diff --git a/spec/compiler/macro/macro_methods_spec.cr b/spec/compiler/macro/macro_methods_spec.cr
index 10ba78d5bdc6..979a507d624d 100644
--- a/spec/compiler/macro/macro_methods_spec.cr
+++ b/spec/compiler/macro/macro_methods_spec.cr
@@ -2726,6 +2726,11 @@ module Crystal
       it "executes else" do
         assert_macro %({{x.else}}), "\"foo\"", {x: MacroIf.new(BoolLiteral.new(true), StringLiteral.new("test"), StringLiteral.new("foo"))}
       end
+
+      it "executes is_unless?" do
+        assert_macro %({{x.is_unless?}}), "true", {x: MacroIf.new(BoolLiteral.new(true), StringLiteral.new("test"), StringLiteral.new("foo"), is_unless: true)}
+        assert_macro %({{x.is_unless?}}), "false", {x: MacroIf.new(BoolLiteral.new(false), StringLiteral.new("test"), StringLiteral.new("foo"), is_unless: false)}
+      end
     end
 
     describe "macro for methods" do
diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 897e5bf7060c..ab8b1e9edfca 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -1156,7 +1156,7 @@ module Crystal
     it_parses "macro foo;bar{% if x %}body{% else %}body2{%end%}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroIf.new("x".var, "body".macro_literal, "body2".macro_literal), "baz;".macro_literal] of ASTNode))
     it_parses "macro foo;bar{% if x %}body{% elsif y %}body2{%end%}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroIf.new("x".var, "body".macro_literal, MacroIf.new("y".var, "body2".macro_literal)), "baz;".macro_literal] of ASTNode))
     it_parses "macro foo;bar{% if x %}body{% elsif y %}body2{% else %}body3{%end%}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroIf.new("x".var, "body".macro_literal, MacroIf.new("y".var, "body2".macro_literal, "body3".macro_literal)), "baz;".macro_literal] of ASTNode))
-    it_parses "macro foo;bar{% unless x %}body{% end %}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroIf.new("x".var, Nop.new, "body".macro_literal), "baz;".macro_literal] of ASTNode))
+    it_parses "macro foo;bar{% unless x %}body{% end %}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroIf.new("x".var, Nop.new, "body".macro_literal, is_unless: true), "baz;".macro_literal] of ASTNode))
 
     it_parses "macro foo;bar{% for x in y %}\\  \n   body{% end %}baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroFor.new(["x".var], "y".var, "body".macro_literal), "baz;".macro_literal] of ASTNode))
     it_parses "macro foo;bar{% for x in y %}\\  \n   body{% end %}\\   baz;end", Macro.new("foo", [] of Arg, Expressions.from(["bar".macro_literal, MacroFor.new(["x".var], "y".var, "body".macro_literal), "baz;".macro_literal] of ASTNode))
diff --git a/spec/std/process/status_spec.cr b/spec/std/process/status_spec.cr
index b56f261c4b5c..6cbabbea5d73 100644
--- a/spec/std/process/status_spec.cr
+++ b/spec/std/process/status_spec.cr
@@ -42,6 +42,16 @@ describe Process::Status do
     status_for(:interrupted).exit_code?.should be_nil
   end
 
