diff --git a/emcc b/emcc
index 3c427677f..490e64088 100755
--- a/emcc
+++ b/emcc
@@ -1967,16 +1967,23 @@ try:
     if len(js_optimizer_extra_info) == 0:
       js_optimizer_extra_info = None
     if len(js_optimizer_queue) > 0 and not(not shared.Settings.ASM_JS and len(js_optimizer_queue) == 1 and js_optimizer_queue[0] == 'last'):
-      if DEBUG != '2':
+
+      def add_opt_args(args):
         if shared.Settings.ASM_JS:
-          js_optimizer_queue = ['asm'] + js_optimizer_queue
+          args = ['asm'] + args
+          if shared.Settings.PRECISE_F32:
+            args = ['asmPreciseF32'] + args
+        return args
+
+      if DEBUG != '2':
+        js_optimizer_queue = add_opt_args(js_optimizer_queue)
         logging.debug('applying js optimization passes: %s', js_optimizer_queue)
         final = shared.Building.js_optimizer(final, js_optimizer_queue, jcache, debug_level >= 4, js_optimizer_extra_info)
         js_transform_tempfiles.append(final)
         if DEBUG: save_intermediate('js_opts')
       else:
         for name in js_optimizer_queue:
-          passes = [name]
+          passes = add_opt_args([name])
           if shared.Settings.ASM_JS:
             passes = ['asm'] + passes
           logging.debug('applying js optimization pass: %s', passes)
diff --git a/tests/test_other.py b/tests/test_other.py
index bc05826ea..47d26416e 100644
--- a/tests/test_other.py
+++ b/tests/test_other.py
@@ -1779,6 +1779,8 @@ f.close()
        ['asm', 'registerize', 'minifyLocals']),
       (path_from_root('tools', 'test-js-optimizer-asm-pre.js'), open(path_from_root('tools', 'test-js-optimizer-asm-pre-output.js')).read(),
        ['asm', 'simplifyExpressions']),
+      (path_from_root('tools', 'test-js-optimizer-asm-pre-f32.js'), open(path_from_root('tools', 'test-js-optimizer-asm-pre-output-f32.js')).read(),
+       ['asm', 'asmPreciseF32', 'simplifyExpressions']),
       (path_from_root('tools', 'test-js-optimizer-asm-last.js'), open(path_from_root('tools', 'test-js-optimizer-asm-last-output.js')).read(),
        ['asm', 'last']),
       (path_from_root('tools', 'test-js-optimizer-asm-relocate.js'), open(path_from_root('tools', 'test-js-optimizer-asm-relocate-output.js')).read(),
diff --git a/tools/js-optimizer.js b/tools/js-optimizer.js
index 2ae3ce47a..905e80fc0 100644
--- a/tools/js-optimizer.js
+++ b/tools/js-optimizer.js
@@ -709,10 +709,6 @@ function simplifyExpressions(ast) {
             }
           }
         });
-        var hasFloat = false;
-        for (var v in asmData.vars) {
-          if (asmData.vars[v] === ASM_FLOAT) hasFloat = true;
-        }
         for (var v in bitcastVars) {
           var info = bitcastVars[v];
           // good variables define only one type, use only one type, have definitions and uses, and define as a different type than they use
@@ -726,7 +722,7 @@ function simplifyExpressions(ast) {
               if (correct === 'HEAP32') {
                 define[3] = ['binary', '|', define[3], ['num', 0]];
               } else {
-                define[3] = ['unary-prefix', '+', define[3]];
+                define[3] = makeAsmCoercion(define[3], asmPreciseF32 ? ASM_FLOAT : ASM_DOUBLE);
               }
               // do we want a simplifybitops on the new values here?
             });
@@ -735,7 +731,7 @@ function simplifyExpressions(ast) {
             });
             var correctType;
             switch(asmData.vars[v]) {
-              case ASM_INT: correctType = hasFloat ? ASM_FLOAT : ASM_DOUBLE; break;
+              case ASM_INT: correctType = asmPreciseF32 ? ASM_FLOAT : ASM_DOUBLE; break;
               case ASM_FLOAT: case ASM_DOUBLE: correctType = ASM_INT; break;
             }
             asmData.vars[v] = correctType;
@@ -5162,7 +5158,7 @@ function asmLastOpts(ast) {
 
 // Passes table
 
-var minifyWhitespace = false, printMetadata = true, asm = false, last = false;
+var minifyWhitespace = false, printMetadata = true, asm = false, asmPreciseF32 = false, last = false;
 
 var passes = {
   // passes
@@ -5191,6 +5187,7 @@ var passes = {
   minifyWhitespace: function() { minifyWhitespace = true },
   noPrintMetadata: function() { printMetadata = false },
   asm: function() { asm = true },
+  asmPreciseF32: function() { asmPreciseF32 = true },
   last: function() { last = true },
 };
 
diff --git a/tools/test-js-optimizer-asm-pre-f32.js b/tools/test-js-optimizer-asm-pre-f32.js
new file mode 100644
index 000000000..a5604ef9c
--- /dev/null
+++ b/tools/test-js-optimizer-asm-pre-f32.js
@@ -0,0 +1,11 @@
+function badf() {
+ var $9 = Math_fround(0);
+ $9 = (HEAP32[tempDoublePtr>>2]=$8,Math_fround(HEAPF32[tempDoublePtr>>2]));
+ HEAPF32[$gep23_asptr>>2] = $9;
+}
+function badf2() {
+ var $9 = 0;
+ $9 = (HEAPF32[tempDoublePtr>>2]=$8,HEAP32[tempDoublePtr>>2]|0);
+ HEAP32[$gep23_asptr>>2] = $9;
+}
+// EMSCRIPTEN_GENERATED_FUNCTIONS: ["a", "b", "rett", "ret2t", "retf", "i32_8", "tempDoublePtr", "boxx", "_main", "badf", "badf2"]
diff --git a/tools/test-js-optimizer-asm-pre-output-f32.js b/tools/test-js-optimizer-asm-pre-output-f32.js
new file mode 100644
index 000000000..42343f555
--- /dev/null
+++ b/tools/test-js-optimizer-asm-pre-output-f32.js
@@ -0,0 +1,11 @@
+function badf() {
+ var $9 = 0;
+ $9 = $8 | 0;
+ HEAP32[$gep23_asptr >> 2] = $9;
+}
+function badf2() {
+ var $9 = Math_fround(0);
+ $9 = Math_fround($8);
+ HEAPF32[$gep23_asptr >> 2] = $9;
+}
+