Implement -ffast-math flag mapping to wasm-opt --fast-math (fixes #21497)

src/settings_internal.js

@@ -260,6 +260,11 @@ var ASYNCIFY_IMPORTS_EXCEPT_JS_LIBS = [];
 
 var WARN_DEPRECATED = true;
 
+// Enable fast math optimizations in wasm-opt when -ffast-math is passed.
+// This enables aggressive floating-point optimizations that may violate
+// IEEE 754 semantics but can improve performance.
+var FAST_MATH = 0;
+
 // WebGL 2 provides new garbage-free entry points to call to WebGL. Use
 // those always when possible.
 // We currently set this to false for certain browser when large memory sizes

test/test_other.py

@@ -15756,3 +15756,30 @@ def has_defined_function(file, func):
     self.assertIn('main.cpp', out)
     self.assertIn('foo.cpp', out)
     self.assertIn('/emsdk/emscripten/system/lib/libc/musl/src/string/strcmp.c', out)
+
+  def test_fast_math_debug_output(self):
+    create_file('test.c', '''
+      #include <math.h>
+      int main() { return (int)(sin(1.0) * 100); }
+    ''')
+
+    err = self.run_process([EMCC, 'test.c', '-v', '-O2', '-ffast-math'], stderr=PIPE).stderr
+    self.assertContained('--fast-math', err)
+
+    err_no_fast = self.run_process([EMCC, 'test.c', '-v', '-O2'], stderr=PIPE).stderr
+    self.assertNotContained('--fast-math', err_no_fast)
+
+  def test_fast_math_size_comparison(self):
+    create_file('math.c', '''
+      #include <math.h>
+      double f(double x) { return sin(x) * cos(x) + sqrt(x); }
+      int main() { return (int)f(1.5); }
+    ''')
+
+    self.run_process([EMCC, 'math.c', '-O2', '-o', 'no_fast.wasm'])
+    no_fast_size = os.path.getsize('no_fast.wasm')
+
+    self.run_process([EMCC, 'math.c', '-O2', '-ffast-math', '-o', 'with_fast.wasm'])
+    with_fast_size = os.path.getsize('with_fast.wasm')
+
+    self.assertLessEqual(with_fast_size, no_fast_size)

tools/building.py

@@ -789,9 +789,12 @@ def minify_wasm_js(js_file, wasm_file, expensive_optimizations, debug_info):
 # get the flags to pass into the very last binaryen tool invocation, that runs
 # the final set of optimizations
 def get_last_binaryen_opts():
-  return [f'--optimize-level={settings.OPT_LEVEL}',
+  opts = [f'--optimize-level={settings.OPT_LEVEL}',
           f'--shrink-level={settings.SHRINK_LEVEL}',
           '--optimize-stack-ir']
+  if settings.FAST_MATH:
+    opts.append('--fast-math')
+  return opts
 
 
 # run binaryen's wasm-metadce to dce both js and wasm
@@ -1258,6 +1261,8 @@ def run_binaryen_command(tool, infile, outfile=None, args=None, debug=False, std
 
 
 def run_wasm_opt(infile, outfile=None, args=[], **kwargs):  # noqa
+  if settings.FAST_MATH:
+    args.append('--fast-math')
   return run_binaryen_command('wasm-opt', infile, outfile, args=args, **kwargs)
 
 

tools/cmdline.py

@@ -294,9 +294,8 @@ def consume_arg_file():
         settings.SHRINK_LEVEL = 0
         settings.DEBUG_LEVEL = max(settings.DEBUG_LEVEL, 1)
       elif requested_level == 'fast':
-        # TODO(https://github.com/emscripten-core/emscripten/issues/21497):
-        # If we ever map `-ffast-math` to `wasm-opt --fast-math` then
-        # then we should enable that too here.
+        # -Ofast typically includes -ffast-math semantics, so enable fast math optimizations
+        settings.FAST_MATH = 1
         requested_level = 3
         settings.SHRINK_LEVEL = 0
       else:
@@ -545,6 +544,8 @@ def consume_arg_file():
       settings.WASM_EXCEPTIONS = 1
     elif arg == '-fignore-exceptions':
       settings.DISABLE_EXCEPTION_CATCHING = 1
+    elif arg == '-ffast-math':
+      settings.FAST_MATH = 1
     elif check_arg('--default-obj-ext'):
       exit_with_error('--default-obj-ext is no longer supported by emcc')
     elif arg.startswith('-fsanitize=cfi'):