+  it "#system_exit_status" do
+    Process::Status.new(exit_status(0)).system_exit_status.should eq 0_u32
+    Process::Status.new(exit_status(1)).system_exit_status.should eq({{ flag?(:unix) ? 0x0100_u32 : 1_u32 }})
+    Process::Status.new(exit_status(127)).system_exit_status.should eq({{ flag?(:unix) ? 0x7f00_u32 : 127_u32 }})
+    Process::Status.new(exit_status(128)).system_exit_status.should eq({{ flag?(:unix) ? 0x8000_u32 : 128_u32 }})
+    Process::Status.new(exit_status(255)).system_exit_status.should eq({{ flag?(:unix) ? 0xFF00_u32 : 255_u32 }})
+
+    status_for(:interrupted).system_exit_status.should eq({% if flag?(:unix) %}Signal::INT.value{% else %}LibC::STATUS_CONTROL_C_EXIT{% end %})
+  end
+
   it "#success?" do
     Process::Status.new(exit_status(0)).success?.should be_true
     Process::Status.new(exit_status(1)).success?.should be_false
diff --git a/src/compiler/crystal/macros.cr b/src/compiler/crystal/macros.cr
index a2ea0aeb85fe..ae6634e83a6f 100644
--- a/src/compiler/crystal/macros.cr
+++ b/src/compiler/crystal/macros.cr
@@ -2361,7 +2361,7 @@ module Crystal::Macros
     end
   end
 
-  # An `if` inside a macro, e.g.
+  # An `if`/`unless` inside a macro, e.g.
   #
   # ```
   # {% if cond %}
@@ -2369,6 +2369,12 @@ module Crystal::Macros
   # {% else %}
   #   puts "Else"
   # {% end %}
+  #
+  # {% unless cond %}
+  #   puts "Then"
+  # {% else %}
+  #   puts "Else"
+  # {% end %}
   # ```
   class MacroIf < ASTNode
     # The condition of the `if` clause.
@@ -2382,6 +2388,10 @@ module Crystal::Macros
     # The `else` branch of the `if`.
     def else : ASTNode
     end
+
+    # Returns `true` if this node represents an `unless` conditional, otherwise returns `false`.
+    def is_unless? : BoolLiteral
+    end
   end
 
   # A `for` loop inside a macro, e.g.
diff --git a/src/compiler/crystal/macros/methods.cr b/src/compiler/crystal/macros/methods.cr
index ab7b353fec45..f2691ba707c9 100644
--- a/src/compiler/crystal/macros/methods.cr
+++ b/src/compiler/crystal/macros/methods.cr
@@ -1541,6 +1541,8 @@ module Crystal
         interpret_check_args { @then }
       when "else"
         interpret_check_args { @else }
+      when "is_unless?"
+        interpret_check_args { BoolLiteral.new @is_unless }
       else
         super
       end
diff --git a/src/compiler/crystal/syntax/ast.cr b/src/compiler/crystal/syntax/ast.cr
index 9ccd8dda1f69..fb443e2e6777 100644
--- a/src/compiler/crystal/syntax/ast.cr
+++ b/src/compiler/crystal/syntax/ast.cr
@@ -2238,8 +2238,9 @@ module Crystal
     property cond : ASTNode
     property then : ASTNode
     property else : ASTNode
+    property? is_unless : Bool
 
-    def initialize(@cond, a_then = nil, a_else = nil)
+    def initialize(@cond, a_then = nil, a_else = nil, @is_unless : Bool = false)
       @then = Expressions.from a_then
       @else = Expressions.from a_else
     end
@@ -2251,10 +2252,10 @@ module Crystal
     end
 
     def clone_without_location
-      MacroIf.new(@cond.clone, @then.clone, @else.clone)
+      MacroIf.new(@cond.clone, @then.clone, @else.clone, @is_unless)
     end
 
-    def_equals_and_hash @cond, @then, @else
+    def_equals_and_hash @cond, @then, @else, @is_unless
   end
 
   # for inside a macro:
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 569bbd4d9409..60a3ec6414a7 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -3505,7 +3505,7 @@ module Crystal
       end
 
       a_then, a_else = a_else, a_then if is_unless
-      MacroIf.new(cond, a_then, a_else).at_end(token_end_location)
+      MacroIf.new(cond, a_then, a_else, is_unless: is_unless).at_end(token_end_location)
     end
 
     def parse_expression_inside_macro
diff --git a/src/crystal/event_loop/epoll.cr b/src/crystal/event_loop/epoll.cr
index 2d7d08ce7c94..371b9039b6b5 100644
--- a/src/crystal/event_loop/epoll.cr
+++ b/src/crystal/event_loop/epoll.cr
@@ -55,7 +55,7 @@ class Crystal::EventLoop::Epoll < Crystal::EventLoop::Polling
   {% end %}
 
   private def system_run(blocking : Bool, & : Fiber ->) : Nil
-    Crystal.trace :evloop, "run", blocking: blocking ? 1 : 0
+    Crystal.trace :evloop, "run", blocking: blocking
 
