diff --git a/mypy/util.py b/mypy/util.py
index 686a71c4331b..ad09395bc343 100644
--- a/mypy/util.py
+++ b/mypy/util.py
@@ -525,7 +525,7 @@ class FancyFormatter:
     def __init__(self, f_out: IO[str], f_err: IO[str], show_error_codes: bool) -> None:
         self.show_error_codes = show_error_codes
         # Check if we are in a human-facing terminal on a supported platform.
-        if sys.platform not in ("linux", "darwin", "win32"):
+        if sys.platform not in ("linux", "darwin", "win32", "emscripten"):
             self.dummy_term = True
             return
         force_color = int(os.getenv("MYPY_FORCE_COLOR", "0"))
@@ -534,6 +534,8 @@ def __init__(self, f_out: IO[str], f_err: IO[str], show_error_codes: bool) -> No
             return
         if sys.platform == "win32":
             self.dummy_term = not self.initialize_win_colors()
+        elif sys.platform == "emscripten":
+            self.dummy_term = not self.initialize_vt100_colors()
         else:
             self.dummy_term = not self.initialize_unix_colors()
         if not self.dummy_term:
@@ -545,6 +547,20 @@ def __init__(self, f_out: IO[str], f_err: IO[str], show_error_codes: bool) -> No
                 "none": "",
             }
 
+    def initialize_vt100_colors(self) -> bool:
+        """Return True if initialization was successful and we can use colors, False otherwise"""
+        # Windows and Emscripten can both use ANSI/VT100 escape sequences for color
+        assert sys.platform in ("win32", "emscripten")
+        self.BOLD = "\033[1m"
+        self.UNDER = "\033[4m"
+        self.BLUE = "\033[94m"
+        self.GREEN = "\033[92m"
+        self.RED = "\033[91m"
+        self.YELLOW = "\033[93m"
+        self.NORMAL = "\033[0m"
+        self.DIM = "\033[2m"
+        return True
+
     def initialize_win_colors(self) -> bool:
         """Return True if initialization was successful and we can use colors, False otherwise"""
         # Windows ANSI escape sequences are only supported on Threshold 2 and above.
@@ -571,14 +587,7 @@ def initialize_win_colors(self) -> bool:
                 | ENABLE_WRAP_AT_EOL_OUTPUT
                 | ENABLE_VIRTUAL_TERMINAL_PROCESSING,
             )
-            self.BOLD = "\033[1m"
-            self.UNDER = "\033[4m"
-            self.BLUE = "\033[94m"
-            self.GREEN = "\033[92m"
-            self.RED = "\033[91m"
-            self.YELLOW = "\033[93m"
-            self.NORMAL = "\033[0m"
-            self.DIM = "\033[2m"
+            self.initialize_vt100_colors()
             return True
         return False
 
diff --git a/mypyc/build.py b/mypyc/build.py
index db548b149946..4f40a6cd0865 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -533,10 +533,8 @@ def mypycify(
             "-Wno-unused-variable",
             "-Wno-unused-command-line-argument",
             "-Wno-unknown-warning-option",
+            "-Wno-unused-but-set-variable",
         ]
-        if "gcc" in compiler.compiler[0] or "gnu-cc" in compiler.compiler[0]:
-            # This flag is needed for gcc but does not exist on clang.
-            cflags += ["-Wno-unused-but-set-variable"]
     elif compiler.compiler_type == "msvc":
         # msvc doesn't have levels, '/O2' is full and '/Od' is disable
         if opt_level == "0":
diff --git a/mypyc/common.py b/mypyc/common.py
index e0202eaa3edc..1ae564338609 100644
--- a/mypyc/common.py
+++ b/mypyc/common.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import sys
+import sysconfig
 from typing import Any, Dict
 from typing_extensions import Final
 
@@ -30,7 +31,16 @@
 # Maximal number of subclasses for a class to trigger fast path in isinstance() checks.
 FAST_ISINSTANCE_MAX_SUBCLASSES: Final = 2
 
-IS_32_BIT_PLATFORM: Final = sys.maxsize < (1 << 31)
+# Size of size_t, if configured.
+SIZEOF_SIZE_T_SYSCONFIG: Final = sysconfig.get_config_var("SIZEOF_SIZE_T")
+
+SIZEOF_SIZE_T: Final = (
+    int(SIZEOF_SIZE_T_SYSCONFIG)
+    if SIZEOF_SIZE_T_SYSCONFIG is not None
+    else (sys.maxsize + 1).bit_length() // 8
+)
+
+IS_32_BIT_PLATFORM: Final = int(SIZEOF_SIZE_T) == 4
 
 PLATFORM_SIZE = 4 if IS_32_BIT_PLATFORM else 8
 
@@ -42,15 +52,16 @@
 IS_MIXED_32_64_BIT_BUILD: Final = sys.platform in ["darwin"] and sys.version_info < (3, 6)
 
 # Maximum value for a short tagged integer.
-MAX_SHORT_INT: Final = sys.maxsize >> 1
+MAX_SHORT_INT: Final = 2 ** (8 * int(SIZEOF_SIZE_T) - 2) - 1
+
 # Minimum value for a short tagged integer.
-MIN_SHORT_INT: Final = -(sys.maxsize >> 1) - 1
+MIN_SHORT_INT: Final = -(MAX_SHORT_INT) - 1
 
 # Maximum value for a short tagged integer represented as a C integer literal.
 #
 # Note: Assume that the compiled code uses the same bit width as mypyc, except for
 #       Python 3.5 on macOS.
-MAX_LITERAL_SHORT_INT: Final = sys.maxsize >> 1 if not IS_MIXED_32_64_BIT_BUILD else 2**30 - 1
+MAX_LITERAL_SHORT_INT: Final = MAX_SHORT_INT if not IS_MIXED_32_64_BIT_BUILD else 2**30 - 1
 MIN_LITERAL_SHORT_INT: Final = -MAX_LITERAL_SHORT_INT - 1
 
 # Decription of the C type used to track definedness of attributes