emit a global const for Math_fround(0) to avoid function call overheads in the fround polyfill

1 files changed, 8 insertions, 1 deletions

diff --git a/src/settings.js b/src/settings.js
index a9a72425..3289eace 100644
--- a/src/settings.js
+++ b/src/settings.js
@@ -124,13 +124,20 @@ var PRECISE_I32_MUL = 1; // If enabled, i32 multiplication is done with full pre
 var PRECISE_F32 = 0; // 0: Use JS numbers for floating-point values. These are 64-bit and do not model C++
                      //    floats exactly, which are 32-bit.
                      // 1: Model C++ floats precisely, using Math.fround, polyfilling when necessary. This
-                     //    can be slow if the polyfill is used on heavy float32 computation.
+                     //    can be slow if the polyfill is used on heavy float32 computation. See note on
+                     //    browser support below.
                      // 2: Model C++ floats precisely using Math.fround if available in the JS engine, otherwise
                      //    use an empty polyfill. This will have much less of a speed penalty than using the full
                      //    polyfill in cases where engine support is not present. In addition, we can
                      //    remove the empty polyfill calls themselves on the client when generating html,
                      //    which should mean that this gives you the best of both worlds of 0 and 1, and is
                      //    therefore recommended.
+                     // XXX Note: To optimize float32-using code, we use the 'const' keyword in the emitted
+                     //           code. This allows us to avoid unnecessary calls to Math.fround, which would
+                     //           slow down engines not yet supporting that function. 'const' is present in
+                     //           all modern browsers, including Firefox, Chrome and Safari, but in IE is only
+                     //           present in IE11 and above. Therefore if you need to support legacy versions of
+                     //           IE, you should not enable PRECISE_F32 1 or 2.
 var SIMD = 0; // Whether to emit SIMD code ( https://github.com/johnmccutchan/ecmascript_simd )
 
 var CLOSURE_COMPILER = 0; // Whether closure compiling is being run on this output