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package transform
// This file implements an escape analysis pass. It looks for calls to
// runtime.alloc and replaces these calls with a stack allocation if the
// allocated value does not escape. It uses the LLVM nocapture flag for
// interprocedural escape analysis.
import (
"fmt"
"go/token"
"regexp"
"tinygo.org/x/go-llvm"
)
// maxStackAlloc is the maximum size of an object that will be allocated on the
// stack. Bigger objects have increased risk of stack overflows and thus will
// always be heap allocated.
//
// TODO: tune this, this is just a random value.
// This value is also used in the compiler when translating ssa.Alloc nodes.
const maxStackAlloc = 256
// OptimizeAllocs tries to replace heap allocations with stack allocations
// whenever possible. It relies on the LLVM 'nocapture' flag for interprocedural
// escape analysis, and within a function looks whether an allocation can escape
// to the heap.
// If printAllocs is non-nil, it indicates the regexp of functions for which a
// heap allocation explanation should be printed (why the object can't be stack
// allocated).
func OptimizeAllocs(mod llvm.Module, printAllocs *regexp.Regexp, logger func(token.Position, string)) {
allocator := mod.NamedFunction("runtime.alloc")
if allocator.IsNil() {
// nothing to optimize
return
}
targetData := llvm.NewTargetData(mod.DataLayout())
defer targetData.Dispose()
i8ptrType := llvm.PointerType(mod.Context().Int8Type(), 0)
builder := mod.Context().NewBuilder()
defer builder.Dispose()
for _, heapalloc := range getUses(allocator) {
logAllocs := printAllocs != nil && printAllocs.MatchString(heapalloc.InstructionParent().Parent().Name())
if heapalloc.Operand(0).IsAConstantInt().IsNil() {
// Do not allocate variable length arrays on the stack.
if logAllocs {
logAlloc(logger, heapalloc, "size is not constant")
}
continue
}
size := heapalloc.Operand(0).ZExtValue()
if size > maxStackAlloc {
// The maximum size for a stack allocation.
if logAllocs {
logAlloc(logger, heapalloc, fmt.Sprintf("object size %d exceeds maximum stack allocation size %d", size, maxStackAlloc))
}
continue
}
if size == 0 {
// If the size is 0, the pointer is allowed to alias other
// zero-sized pointers. Use the pointer to the global that would
// also be returned by runtime.alloc.
zeroSizedAlloc := mod.NamedGlobal("runtime.zeroSizedAlloc")
if !zeroSizedAlloc.IsNil() {
heapalloc.ReplaceAllUsesWith(zeroSizedAlloc)
heapalloc.EraseFromParentAsInstruction()
}
continue
}
// In general the pattern is:
// %0 = call i8* @runtime.alloc(i32 %size, i8* null)
// %1 = bitcast i8* %0 to type*
// (use %1 only)
// But the bitcast might sometimes be dropped when allocating an *i8.
// The 'bitcast' variable below is thus usually a bitcast of the
// heapalloc but not always.
bitcast := heapalloc // instruction that creates the value
if uses := getUses(heapalloc); len(uses) == 1 && !uses[0].IsABitCastInst().IsNil() {
// getting only bitcast use
bitcast = uses[0]
}
if at := valueEscapesAt(bitcast); !at.IsNil() {
if logAllocs {
atPos := getPosition(at)
msg := "escapes at unknown line"
if atPos.Line != 0 {
msg = fmt.Sprintf("escapes at line %d", atPos.Line)
}
logAlloc(logger, heapalloc, msg)
}
continue
}
// The pointer value does not escape.
// Determine the appropriate alignment of the alloca. The size of the
// allocation gives us a hint what the alignment should be.
var alignment int
if size%2 != 0 {
alignment = 1
} else if size%4 != 0 {
alignment = 2
} else if size%8 != 0 {
alignment = 4
} else {
alignment = 8
}
if pointerAlignment := targetData.ABITypeAlignment(i8ptrType); pointerAlignment < alignment {
// Use min(alignment, alignof(void*)) as the alignment.
alignment = pointerAlignment
}
// Insert alloca in the entry block. Do it here so that mem2reg can
// promote it to a SSA value.
fn := bitcast.InstructionParent().Parent()
builder.SetInsertPointBefore(fn.EntryBasicBlock().FirstInstruction())
allocaType := llvm.ArrayType(mod.Context().Int8Type(), int(size))
alloca := builder.CreateAlloca(allocaType, "stackalloc.alloca")
alloca.SetAlignment(alignment)
// Zero the allocation inside the block where the value was originally allocated.
zero := llvm.ConstNull(alloca.Type().ElementType())
builder.SetInsertPointBefore(bitcast)
store := builder.CreateStore(zero, alloca)
store.SetAlignment(alignment)
// Replace heap alloc bitcast with stack alloc bitcast.
stackalloc := builder.CreateBitCast(alloca, bitcast.Type(), "stackalloc")
bitcast.ReplaceAllUsesWith(stackalloc)
if heapalloc != bitcast {
bitcast.EraseFromParentAsInstruction()
}
heapalloc.EraseFromParentAsInstruction()
}
}
// valueEscapesAt returns the instruction where the given value may escape and a
// nil llvm.Value if it definitely doesn't. The value must be an instruction.
func valueEscapesAt(value llvm.Value) llvm.Value {
uses := getUses(value)
for _, use := range uses {
if use.IsAInstruction().IsNil() {
panic("expected instruction use")
}
switch use.InstructionOpcode() {
case llvm.GetElementPtr:
if at := valueEscapesAt(use); !at.IsNil() {
return at
}
case llvm.BitCast:
// A bitcast escapes if the casted-to value escapes.
if at := valueEscapesAt(use); !at.IsNil() {
return at
}
case llvm.Load:
// Load does not escape.
case llvm.Store:
// Store only escapes when the value is stored to, not when the
// value is stored into another value.
if use.Operand(0) == value {
return use
}
case llvm.Call:
if !hasFlag(use, value, "nocapture") {
return use
}
case llvm.ICmp:
// Comparing pointers don't let the pointer escape.
// This is often a compiler-inserted nil check.
default:
// Unknown instruction, might escape.
return use
}
}
// Checked all uses, and none let the pointer value escape.
return llvm.Value{}
}
// logAlloc prints a message to stderr explaining why the given object had to be
// allocated on the heap.
func logAlloc(logger func(token.Position, string), allocCall llvm.Value, reason string) {
logger(getPosition(allocCall), "object allocated on the heap: "+reason)
}
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