runtime: add "end of heap" to detect out of memory

This can be used in the future to trigger garbage collection. For now,
it provides a more useful error message in case the heap is completely
filled up.

src/runtime/arch_avr.go

@@ -14,7 +14,13 @@ const TargetBits = 8
 //go:extern _heap_start
 var heapStartSymbol unsafe.Pointer
 
-var heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+//go:extern _heap_end
+var heapEndSymbol unsafe.Pointer
+
+var (
+	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+	heapEnd   = uintptr(unsafe.Pointer(&heapEndSymbol))
+)
 
 // Align on a word boundary.
 func align(ptr uintptr) uintptr {

src/runtime/arch_tinygoarm.go

@@ -14,7 +14,13 @@ const TargetBits = 32
 //go:extern _heap_start
 var heapStartSymbol unsafe.Pointer
 
-var heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+//go:extern _heap_end
+var heapEndSymbol unsafe.Pointer
+
+var (
+	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+	heapEnd   = uintptr(unsafe.Pointer(&heapEndSymbol))
+)
 
 // Align on word boundary.
 func align(ptr uintptr) uintptr {

src/runtime/arch_wasm.go

@@ -14,7 +14,12 @@ const TargetBits = 32
 //go:extern __heap_base
 var heapStartSymbol unsafe.Pointer
 
-var heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+var (
+	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+	heapEnd   = (heapStart + wasmPageSize - 1) & (wasmPageSize - 1) // conservative guess: one page of heap memory
+)
+
+const wasmPageSize = 64 * 1024
 
 // Align on word boundary.
 func align(ptr uintptr) uintptr {

src/runtime/gc_dumb.go

@@ -20,6 +20,9 @@ func alloc(size uintptr) unsafe.Pointer {
 	size = align(size)
 	addr := heapptr
 	heapptr += size
+	if heapptr >= heapEnd {
+		runtimePanic("out of memory")
+	}
 	for i := uintptr(0); i < uintptr(size); i += 4 {
 		ptr := (*uint32)(unsafe.Pointer(addr + i))
 		*ptr = 0

src/runtime/runtime_unix.go

@@ -14,7 +14,10 @@ func _Cfunc_clock_gettime(clk_id uint, ts *timespec)
 
 const heapSize = 1 * 1024 * 1024 // 1MB to start
 
-var heapStart = uintptr(_Cfunc_malloc(heapSize))
+var (
+	heapStart = uintptr(_Cfunc_malloc(heapSize))
+	heapEnd   = heapStart + heapSize
+)
 
 type timeUnit int64