenerOn {
+		begin *= 3
+	}
+	c.memoryGrowSig = ssa.Signature{
+		ID: begin,
+		// Takes execution context and the page size to grow.
+		Params: []ssa.Type{ssa.TypeI64, ssa.TypeI32},
+		// Returns the previous page size.
+		Results: []ssa.Type{ssa.TypeI32},
+	}
+	c.ssaBuilder.DeclareSignature(&c.memoryGrowSig)
+
+	c.checkModuleExitCodeSig = ssa.Signature{
+		ID: c.memoryGrowSig.ID + 1,
+		// Only takes execution context.
+		Params: []ssa.Type{ssa.TypeI64},
+	}
+	c.ssaBuilder.DeclareSignature(&c.checkModuleExitCodeSig)
+
+	c.tableGrowSig = ssa.Signature{
+		ID:     c.checkModuleExitCodeSig.ID + 1,
+		Params: []ssa.Type{ssa.TypeI64 /* exec context */, ssa.TypeI32 /* table index */, ssa.TypeI32 /* num */, ssa.TypeI64 /* ref */},
+		// Returns the previous size.
+		Results: []ssa.Type{ssa.TypeI32},
+	}
+	c.ssaBuilder.DeclareSignature(&c.tableGrowSig)
+
+	c.refFuncSig = ssa.Signature{
+		ID:     c.tableGrowSig.ID + 1,
+		Params: []ssa.Type{ssa.TypeI64 /* exec context */, ssa.TypeI32 /* func index */},
+		// Returns the function reference.
+		Results: []ssa.Type{ssa.TypeI64},
+	}
+	c.ssaBuilder.DeclareSignature(&c.refFuncSig)
+
+	c.memmoveSig = ssa.Signature{
+		ID: c.refFuncSig.ID + 1,
+		// dst, src, and the byte count.
+		Params: []ssa.Type{ssa.TypeI64, ssa.TypeI64, ssa.TypeI64},
+	}
+
+	c.ssaBuilder.DeclareSignature(&c.memmoveSig)
+
+	c.memoryWait32Sig = ssa.Signature{
+		ID: c.memmoveSig.ID + 1,
+		// exec context, timeout, expected, addr
+		Params: []ssa.Type{ssa.TypeI64, ssa.TypeI64, ssa.TypeI32, ssa.TypeI64},
+		// Returns the status.
+		Results: []ssa.Type{ssa.TypeI32},
+	}
+	c.ssaBuilder.DeclareSignature(&c.memoryWait32Sig)
+
+	c.memoryWait64Sig = ssa.Signature{
+		ID: c.memoryWait32Sig.ID + 1,
+		// exec context, timeout, expected, addr
+		Params: []ssa.Type{ssa.TypeI64, ssa.TypeI64, ssa.TypeI64, ssa.TypeI64},
+		// Returns the status.
+		Results: []ssa.Type{ssa.TypeI32},
+	}
+	c.ssaBuilder.DeclareSignature(&c.memoryWait64Sig)
+
+	c.memoryNotifySig = ssa.Signature{
+		ID: c.memoryWait64Sig.ID + 1,
+		// exec context, count, addr
+		Params: []ssa.Type{ssa.TypeI64, ssa.TypeI32, ssa.TypeI64},
+		// Returns the number notified.
+		Results: []ssa.Type{ssa.TypeI32},
+	}
+	c.ssaBuilder.DeclareSignature(&c.memoryNotifySig)
+}
+
+// SignatureForWasmFunctionType returns the ssa.Signature for the given wasm.FunctionType.
+func SignatureForWasmFunctionType(typ *wasm.FunctionType) ssa.Signature {
+	sig := ssa.Signature{
+		// +2 to pass moduleContextPtr and executionContextPtr. See the inline comment LowerToSSA.
+		Params:  make([]ssa.Type, len(typ.Params)+2),
+		Results: make([]ssa.Type, len(typ.Results)),
+	}
+	sig.Params[0] = executionContextPtrTyp
+	sig.Params[1] = moduleContextPtrTyp
+	for j, typ := range typ.Params {
+		sig.Params[j+2] = WasmTypeToSSAType(typ)
+	}
+	for j, typ := range typ.Results {
+		sig.Results[j] = WasmTypeToSSAType(typ)
+	}
+	return sig
+}
+
+// Init initializes the state of frontendCompiler and make it ready for a next function.
