463 строки
16 КиБ
Go
463 строки
16 КиБ
Go
package compiler
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// This file provides IR transformations necessary for precise and portable
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// garbage collectors.
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import (
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"go/token"
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"math/big"
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"golang.org/x/tools/go/ssa"
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"tinygo.org/x/go-llvm"
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)
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// trackExpr inserts pointer tracking intrinsics for the GC if the expression is
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// one of the expressions that need this.
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func (c *Compiler) trackExpr(frame *Frame, expr ssa.Value, value llvm.Value) {
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// There are uses of this expression, Make sure the pointers
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// are tracked during GC.
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switch expr := expr.(type) {
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case *ssa.Alloc, *ssa.MakeChan, *ssa.MakeMap:
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// These values are always of pointer type in IR.
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c.trackPointer(value)
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case *ssa.Call, *ssa.Convert, *ssa.MakeClosure, *ssa.MakeInterface, *ssa.MakeSlice, *ssa.Next:
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if !value.IsNil() {
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c.trackValue(value)
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}
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case *ssa.Select:
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if alloca, ok := frame.selectRecvBuf[expr]; ok {
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if alloca.IsAUndefValue().IsNil() {
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c.trackPointer(alloca)
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}
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}
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case *ssa.UnOp:
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switch expr.Op {
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case token.MUL:
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// Pointer dereference.
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c.trackValue(value)
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case token.ARROW:
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// Channel receive operator.
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// It's not necessary to look at commaOk here, because in that
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// case it's just an aggregate and trackValue will extract the
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// pointer in there (if there is one).
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c.trackValue(value)
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}
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}
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}
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// trackValue locates pointers in a value (possibly an aggregate) and tracks the
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// individual pointers
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func (c *Compiler) trackValue(value llvm.Value) {
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typ := value.Type()
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switch typ.TypeKind() {
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case llvm.PointerTypeKind:
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c.trackPointer(value)
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case llvm.StructTypeKind:
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if !typeHasPointers(typ) {
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return
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}
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numElements := typ.StructElementTypesCount()
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for i := 0; i < numElements; i++ {
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subValue := c.builder.CreateExtractValue(value, i, "")
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c.trackValue(subValue)
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}
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case llvm.ArrayTypeKind:
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if !typeHasPointers(typ) {
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return
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}
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numElements := typ.ArrayLength()
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for i := 0; i < numElements; i++ {
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subValue := c.builder.CreateExtractValue(value, i, "")
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c.trackValue(subValue)
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}
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}
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}
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// trackPointer creates a call to runtime.trackPointer, bitcasting the poitner
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// first if needed. The input value must be of LLVM pointer type.
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func (c *Compiler) trackPointer(value llvm.Value) {
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if value.Type() != c.i8ptrType {
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value = c.builder.CreateBitCast(value, c.i8ptrType, "")
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}
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c.createRuntimeCall("trackPointer", []llvm.Value{value}, "")
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}
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// typeHasPointers returns whether this type is a pointer or contains pointers.
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// If the type is an aggregate type, it will check whether there is a pointer
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// inside.
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func typeHasPointers(t llvm.Type) bool {
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switch t.TypeKind() {
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case llvm.PointerTypeKind:
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return true
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case llvm.StructTypeKind:
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for _, subType := range t.StructElementTypes() {
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if typeHasPointers(subType) {
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return true
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}
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}
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return false
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case llvm.ArrayTypeKind:
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if typeHasPointers(t.ElementType()) {
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return true
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}
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return false
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default:
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return false
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}
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}
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// makeGCStackSlots converts all calls to runtime.trackPointer to explicit
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// stores to stack slots that are scannable by the GC.
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func (c *Compiler) makeGCStackSlots() bool {
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// Check whether there are allocations at all.
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alloc := c.mod.NamedFunction("runtime.alloc")
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if alloc.IsNil() {
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// Nothing to. Make sure all remaining bits and pieces for stack
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// chains are neutralized.
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for _, call := range getUses(c.mod.NamedFunction("runtime.trackPointer")) {
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call.EraseFromParentAsInstruction()
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}
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stackChainStart := c.mod.NamedGlobal("runtime.stackChainStart")
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if !stackChainStart.IsNil() {
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stackChainStart.SetInitializer(llvm.ConstNull(stackChainStart.Type().ElementType()))
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stackChainStart.SetGlobalConstant(true)
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}
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return false
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}
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trackPointer := c.mod.NamedFunction("runtime.trackPointer")
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if trackPointer.IsNil() || trackPointer.FirstUse().IsNil() {
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return false // nothing to do
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}
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// Look at *all* functions to see whether they are free of function pointer
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// calls.
