Fix bug with UCLO instruction not correctly emitted in while loop

Reported on github from tests:

https://github.com/openresty/luajit2-test-suite/blob/master/test/misc/uclo.lua

The uclo-3 test is still failing and needs another fix.

lang/bytecode.lua

@@ -943,6 +943,8 @@ local function locate_jump(self, name)
         return NO_JMP
     end
 end
+-- Emit a jump instruction, JMP or UCLO, to the given "name" location
+-- using 'basereg' register.
 function Proto.__index:scope_jump(name, basereg, need_uclo)
     local jump = locate_jump(self, name)
     return self:emit(need_uclo and BC.UCLO or BC.JMP, basereg, jump)
@@ -997,11 +999,18 @@ function Proto.__index:goto_label(label_name)
         return true, label
     end
 end
+-- Add information in the current scope about the loop, its exit location,
+-- save register. 'cont' should be the register for continue instruction
+-- but is no longer used.
 function Proto.__index:loop_register(exit, exit_reg, cont)
     self.scope.loop_exit = exit
     self.scope.loop_basereg = exit_reg
     self.scope.loop_cont = cont
 end
+
+-- Looks in current and outer scopes for the closer loop and returns:
+-- loop's base register, loop's continue register and if uclo is needed.
+-- The loop_cont value is not really used.
 function Proto.__index:current_loop(cont)
     local scope = self.scope
     local need_uclo = false

lang/generator.lua

@@ -741,17 +741,19 @@ function StatementRule:AssignmentExpression(node)
     self.ctx.freereg = free
 end
 function StatementRule:WhileStatement(node)
-    local free = self.ctx.freereg
-    local loop, exit = genid(), genid()
-    self:loop_enter(exit, free)
-    self.ctx:here(loop)
-    self:test_emit(node.test, exit, free)
-    self.ctx:loop(exit)
+    local register_save = self.ctx.freereg
+    local loop_begin_location, loop_exit_location = genid(), genid()
+    self:loop_enter(loop_exit_location, register_save)
+    self.ctx:here(loop_begin_location)
+    self:test_emit(node.test, loop_exit_location, register_save)
+    self.ctx:loop(loop_exit_location)
     self:block_emit(node.body)
-    self.ctx:jump(loop, free)
-    self.ctx:here(exit)
-    self:loop_leave(node.lastline)
-    self.ctx.freereg = free
+    self.ctx:scope_jump(loop_begin_location, register_save, self.ctx.scope.need_uclo)
+    self.ctx:here(loop_exit_location)
+    self.ctx:fscope_end()
+    self.ctx:leave()
+    if node.lastline then self.ctx:line(node.lastline) end
+    self.ctx.freereg = register_save
 end
 function StatementRule:RepeatStatement(node)
     local free = self.ctx.freereg

tests/uclo-1.lua

@@ -0,0 +1,8 @@
+local z1
+local i = 1
+while i <= 2 do
+  local j = i
+  z1 = z1 or function() return j end
+  i = i + 1
+end
+print(z1())

tests/uclo-2.lua

@@ -0,0 +1,7 @@
+local z1, z2
+for i=1,10 do
+    local function f() return i end
+    if z1 then z2 = f else z1 = f end
+end
+print(z1())
+print(z2())

tests/uclo-3.lua

@@ -0,0 +1,10 @@
+local z1, z2
+local i = 1
+repeat
+    local j = i
+    local function f() return j end
+    if z1 then z2 = f else z1 = f end
+    i = i + 1
+until i > 10
+print(z1())
+print(z2())

tests/uclo-4.lua

@@ -0,0 +1,9 @@
+local x = 1
+local function f()
+local y = 0
+for i=1,100 do y=y+x end
+    return y
+end
+print(f())
+x = 2
+print(f())