     # wait for events (indefinitely when blocking)
     buffer = uninitialized LibC::EpollEvent[128]
diff --git a/src/crystal/event_loop/kqueue.cr b/src/crystal/event_loop/kqueue.cr
index 52a7701ef2b1..47f00ddc9e89 100644
--- a/src/crystal/event_loop/kqueue.cr
+++ b/src/crystal/event_loop/kqueue.cr
@@ -73,7 +73,7 @@ class Crystal::EventLoop::Kqueue < Crystal::EventLoop::Polling
   private def system_run(blocking : Bool, & : Fiber ->) : Nil
     buffer = uninitialized LibC::Kevent[128]
 
-    Crystal.trace :evloop, "run", blocking: blocking ? 1 : 0
+    Crystal.trace :evloop, "run", blocking: blocking
     timeout = blocking ? nil : Time::Span.zero
     kevents = @kqueue.wait(buffer.to_slice, timeout)
 
diff --git a/src/crystal/main.cr b/src/crystal/main.cr
index 625238229c58..9b4384f16a8c 100644
--- a/src/crystal/main.cr
+++ b/src/crystal/main.cr
@@ -132,8 +132,8 @@ end
 
 {% if flag?(:win32) %}
   require "./system/win32/wmain"
-{% end %}
-
-{% if flag?(:wasi) %}
+{% elsif flag?(:wasi) %}
   require "./system/wasi/main"
+{% else %}
+  require "./system/unix/main"
 {% end %}
diff --git a/src/crystal/scheduler.cr b/src/crystal/scheduler.cr
index ad0f2a55672e..efee6b3c06f1 100644
--- a/src/crystal/scheduler.cr
+++ b/src/crystal/scheduler.cr
@@ -66,7 +66,7 @@ class Crystal::Scheduler
   end
 
   def self.sleep(time : Time::Span) : Nil
-    Crystal.trace :sched, "sleep", for: time.total_nanoseconds.to_i64!
+    Crystal.trace :sched, "sleep", for: time
     Thread.current.scheduler.sleep(time)
   end
 
@@ -225,7 +225,7 @@ class Crystal::Scheduler
       pending = Atomic(Int32).new(count - 1)
       @@workers = Array(Thread).new(count) do |i|
         if i == 0
-          worker_loop = Fiber.new(name: "Worker Loop") { Thread.current.scheduler.run_loop }
+          worker_loop = Fiber.new(name: "worker-loop") { Thread.current.scheduler.run_loop }
           worker_loop.set_current_thread
           Thread.current.scheduler.enqueue worker_loop
           Thread.current
@@ -272,7 +272,7 @@ class Crystal::Scheduler
 
   # Background loop to cleanup unused fiber stacks.
   def spawn_stack_pool_collector
-    fiber = Fiber.new(name: "Stack pool collector", &->@stack_pool.collect_loop)
+    fiber = Fiber.new(name: "stack-pool-collector", &->@stack_pool.collect_loop)
     {% if flag?(:preview_mt) %} fiber.set_current_thread {% end %}
     enqueue(fiber)
   end
diff --git a/src/crystal/system/thread.cr b/src/crystal/system/thread.cr
index 0d6f5077633a..92136d1f3989 100644
--- a/src/crystal/system/thread.cr
+++ b/src/crystal/system/thread.cr
@@ -74,6 +74,10 @@ class Thread
     @@threads.try(&.unsafe_each { |thread| yield thread })
   end
 