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// This takes less than 5ms for ~100kB of WebAssembly but would perhaps be
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// faster when written in C++ (to avoid the CGo overhead).
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funcsWithFPCall := map[llvm.Value]struct{}{}
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n := 0
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for fn := c.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
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n++
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if _, ok := funcsWithFPCall[fn]; ok {
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continue // already found
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}
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done := false
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for bb := fn.FirstBasicBlock(); !bb.IsNil() && !done; bb = llvm.NextBasicBlock(bb) {
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for call := bb.FirstInstruction(); !call.IsNil() && !done; call = llvm.NextInstruction(call) {
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if call.IsACallInst().IsNil() {
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continue // only looking at calls
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}
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called := call.CalledValue()
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if !called.IsAFunction().IsNil() {
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continue // only looking for function pointers
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}
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funcsWithFPCall[fn] = struct{}{}
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markParentFunctions(funcsWithFPCall, fn)
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done = true
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}
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}
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}
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// Determine which functions need stack objects. Many leaf functions don't
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// need it: it only causes overhead for them.
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// Actually, in one test it was only able to eliminate stack object from 12%
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// of functions that had a call to runtime.trackPointer (8 out of 68
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// functions), so this optimization is not as big as it may seem.
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allocatingFunctions := map[llvm.Value]struct{}{} // set of allocating functions
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// Work from runtime.alloc and trace all parents to check which functions do
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// a heap allocation (and thus which functions do not).
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markParentFunctions(allocatingFunctions, alloc)
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// Also trace all functions that call a function pointer.
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for fn := range funcsWithFPCall {
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// Assume that functions that call a function pointer do a heap
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// allocation as a conservative guess because the called function might
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// do a heap allocation.
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allocatingFunctions[fn] = struct{}{}
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markParentFunctions(allocatingFunctions, fn)
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}
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// Collect some variables used below in the loop.
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stackChainStart := c.mod.NamedGlobal("runtime.stackChainStart")
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if stackChainStart.IsNil() {
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// This may be reached in a weird scenario where we call runtime.alloc but the garbage collector is unreachable.
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// This can be accomplished by allocating 0 bytes.
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// There is no point in tracking anything.
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for _, use := range getUses(trackPointer) {
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use.EraseFromParentAsInstruction()
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}
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return false
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}
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stackChainStartType := stackChainStart.Type().ElementType()
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stackChainStart.SetInitializer(llvm.ConstNull(stackChainStartType))
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// Iterate until runtime.trackPointer has no uses left.
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for use := trackPointer.FirstUse(); !use.IsNil(); use = trackPointer.FirstUse() {
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// Pick the first use of runtime.trackPointer.
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call := use.User()
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if call.IsACallInst().IsNil() {
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panic("expected runtime.trackPointer use to be a call")
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}
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// Pick the parent function.
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fn := call.InstructionParent().Parent()
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if _, ok := allocatingFunctions[fn]; !ok {
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// This function nor any of the functions it calls (recursively)
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// allocate anything from the heap, so it will not trigger a garbage
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// collection cycle. Thus, it does not need to track local pointer
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// values.
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// This is a useful optimization but not as big as you might guess,
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// as described above (it avoids stack objects for ~12% of
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// functions).
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call.EraseFromParentAsInstruction()
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continue
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}
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// Find all calls to runtime.trackPointer in this function.
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var calls []llvm.Value
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var returns []llvm.Value
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for bb := fn.FirstBasicBlock(); !bb.IsNil(); bb = llvm.NextBasicBlock(bb) {
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for inst := bb.FirstInstruction(); !inst.IsNil(); inst = llvm.NextInstruction(inst) {
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switch inst.InstructionOpcode() {
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case llvm.Call:
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if inst.CalledValue() == trackPointer {
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calls = append(calls, inst)
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}
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case llvm.Ret:
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returns = append(returns, inst)
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}
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}
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}
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// Determine what to do with each call.
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var allocas, pointers []llvm.Value
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for _, call := range calls {
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ptr := call.Operand(0)
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call.EraseFromParentAsInstruction()
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if ptr.IsAInstruction().IsNil() {
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continue
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}
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// Some trivial optimizations.
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if ptr.IsAInstruction().IsNil() {
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continue
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}
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switch ptr.InstructionOpcode() {
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case llvm.PHI, llvm.GetElementPtr:
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// These values do not create new values: the values already
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// existed locally in this function so must have been tracked
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// already.
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continue
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case llvm.ExtractValue, llvm.BitCast:
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// These instructions do not create new values, but their
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// original value may not be tracked. So keep tracking them for
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// now.
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// With more analysis, it should be possible to optimize a
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// significant chunk of these away.