+func (c *Compiler) Init(idx, typIndex wasm.Index, typ *wasm.FunctionType, localTypes []wasm.ValueType, body []byte, needListener bool, bodyOffsetInCodeSection uint64) {
+	c.ssaBuilder.Init(c.signatures[typ])
+	c.loweringState.reset()
+
+	c.wasmFunctionTypeIndex = typIndex
+	c.wasmLocalFunctionIndex = idx
+	c.wasmFunctionTyp = typ
+	c.wasmFunctionLocalTypes = localTypes
+	c.wasmFunctionBody = body
+	c.wasmFunctionBodyOffsetInCodeSection = bodyOffsetInCodeSection
+	c.needListener = needListener
+	c.clearSafeBounds()
+	c.varLengthKnownSafeBoundWithIDPool.Reset()
+	c.knownSafeBoundsAtTheEndOfBlocks = c.knownSafeBoundsAtTheEndOfBlocks[:0]
+}
+
+// Note: this assumes 64-bit platform (I believe we won't have 32-bit backend ;)).
+const executionContextPtrTyp, moduleContextPtrTyp = ssa.TypeI64, ssa.TypeI64
+
+// LowerToSSA lowers the current function to SSA function which will be held by ssaBuilder.
+// After calling this, the caller will be able to access the SSA info in *Compiler.ssaBuilder.
+//
+// Note that this only does the naive lowering, and do not do any optimization, instead the caller is expected to do so.
+func (c *Compiler) LowerToSSA() {
+	builder := c.ssaBuilder
+
+	// Set up the entry block.
+	entryBlock := builder.AllocateBasicBlock()
+	builder.SetCurrentBlock(entryBlock)
+
+	// Functions always take two parameters in addition to Wasm-level parameters:
+	//
+	//  1. executionContextPtr: pointer to the *executionContext in wazevo package.
+	//    This will be used to exit the execution in the face of trap, plus used for host function calls.
+	//
+	// 	2. moduleContextPtr: pointer to the *moduleContextOpaque in wazevo package.
+	//	  This will be used to access memory, etc. Also, this will be used during host function calls.
+	//
+	// Note: it's clear that sometimes a function won't need them. For example,
+	//  if the function doesn't trap and doesn't make function call, then
+	// 	we might be able to eliminate the parameter. However, if that function
+	//	can be called via call_indirect, then we cannot eliminate because the
+	//  signature won't match with the expected one.
+	// TODO: maybe there's some way to do this optimization without glitches, but so far I have no clue about the feasibility.
+	//
+	// Note: In Wasmtime or many other runtimes, moduleContextPtr is called "vmContext". Also note that `moduleContextPtr`
+	//  is wazero-specific since other runtimes can naturally use the OS-level signal to do this job thanks to the fact that
+	//  they can use native stack vs wazero cannot use Go-routine stack and have to use Go-runtime allocated []byte as a stack.
+	c.execCtxPtrValue = entryBlock.AddParam(builder, executionContextPtrTyp)
+	c.moduleCtxPtrValue = entryBlock.AddParam(builder, moduleContextPtrTyp)
+	builder.AnnotateValue(c.execCtxPtrValue, "exec_ctx")
+	builder.AnnotateValue(c.moduleCtxPtrValue, "module_ctx")
+
+	for i, typ := range c.wasmFunctionTyp.Params {
+		st := WasmTypeToSSAType(typ)
+		variable := builder.DeclareVariable(st)
+		value := entryBlock.AddParam(builder, st)
+		builder.DefineVariable(variable, value, entryBlock)
+		c.setWasmLocalVariable(wasm.Index(i), variable)
+	}
+	c.declareWasmLocals(entryBlock)
+	c.declareNecessaryVariables()
+
+	c.lowerBody(entryBlock)
+}
+
+// localVariable returns the SSA variable for the given Wasm local index.
+func (c *Compiler) localVariable(index wasm.Index) ssa.Variable {
+	return c.wasmLocalToVariable[index]
+}
+
+func (c *Compiler) setWasmLocalVariable(index wasm.Index, variable ssa.Variable) {
+	idx := int(index)
+	if idx >= len(c.wasmLocalToVariable) {
+		c.wasmLocalToVariable = append(c.wasmLocalToVariable, make([]ssa.Variable, idx+1-len(c.wasmLocalToVariable))...)
+	}
+	c.wasmLocalToVariable[idx] = variable
+}
+
+// declareWasmLocals declares the SSA variables for the Wasm locals.