+  def self.each(&)
+    threads.each { |thread| yield thread }
+  end
+
   def self.lock : Nil
     threads.@mutex.lock
   end
@@ -182,6 +186,11 @@ class Thread
     self.system_name = name
   end
 
+  # Changes the Thread#name property but doesn't update the system name. Useful
+  # on the main thread where we'd change the process name (e.g. top, ps, ...).
+  def internal_name=(@name : String)
+  end
+
   # Holds the GC thread handler
   property gc_thread_handler : Void* = Pointer(Void).null
 
diff --git a/src/crystal/system/thread_linked_list.cr b/src/crystal/system/thread_linked_list.cr
index b6f3ccf65d4e..f43dd04fedc2 100644
--- a/src/crystal/system/thread_linked_list.cr
+++ b/src/crystal/system/thread_linked_list.cr
@@ -24,6 +24,13 @@ class Thread
       end
     end
 
+    # Safely iterates the list.
+    def each(&) : Nil
+      @mutex.synchronize do
+        unsafe_each { |node| yield node }
+      end
+    end
+
     # Appends a node to the tail of the list. The operation is thread-safe.
     #
     # There are no guarantees that a node being pushed will be iterated by
@@ -47,16 +54,21 @@ class Thread
     # `#unsafe_each` until the method has returned.
     def delete(node : T) : Nil
       @mutex.synchronize do
-        if previous = node.previous
-          previous.next = node.next
+        previous = node.previous
+        _next = node.next
+
+        if previous
+          node.previous = nil
+          previous.next = _next
         else
-          @head = node.next
+          @head = _next
         end
 
-        if _next = node.next
-          _next.previous = node.previous
+        if _next
+          node.next = nil
+          _next.previous = previous
         else
-          @tail = node.previous
+          @tail = previous
         end
       end
     end
diff --git a/src/crystal/system/unix/main.cr b/src/crystal/system/unix/main.cr
new file mode 100644
index 000000000000..1592a6342002
--- /dev/null
+++ b/src/crystal/system/unix/main.cr
@@ -0,0 +1,11 @@
+require "c/stdlib"
+
+# Prefer explicit exit over returning the status, so we are free to resume the
+# main thread's fiber on any thread, without occuring a weird behavior where
+# another thread returns from main when the caller might expect the main thread
+# to be the one returning.
+
+fun main(argc : Int32, argv : UInt8**) : Int32
+  status = Crystal.main(argc, argv)
+  LibC.exit(status)
+end
diff --git a/src/crystal/system/unix/signal.cr b/src/crystal/system/unix/signal.cr
index 802cb418db15..a68108ad327a 100644
--- a/src/crystal/system/unix/signal.cr
+++ b/src/crystal/system/unix/signal.cr
@@ -82,7 +82,7 @@ module Crystal::System::Signal
   end
 
   private def self.start_loop
-    spawn(name: "Signal Loop") do
+    spawn(name: "signal-loop") do
       loop do
         value = reader.read_bytes(Int32)
       rescue IO::Error
diff --git a/src/crystal/system/unix/time.cr b/src/crystal/system/unix/time.cr
index 2ead3bdb0fa2..5ffcc6f373a2 100644
--- a/src/crystal/system/unix/time.cr
+++ b/src/crystal/system/unix/time.cr
@@ -39,9 +39,8 @@ module Crystal::System::Time
       nanoseconds = total_nanoseconds.remainder(NANOSECONDS_PER_SECOND)
       {seconds.to_i64, nanoseconds.to_i32}
     {% else %}
-      if LibC.clock_gettime(LibC::CLOCK_MONOTONIC, out tp) == 1
-        raise RuntimeError.from_errno("clock_gettime(CLOCK_MONOTONIC)")
-      end
+      ret = LibC.clock_gettime(LibC::CLOCK_MONOTONIC, out tp)
+      raise RuntimeError.from_errno("clock_gettime(CLOCK_MONOTONIC)") unless ret == 0
       {tp.tv_sec.to_i64, tp.tv_nsec.to_i32}
     {% end %}
   end
diff --git a/src/crystal/system/wasi/main.cr b/src/crystal/system/wasi/main.cr
index 57ffd5f3f43c..9a3394809271 100644
--- a/src/crystal/system/wasi/main.cr
+++ b/src/crystal/system/wasi/main.cr
@@ -27,7 +27,8 @@ fun _start
   LibWasi.proc_exit(status) if status != 0
 end
 
