main: add -print-allocs flag that lets you print all heap allocations

This flag, if set, is a regexp for function names. If there are heap
allocations in the matching function names, these heap allocations will
be printed with an explanation why the heap allocation exists (and why
the object can't be stack allocated).

transform/allocs.go

@@ -6,6 +6,10 @@ package transform
 // interprocedural escape analysis.
 
 import (
+	"fmt"
+	"go/token"
+	"regexp"
+
 	"tinygo.org/x/go-llvm"
 )
 
@@ -20,7 +24,10 @@ const maxStackAlloc = 256
 // 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.
-func OptimizeAllocs(mod llvm.Module) {
+// 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
@@ -32,14 +39,21 @@ func OptimizeAllocs(mod llvm.Module) {
 	builder := mod.Context().NewBuilder()
 
 	for _, heapalloc := range getUses(allocator) {
+		logAllocs := printAllocs != nil && printAllocs.MatchString(heapalloc.InstructionParent().Parent().Name())
 		if heapalloc.Operand(0).IsAConstant().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
 		}
 
@@ -68,7 +82,15 @@ func OptimizeAllocs(mod llvm.Module) {
 			bitcast = uses[0]
 		}
 
-		if mayEscape(bitcast) {
+		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.
@@ -97,9 +119,9 @@ func OptimizeAllocs(mod llvm.Module) {
 	}
 }
 
-// mayEscape returns whether the value might escape. It returns true if it might
-// escape, and false if it definitely doesn't. The value must be an instruction.
-func mayEscape(value llvm.Value) bool {
+// 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() {
@@ -107,13 +129,13 @@ func mayEscape(value llvm.Value) bool {
 		}
 		switch use.InstructionOpcode() {
 		case llvm.GetElementPtr:
-			if mayEscape(use) {
-				return true
+			if at := valueEscapesAt(use); !at.IsNil() {
+				return at
 			}
 		case llvm.BitCast:
 			// A bitcast escapes if the casted-to value escapes.
-			if mayEscape(use) {
-				return true
+			if at := valueEscapesAt(use); !at.IsNil() {
+				return at
 			}
 		case llvm.Load:
 			// Load does not escape.
@@ -121,21 +143,27 @@ func mayEscape(value llvm.Value) bool {
 			// Store only escapes when the value is stored to, not when the
 			// value is stored into another value.
 			if use.Operand(0) == value {
-				return true
+				return use
 			}
 		case llvm.Call:
 			if !hasFlag(use, value, "nocapture") {
-				return true
+				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 true
+			return use
 		}
 	}
 
 	// Checked all uses, and none let the pointer value escape.
-	return false
+	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)
 }