Fix inaccurate round while being fast

This patch applies a clever rounding approach to fix subtle correctness
bugs in the implementation of float rounding. It uses copysign to avoid
branching, and with the hope of using llvm intrinsics where available.

Work in progress, tests are missing.

Fixes emscripten-core#7306

Author: raphlinus

src/library.js

@@ -1566,30 +1566,34 @@ LibraryManager.library = {
 
   round__asm: true,
   round__sig: 'dd',
+  round__deps: ['llvm_copysign_f64'],
   round: function(d) {
     d = +d;
-    return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
+    return +_llvm_copysign_f64(+Math_floor((+Math_abs(d) + +0.2499999999999999) + +0.2500000000000001), d);
   },
 
   roundf__asm: true,
   roundf__sig: 'ff',
+  roundf__deps: ['llvm_copysign_f64'],
   roundf: function(d) {
     d = +d;
-    return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
+    return +_llvm_copysign_f64(+Math_floor((+Math_abs(d) + +0.2499999999999999) + +0.2500000000000001), d);
   },
 
   llvm_round_f64__asm: true,
   llvm_round_f64__sig: 'dd',
+  llvm_round_f64__deps: ['llvm_copysign_f64'],
   llvm_round_f64: function(d) {
     d = +d;
-    return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
+    return +_llvm_copysign_f64(+Math_floor((+Math_abs(d) + +0.2499999999999999) + +0.2500000000000001), d);
   },
 
   llvm_round_f32__asm: true,
   llvm_round_f32__sig: 'ff',
+  llvm_round_f32__deps: ['llvm_copysign_f64'],
   llvm_round_f32: function(f) {
     f = +f;
-    return f >= +0 ? +Math_floor(f + +0.5) : +Math_ceil(f - +0.5); // TODO: use fround?
+    return +_llvm_copysign_f64(+Math_floor((+Math_abs(d) + +0.2499999999999999) + +0.2500000000000001), d);
   },
 
   rintf__asm: true,