-# `__main_argc_argv` is called by wasi-libc's `__main_void` with the program arguments.
+# `__main_argc_argv` is called by wasi-libc's `__main_void` with the program
+# arguments.
 fun __main_argc_argv(argc : Int32, argv : UInt8**) : Int32
   main(argc, argv)
 end
diff --git a/src/crystal/system/win32/process.cr b/src/crystal/system/win32/process.cr
index 5eb02d826c3b..5249491bbd3f 100644
--- a/src/crystal/system/win32/process.cr
+++ b/src/crystal/system/win32/process.cr
@@ -203,7 +203,7 @@ struct Crystal::System::Process
   def self.start_interrupt_loop : Nil
     return unless @@setup_interrupt_handler.test_and_set
 
-    spawn(name: "Interrupt signal loop") do
+    spawn(name: "interrupt-signal-loop") do
       while true
         @@interrupt_count.wait { sleep 50.milliseconds }
 
diff --git a/src/crystal/system/win32/wmain.cr b/src/crystal/system/win32/wmain.cr
index 2120bfc06bfc..b1726f90329b 100644
--- a/src/crystal/system/win32/wmain.cr
+++ b/src/crystal/system/win32/wmain.cr
@@ -12,17 +12,13 @@ require "c/stdlib"
 lib LibCrystalMain
 end
 
-# The actual entry point for Windows executables. This is necessary because
-# *argv* (and Win32's `GetCommandLineA`) mistranslate non-ASCII characters to
-# Windows-1252, so `PROGRAM_NAME` and `ARGV` would be garbled; to avoid that, we
-# use this Windows-exclusive entry point which contains the correctly encoded
-# UTF-16 *argv*, convert it to UTF-8, and then forward it to the original
-# `main`.
+# The actual entry point for Windows executables.
 #
-# The different main functions in `src/crystal/main.cr` need not be aware that
-# such an alternate entry point exists, nor that the original command line was
-# not UTF-8. Thus all other aspects of program initialization still occur there,
-# and uses of those main functions continue to work across platforms.
+# This is necessary because *argv* (and Win32's `GetCommandLineA`) mistranslate
+# non-ASCII characters to Windows-1252, so `PROGRAM_NAME` and `ARGV` would be
+# garbled; to avoid that, we use this Windows-exclusive entry point which
+# contains the correctly encoded UTF-16 *argv*, convert it to UTF-8, and then
+# forward it to the original `main`.
 #
 # NOTE: we cannot use anything from the standard library here, including the GC.
 fun wmain(argc : Int32, argv : UInt16**) : Int32
@@ -46,5 +42,9 @@ fun wmain(argc : Int32, argv : UInt16**) : Int32
   end
   LibC.free(utf8_argv)
 
-  status
+  # prefer explicit exit over returning the status, so we are free to resume the
+  # main thread's fiber on any thread, without occuring a weird behavior where
+  # another thread returns from main when the caller might expect the main
+  # thread to be the one returning.
+  LibC.exit(status)
 end
diff --git a/src/crystal/tracing.cr b/src/crystal/tracing.cr
index 684680b10b28..d9508eda85a8 100644
--- a/src/crystal/tracing.cr
+++ b/src/crystal/tracing.cr
@@ -51,37 +51,58 @@ module Crystal
           @size = 0
         end
 
-        def write(bytes : Bytes) : Nil
+        def write(value : Bytes) : Nil
           pos = @size
           remaining = N - pos
           return if remaining == 0
 