+func (c *Compiler) declareWasmLocals(entry ssa.BasicBlock) {
+	localCount := wasm.Index(len(c.wasmFunctionTyp.Params))
+	for i, typ := range c.wasmFunctionLocalTypes {
+		st := WasmTypeToSSAType(typ)
+		variable := c.ssaBuilder.DeclareVariable(st)
+		c.setWasmLocalVariable(wasm.Index(i)+localCount, variable)
+
+		zeroInst := c.ssaBuilder.AllocateInstruction()
+		switch st {
+		case ssa.TypeI32:
+			zeroInst.AsIconst32(0)
+		case ssa.TypeI64:
+			zeroInst.AsIconst64(0)
+		case ssa.TypeF32:
+			zeroInst.AsF32const(0)
+		case ssa.TypeF64:
+			zeroInst.AsF64const(0)
+		case ssa.TypeV128:
+			zeroInst.AsVconst(0, 0)
+		default:
+			panic("TODO: " + wasm.ValueTypeName(typ))
+		}
+
+		c.ssaBuilder.InsertInstruction(zeroInst)
+		value := zeroInst.Return()
+		c.ssaBuilder.DefineVariable(variable, value, entry)
+	}
+}
+
+func (c *Compiler) declareNecessaryVariables() {
+	if c.needMemory = c.m.MemorySection != nil; c.needMemory {
+		c.memoryShared = c.m.MemorySection.IsShared
+	} else if c.needMemory = c.m.ImportMemoryCount > 0; c.needMemory {
+		for _, imp := range c.m.ImportSection {
+			if imp.Type == wasm.ExternTypeMemory {
+				c.memoryShared = imp.DescMem.IsShared
+				break
+			}
+		}
+	}
+
+	if c.needMemory {
+		c.memoryBaseVariable = c.ssaBuilder.DeclareVariable(ssa.TypeI64)
+		c.memoryLenVariable = c.ssaBuilder.DeclareVariable(ssa.TypeI64)
+	}
+
+	c.globalVariables = c.globalVariables[:0]
+	c.mutableGlobalVariablesIndexes = c.mutableGlobalVariablesIndexes[:0]
+	c.globalVariablesTypes = c.globalVariablesTypes[:0]
+	for _, imp := range c.m.ImportSection {
+		if imp.Type == wasm.ExternTypeGlobal {
+			desc := imp.DescGlobal
+			c.declareWasmGlobal(desc.ValType, desc.Mutable)
+		}
+	}
+	for _, g := range c.m.GlobalSection {
+		desc := g.Type
+		c.declareWasmGlobal(desc.ValType, desc.Mutable)
+	}
+
+	// TODO: add tables.
+}
+
+func (c *Compiler) declareWasmGlobal(typ wasm.ValueType, mutable bool) {
+	var st ssa.Type
+	switch typ {
+	case wasm.ValueTypeI32:
+		st = ssa.TypeI32
+	case wasm.ValueTypeI64,
+		// Both externref and funcref are represented as I64 since we only support 64-bit platforms.
+		wasm.ValueTypeExternref, wasm.ValueTypeFuncref:
+		st = ssa.TypeI64
+	case wasm.ValueTypeF32:
+		st = ssa.TypeF32
+	case wasm.ValueTypeF64:
+		st = ssa.TypeF64
+	case wasm.ValueTypeV128:
+		st = ssa.TypeV128
+	default:
+		panic("TODO: " + wasm.ValueTypeName(typ))
+	}
+	v := c.ssaBuilder.DeclareVariable(st)
+	index := wasm.Index(len(c.globalVariables))
+	c.globalVariables = append(c.globalVariables, v)
+	c.globalVariablesTypes = append(c.globalVariablesTypes, st)
+	if mutable {
+		c.mutableGlobalVariablesIndexes = append(c.mutableGlobalVariablesIndexes, index)
+	}
+}
+
+// WasmTypeToSSAType converts wasm.ValueType to ssa.Type.
+func WasmTypeToSSAType(vt wasm.ValueType) ssa.Type {
+	switch vt {
+	case wasm.ValueTypeI32:
+		return ssa.TypeI32
+	case wasm.ValueTypeI64,
+		// Both externref and funcref are represented as I64 since we only support 64-bit platforms.