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case llvm.Call, llvm.Load, llvm.IntToPtr:
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// These create new values so must be stored locally. But
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// perhaps some of these can be fused when they actually refer
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// to the same value.
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default:
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// Ambiguous. These instructions are uncommon, but perhaps could
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// be optimized if needed.
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}
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if !ptr.IsAAllocaInst().IsNil() {
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if typeHasPointers(ptr.Type().ElementType()) {
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allocas = append(allocas, ptr)
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}
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} else {
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pointers = append(pointers, ptr)
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}
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}
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if len(allocas) == 0 && len(pointers) == 0 {
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// This function does not need to keep track of stack pointers.
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continue
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}
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// Determine the type of the required stack slot.
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fields := []llvm.Type{
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stackChainStartType, // Pointer to parent frame.
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c.uintptrType, // Number of elements in this frame.
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}
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for _, alloca := range allocas {
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fields = append(fields, alloca.Type().ElementType())
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}
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for _, ptr := range pointers {
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fields = append(fields, ptr.Type())
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}
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stackObjectType := c.ctx.StructType(fields, false)
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// Create the stack object at the function entry.
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c.builder.SetInsertPointBefore(fn.EntryBasicBlock().FirstInstruction())
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stackObject := c.builder.CreateAlloca(stackObjectType, "gc.stackobject")
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initialStackObject := llvm.ConstNull(stackObjectType)
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numSlots := (c.targetData.TypeAllocSize(stackObjectType) - c.targetData.TypeAllocSize(c.i8ptrType)*2) / uint64(c.targetData.ABITypeAlignment(c.uintptrType))
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numSlotsValue := llvm.ConstInt(c.uintptrType, numSlots, false)
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initialStackObject = llvm.ConstInsertValue(initialStackObject, numSlotsValue, []uint32{1})
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c.builder.CreateStore(initialStackObject, stackObject)
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// Update stack start.
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parent := c.builder.CreateLoad(stackChainStart, "")
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gep := c.builder.CreateGEP(stackObject, []llvm.Value{
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llvm.ConstInt(c.ctx.Int32Type(), 0, false),
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llvm.ConstInt(c.ctx.Int32Type(), 0, false),
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}, "")
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c.builder.CreateStore(parent, gep)
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stackObjectCast := c.builder.CreateBitCast(stackObject, stackChainStartType, "")
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c.builder.CreateStore(stackObjectCast, stackChainStart)
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// Replace all independent allocas with GEPs in the stack object.
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for i, alloca := range allocas {
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gep := c.builder.CreateGEP(stackObject, []llvm.Value{
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llvm.ConstInt(c.ctx.Int32Type(), 0, false),
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llvm.ConstInt(c.ctx.Int32Type(), uint64(2+i), false),
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}, "")
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alloca.ReplaceAllUsesWith(gep)
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alloca.EraseFromParentAsInstruction()
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}
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// Do a store to the stack object after each new pointer that is created.
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for i, ptr := range pointers {
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c.builder.SetInsertPointBefore(llvm.NextInstruction(ptr))
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gep := c.builder.CreateGEP(stackObject, []llvm.Value{
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llvm.ConstInt(c.ctx.Int32Type(), 0, false),
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llvm.ConstInt(c.ctx.Int32Type(), uint64(2+len(allocas)+i), false),
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}, "")
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c.builder.CreateStore(ptr, gep)
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}
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// Make sure this stack object is popped from the linked list of stack
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// objects at return.