-          n = bytes.size.clamp(..remaining)
-          bytes.to_unsafe.copy_to(@buf.to_unsafe + pos, n)
+          n = value.size.clamp(..remaining)
+          value.to_unsafe.copy_to(@buf.to_unsafe + pos, n)
           @size = pos + n
         end
 
-        def write(string : String) : Nil
-          write string.to_slice
+        def write(value : String) : Nil
+          write value.to_slice
         end
 
-        def write(fiber : Fiber) : Nil
-          write fiber.as(Void*)
-          write ":"
-          write fiber.name || "?"
+        def write(value : Char) : Nil
+          chars = uninitialized UInt8[4]
+          i = 0
+          value.each_byte do |byte|
+            chars[i] = byte
+            i += 1
+          end
+          write chars.to_slice[0, i]
+        end
+
+        def write(value : Fiber) : Nil
+          write value.as(Void*)
+          write ':'
+          write value.name || '?'
         end
 
-        def write(ptr : Pointer) : Nil
+        def write(value : Pointer) : Nil
           write "0x"
-          System.to_int_slice(ptr.address, 16, true, 2) { |bytes| write(bytes) }
+          System.to_int_slice(value.address, 16, true, 2) { |bytes| write(bytes) }
+        end
+
+        def write(value : Int::Signed) : Nil
+          System.to_int_slice(value, 10, true, 2) { |bytes| write(bytes) }
+        end
+
+        def write(value : Int::Unsigned) : Nil
+          System.to_int_slice(value, 10, false, 2) { |bytes| write(bytes) }
+        end
+
+        def write(value : Time::Span) : Nil
+          write(value.seconds * Time::NANOSECONDS_PER_SECOND + value.nanoseconds)
         end
 
-        def write(int : Int::Signed) : Nil
-          System.to_int_slice(int, 10, true, 2) { |bytes| write(bytes) }
+        def write(value : Bool) : Nil
+          write value ? '1' : '0'
         end
 
-        def write(uint : Int::Unsigned) : Nil
-          System.to_int_slice(uint, 10, false, 2) { |bytes| write(bytes) }
+        def write(value : Nil) : Nil
         end
 
         def to_slice : Bytes
diff --git a/src/fiber.cr b/src/fiber.cr
index 2b596a16017c..55745666c66d 100644
--- a/src/fiber.cr
+++ b/src/fiber.cr
@@ -78,6 +78,11 @@ class Fiber
     @@fibers.try(&.unsafe_each { |fiber| yield fiber })
   end
 
+  # :nodoc:
+  def self.each(&)
+    fibers.each { |fiber| yield fiber }
+  end
+
   # Creates a new `Fiber` instance.
   #
   # When the fiber is executed, it runs *proc* in its context.
diff --git a/src/process/status.cr b/src/process/status.cr
index db759bd1c178..268d2f9e52d6 100644
--- a/src/process/status.cr
+++ b/src/process/status.cr
@@ -104,10 +104,18 @@ end
 class Process::Status
   # Platform-specific exit status code, which usually contains either the exit code or a termination signal.
   # The other `Process::Status` methods extract the values from `exit_status`.
+  @[Deprecated("Use `#exit_reason`, `#exit_code`, or `#system_exit_status` instead")]
   def exit_status : Int32
     @exit_status.to_i32!
   end
 
+  # Returns the exit status as indicated by the operating system.
+  #
+  # It can encode exit codes and termination signals and is platform-specific.
+  def system_exit_status : UInt32
+    @exit_status.to_u32!
+  end
+
   {% if flag?(:win32) %}
     # :nodoc:
     def initialize(@exit_status : UInt32)
@@ -261,7 +269,7 @@ class Process::Status
       # define __WEXITSTATUS(status) (((status) & 0xff00) >> 8)
       (@exit_status & 0xff00) >> 8
     {% else %}
-      exit_status
+      @exit_status.to_i32!
     {% end %}
   end