+		wasm.ValueTypeExternref, wasm.ValueTypeFuncref:
+		return ssa.TypeI64
+	case wasm.ValueTypeF32:
+		return ssa.TypeF32
+	case wasm.ValueTypeF64:
+		return ssa.TypeF64
+	case wasm.ValueTypeV128:
+		return ssa.TypeV128
+	default:
+		panic("TODO: " + wasm.ValueTypeName(vt))
+	}
+}
+
+// addBlockParamsFromWasmTypes adds the block parameters to the given block.
+func (c *Compiler) addBlockParamsFromWasmTypes(tps []wasm.ValueType, blk ssa.BasicBlock) {
+	for _, typ := range tps {
+		st := WasmTypeToSSAType(typ)
+		blk.AddParam(c.ssaBuilder, st)
+	}
+}
+
+// formatBuilder outputs the constructed SSA function as a string with a source information.
+func (c *Compiler) formatBuilder() string {
+	return c.ssaBuilder.Format()
+}
+
+// SignatureForListener returns the signatures for the listener functions.
+func SignatureForListener(wasmSig *wasm.FunctionType) (*ssa.Signature, *ssa.Signature) {
+	beforeSig := &ssa.Signature{}
+	beforeSig.Params = make([]ssa.Type, len(wasmSig.Params)+2)
+	beforeSig.Params[0] = ssa.TypeI64 // Execution context.
+	beforeSig.Params[1] = ssa.TypeI32 // Function index.
+	for i, p := range wasmSig.Params {
+		beforeSig.Params[i+2] = WasmTypeToSSAType(p)
+	}
+	afterSig := &ssa.Signature{}
+	afterSig.Params = make([]ssa.Type, len(wasmSig.Results)+2)
+	afterSig.Params[0] = ssa.TypeI64 // Execution context.
+	afterSig.Params[1] = ssa.TypeI32 // Function index.
+	for i, p := range wasmSig.Results {
+		afterSig.Params[i+2] = WasmTypeToSSAType(p)
+	}
+	return beforeSig, afterSig
+}
+
+// isBoundSafe returns true if the given value is known to be safe to access up to the given bound.
+func (c *Compiler) getKnownSafeBound(v ssa.ValueID) *knownSafeBound {
+	if int(v) >= len(c.knownSafeBounds) {
+		return nil
+	}
+	return &c.knownSafeBounds[v]
+}
+
+// recordKnownSafeBound records the given safe bound for the given value.
+func (c *Compiler) recordKnownSafeBound(v ssa.ValueID, safeBound uint64, absoluteAddr ssa.Value) {
+	if int(v) >= len(c.knownSafeBounds) {
+		c.knownSafeBounds = append(c.knownSafeBounds, make([]knownSafeBound, v+1)...)
+	}
+
+	if exiting := c.knownSafeBounds[v]; exiting.bound == 0 {
+		c.knownSafeBounds[v] = knownSafeBound{
+			bound:        safeBound,
+			absoluteAddr: absoluteAddr,
+		}
+		c.knownSafeBoundsSet = append(c.knownSafeBoundsSet, v)
+	} else if safeBound > exiting.bound {
+		c.knownSafeBounds[v].bound = safeBound
+	}
+}
+
+// clearSafeBounds clears the known safe bounds.
+func (c *Compiler) clearSafeBounds() {
+	for _, v := range c.knownSafeBoundsSet {
+		ptr := &c.knownSafeBounds[v]
+		ptr.bound = 0
+		ptr.absoluteAddr = ssa.ValueInvalid
+	}
+	c.knownSafeBoundsSet = c.knownSafeBoundsSet[:0]
+}
+
+// resetAbsoluteAddressInSafeBounds resets the absolute addresses recorded in the known safe bounds.
+func (c *Compiler) resetAbsoluteAddressInSafeBounds() {
+	for _, v := range c.knownSafeBoundsSet {
+		ptr := &c.knownSafeBounds[v]
+		ptr.absoluteAddr = ssa.ValueInvalid
+	}
+}
+
+func (k *knownSafeBound) valid() bool {
+	return k != nil && k.bound > 0
+}
+
+func (c *Compiler) allocateVarLengthValues(_cap int, vs ...ssa.Value) ssa.Values {
+	builder := c.ssaBuilder
+	pool := builder.VarLengthPool()
+	args := pool.Allocate(_cap)
+	args = args.Append(builder.VarLengthPool(), vs...)
