Add --fast-math to binaryen passes when linking with -ffast-math

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/other/test_fast_math.c

@@ -0,0 +1,47 @@
+#include <math.h>
+#include <stdio.h>
+
+static double mandelbrot_iter(double cx, double cy, int max_iter) {
+  double x = 0.0, y = 0.0;
+  for (int i = 0; i < max_iter; i++) {
+    if (x*x + y*y > 4.0) return (double)i;
+    double tx = x*x - y*y + cx;
+    y = 2.0*x*y + cy;
+    x = tx;
+  }
+  return (double)max_iter;
+}
+
+static double newton_raphson(double x, int iterations) {
+  for (int i = 0; i < iterations; i++) {
+    double fx = x*x*x - x - 1.0;
+    double fpx = 3.0*x*x - 1.0;
+    if (fabs(fpx) < 1e-10) break;
+    x = x - fx / fpx;
+  }
+  return x;
+}
+
+int main() {
+  double result = 0.0;
+
+  for (int i = 0; i < 100; i++) {
+    double x = (i - 50) * 0.02;
+    for (int j = 0; j < 100; j++) {
+      double y = (j - 50) * 0.02;
+      result += mandelbrot_iter(x, y, 50);
+    }
+  }
+
+  for (int i = 0; i < 50; i++) {
+    result += newton_raphson(1.5 + i * 0.1, 20);
+  }
+
+  for (int i = 0; i < 1000; i++) {
+    double angle = i * 0.01;
+    result += sin(angle) * cos(angle) + tan(angle);
+  }
+
+  printf("Result: %f\n", result);
+  return 0;
+}

test/test_other.py

@@ -15202,6 +15202,13 @@ def has_defined_function(file, func):
     self.assertIn('foo.cpp', out)
     self.assertIn('/emsdk/emscripten/system/lib/libc/musl/src/string/strcmp.c', out)
 
+  def test_binaryen_fast_math(self):
+    # Use a simple input; contents don't matter for -v flag inspection
+    err = self.run_process([EMCC, test_file('hello_world.c'), '-v', '-O2', '-ffast-math'], stderr=PIPE).stderr
+    self.assertContained('--fast-math', err)
+
+    err_no_fast = self.run_process([EMCC, test_file('hello_world.c'), '-v', '-O2'], stderr=PIPE).stderr
+    self.assertNotContained('--fast-math', err_no_fast)
   def test_relocatable(self):
     # This setting is due for removal:
     # https://github.com/emscripten-core/emscripten/issues/25262

tools/cmdline.py

@@ -301,9 +301,8 @@ 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
+        settings.FAST_MATH = 1
         requested_level = 3
         settings.SHRINK_LEVEL = 0
       else:
@@ -552,6 +551,8 @@ 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'):

tools/link.py

@@ -392,6 +392,8 @@ def get_binaryen_passes():
       passes += ['--pass-arg=post-emscripten-side-module']
   if optimizing:
     passes += [building.opt_level_to_str(settings.OPT_LEVEL, settings.SHRINK_LEVEL)]
+  if settings.FAST_MATH:
+    passes += ['--fast-math']
   # when optimizing, use the fact that low memory is never used (1024 is a
   # hardcoded value in the binaryen pass). we also cannot do it when the stack
   # is first, as then the stack is in the low memory that should be unused.