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for _, ret := range returns {
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c.builder.SetInsertPointBefore(ret)
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c.builder.CreateStore(parent, stackChainStart)
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}
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}
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return true
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}
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func (c *Compiler) addGlobalsBitmap() bool {
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if c.mod.NamedGlobal("runtime.trackedGlobalsStart").IsNil() {
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return false // nothing to do: no GC in use
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}
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var trackedGlobals []llvm.Value
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var trackedGlobalTypes []llvm.Type
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for global := c.mod.FirstGlobal(); !global.IsNil(); global = llvm.NextGlobal(global) {
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if global.IsDeclaration() {
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continue
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}
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typ := global.Type().ElementType()
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ptrs := c.getPointerBitmap(typ, global.Name())
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if ptrs.BitLen() == 0 {
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continue
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}
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trackedGlobals = append(trackedGlobals, global)
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trackedGlobalTypes = append(trackedGlobalTypes, typ)
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}
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globalsBundleType := c.ctx.StructType(trackedGlobalTypes, false)
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globalsBundle := llvm.AddGlobal(c.mod, globalsBundleType, "tinygo.trackedGlobals")
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globalsBundle.SetLinkage(llvm.InternalLinkage)
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globalsBundle.SetUnnamedAddr(true)
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initializer := llvm.Undef(globalsBundleType)
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for i, global := range trackedGlobals {
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initializer = llvm.ConstInsertValue(initializer, global.Initializer(), []uint32{uint32(i)})
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gep := llvm.ConstGEP(globalsBundle, []llvm.Value{
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llvm.ConstInt(c.ctx.Int32Type(), 0, false),
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llvm.ConstInt(c.ctx.Int32Type(), uint64(i), false),
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})
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global.ReplaceAllUsesWith(gep)
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global.EraseFromParentAsGlobal()
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}
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globalsBundle.SetInitializer(initializer)
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trackedGlobalsStart := llvm.ConstPtrToInt(globalsBundle, c.uintptrType)
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c.mod.NamedGlobal("runtime.trackedGlobalsStart").SetInitializer(trackedGlobalsStart)
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alignment := c.targetData.PrefTypeAlignment(c.i8ptrType)
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trackedGlobalsLength := llvm.ConstInt(c.uintptrType, c.targetData.TypeAllocSize(globalsBundleType)/uint64(alignment), false)
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c.mod.NamedGlobal("runtime.trackedGlobalsLength").SetInitializer(trackedGlobalsLength)
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bitmapBytes := c.getPointerBitmap(globalsBundleType, "globals bundle").Bytes()
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bitmapValues := make([]llvm.Value, len(bitmapBytes))
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for i, b := range bitmapBytes {
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bitmapValues[len(bitmapBytes)-i-1] = llvm.ConstInt(c.ctx.Int8Type(), uint64(b), false)
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}
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bitmapArray := llvm.ConstArray(c.ctx.Int8Type(), bitmapValues)
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bitmapNew := llvm.AddGlobal(c.mod, bitmapArray.Type(), "runtime.trackedGlobalsBitmap.tmp")
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bitmapOld := c.mod.NamedGlobal("runtime.trackedGlobalsBitmap")
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bitmapOld.ReplaceAllUsesWith(llvm.ConstBitCast(bitmapNew, bitmapOld.Type()))
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bitmapNew.SetInitializer(bitmapArray)
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bitmapNew.SetName("runtime.trackedGlobalsBitmap")
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return true // the IR was changed
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}
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func (c *Compiler) getPointerBitmap(typ llvm.Type, name string) *big.Int {
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alignment := c.targetData.PrefTypeAlignment(c.i8ptrType)
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switch typ.TypeKind() {
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case llvm.IntegerTypeKind, llvm.FloatTypeKind, llvm.DoubleTypeKind:
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return big.NewInt(0)
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case llvm.PointerTypeKind:
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return big.NewInt(1)
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case llvm.StructTypeKind:
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ptrs := big.NewInt(0)
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for i, subtyp := range typ.StructElementTypes() {
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subptrs := c.getPointerBitmap(subtyp, name)
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if subptrs.BitLen() == 0 {
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continue
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}
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offset := c.targetData.ElementOffset(typ, i)
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if offset%uint64(alignment) != 0 {
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panic("precise GC: global contains unaligned pointer: " + name)
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}
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subptrs.Lsh(subptrs, uint(offset)/uint(alignment))
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ptrs.Or(ptrs, subptrs)
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}
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return ptrs
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case llvm.ArrayTypeKind:
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subtyp := typ.ElementType()
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subptrs := c.getPointerBitmap(subtyp, name)
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ptrs := big.NewInt(0)
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if subptrs.BitLen() == 0 {
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return ptrs
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}
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elementSize := c.targetData.TypeAllocSize(subtyp)
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for i := 0; i < typ.ArrayLength(); i++ {
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ptrs.Lsh(ptrs, uint(elementSize)/uint(alignment))
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ptrs.Or(ptrs, subptrs)
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}
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return ptrs
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default:
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panic("unknown type kind of global: " + name)
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}
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}
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// markParentFunctions traverses all parent function calls (recursively) and
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// adds them to the set of marked functions. It only considers function calls:
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// any other uses of such a function is ignored.
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func markParentFunctions(marked map[llvm.Value]struct{}, fn llvm.Value) {
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worklist := []llvm.Value{fn}
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for len(worklist) != 0 {
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fn := worklist[len(worklist)-1]
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worklist = worklist[:len(worklist)-1]
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for _, use := range getUses(fn) {
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if use.IsACallInst().IsNil() || use.CalledValue() != fn {
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// Not the parent function.
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continue
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}
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parent := use.InstructionParent().Parent()
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if _, ok := marked[parent]; !ok {
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marked[parent] = struct{}{}
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worklist = append(worklist, parent)
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}
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}
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}
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}
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