+	return args
+}
+
+func (c *Compiler) finalizeKnownSafeBoundsAtTheEndOfBlock(bID ssa.BasicBlockID) {
+	_bID := int(bID)
+	if l := len(c.knownSafeBoundsAtTheEndOfBlocks); _bID >= l {
+		c.knownSafeBoundsAtTheEndOfBlocks = append(c.knownSafeBoundsAtTheEndOfBlocks,
+			make([]knownSafeBoundsAtTheEndOfBlock, _bID+1-len(c.knownSafeBoundsAtTheEndOfBlocks))...)
+		for i := l; i < len(c.knownSafeBoundsAtTheEndOfBlocks); i++ {
+			c.knownSafeBoundsAtTheEndOfBlocks[i] = knownSafeBoundsAtTheEndOfBlockNil
+		}
+	}
+	p := &c.varLengthKnownSafeBoundWithIDPool
+	size := len(c.knownSafeBoundsSet)
+	allocated := c.varLengthKnownSafeBoundWithIDPool.Allocate(size)
+	// Sort the known safe bounds by the value ID so that we can use the intersection algorithm in initializeCurrentBlockKnownBounds.
+	sortSSAValueIDs(c.knownSafeBoundsSet)
+	for _, vID := range c.knownSafeBoundsSet {
+		kb := c.knownSafeBounds[vID]
+		allocated = allocated.Append(p, knownSafeBoundWithID{
+			knownSafeBound: kb,
+			id:             vID,
+		})
+	}
+	c.knownSafeBoundsAtTheEndOfBlocks[bID] = allocated
+	c.clearSafeBounds()
+}
+
+func (c *Compiler) initializeCurrentBlockKnownBounds() {
+	currentBlk := c.ssaBuilder.CurrentBlock()
+	switch preds := currentBlk.Preds(); preds {
+	case 0:
+	case 1:
+		pred := currentBlk.Pred(0).ID()
+		for _, kb := range c.getKnownSafeBoundsAtTheEndOfBlocks(pred).View() {
+			// Unless the block is sealed, we cannot assume the absolute address is valid:
+			// later we might add another predecessor that has no visibility of that value.
+			addr := ssa.ValueInvalid
+			if currentBlk.Sealed() {
+				addr = kb.absoluteAddr
+			}
+			c.recordKnownSafeBound(kb.id, kb.bound, addr)
+		}
+	default:
+		c.pointers = c.pointers[:0]
+		c.bounds = c.bounds[:0]
+		for i := 0; i < preds; i++ {
+			c.bounds = append(c.bounds, c.getKnownSafeBoundsAtTheEndOfBlocks(currentBlk.Pred(i).ID()).View())
+			c.pointers = append(c.pointers, 0)
+		}
+
+		// If there are multiple predecessors, we need to find the intersection of the known safe bounds.
+
+	outer:
+		for {
+			smallestID := ssa.ValueID(math.MaxUint32)
+			for i, ptr := range c.pointers {
+				if ptr >= len(c.bounds[i]) {
+					break outer
+				}
+				cb := &c.bounds[i][ptr]
+				if id := cb.id; id < smallestID {
+					smallestID = cb.id
+				}
+			}
+
+			// Check if current elements are the same across all lists.
+			same := true
+			minBound := uint64(math.MaxUint64)
+			for i := 0; i < preds; i++ {
+				cb := &c.bounds[i][c.pointers[i]]
+				if cb.id != smallestID {
+					same = false
+					break
+				} else {
+					if cb.bound < minBound {
+						minBound = cb.bound
+					}
+				}
+			}
+
+			if same { // All elements are the same.
+				// Absolute address cannot be used in the intersection since the value might be only defined in one of the predecessors.
+				c.recordKnownSafeBound(smallestID, minBound, ssa.ValueInvalid)
+			}
+
+			// Move pointer(s) for the smallest ID forward (if same, move all).
+			for i := 0; i < preds; i++ {
+				cb := &c.bounds[i][c.pointers[i]]
+				if cb.id == smallestID {
+					c.pointers[i]++
+				}
+			}
+		}
+	}
+}
+
+func (c *Compiler) getKnownSafeBoundsAtTheEndOfBlocks(id ssa.BasicBlockID) knownSafeBoundsAtTheEndOfBlock {
+	if int(id) >= len(c.knownSafeBoundsAtTheEndOfBlocks) {
+		return knownSafeBoundsAtTheEndOfBlockNil
+	}
+	return c.knownSafeBoundsAtTheEndOfBlocks[id]
+}