branchless dateFromEpoch

Author: MaxGraey

std/assembly/date.ts

@@ -332,17 +332,15 @@ function invalidDate(millis: i64): bool {
 // Paper: https://arxiv.org/pdf/2102.06959.pdf
 function dateFromEpoch(ms: i64): i32 {
   let da = (<i32>floorDiv(ms, MILLIS_PER_DAY) * 4 + EPOCH_OFFSET * 4) | 3;
-  let q0 = floorDiv(da, DAYS_PER_EPOCH); // [0, 146096]
+  let q0 = floorDiv(da, DAYS_PER_EPOCH);  // [0, 146096]
   let r1 = <u32>da - q0 * DAYS_PER_EPOCH;
   let u1 = u64(r1 | 3) * 2939745;
   let dm1 = <u32>u1 / 11758980;
   let n1 = 2141 * dm1 + 197913;
   let year = 100 * q0 + i32(u1 >>> 32);
-  let mo = n1 >>> 16;
-  _day = (n1 & 0xFFFF) / 2141 + 1; // [1, 31]
-  if (dm1 >= 306) { mo -= 12; ++year; }
-  _month = mo; // [1, 12]
-  return year;
+  _day = (n1 & 0xFFFF) / 2141 + 1;              // [1, 31]
+  _month = (n1 >>> 16) - (dm1 >= 306 ? 12 : 0); // [1, 12]
+  return year + (dm1 >= 306 ?  1 : 0);
 }
 
 // http://howardhinnant.github.io/date_algorithms.html#days_from_civil

tests/compiler/std/date.debug.wat

@@ -309,7 +309,6 @@
   (local $dm1 i32)
   (local $n1 i32)
   (local $year i32)
-  (local $mo i32)
   block $~lib/date/floorDiv<i64>|inlined.0 (result i64)
    local.get $ms
    local.set $a
@@ -398,33 +397,36 @@
   i32.add
   local.set $year
   local.get $n1
-  i32.const 16
-  i32.shr_u
-  local.set $mo
-  local.get $n1
   i32.const 65535
   i32.and
   i32.const 2141
   i32.div_s
   i32.const 1
   i32.add
   global.set $~lib/date/_day
+  local.get $n1
+  i32.const 16
+  i32.shr_u
   local.get $dm1
   i32.const 306
   i32.ge_u
-  if
-   local.get $mo
+  if (result i32)
    i32.const 12
-   i32.sub
-   local.set $mo
-   local.get $year
-   i32.const 1
-   i32.add
-   local.set $year
+  else
+   i32.const 0
   end
-  local.get $mo
+  i32.sub
   global.set $~lib/date/_month
   local.get $year
+  local.get $dm1
+  i32.const 306
+  i32.ge_u
+  if (result i32)
+   i32.const 1
+  else
+   i32.const 0
+  end
+  i32.add
   return
  )
  (func $~lib/date/Date#set:year (param $this i32) (param $year i32)

tests/compiler/std/date.release.wat

@@ -5395,7 +5395,6 @@
   (local $1 i32)
   (local $2 i32)
   (local $3 i32)
-  (local $4 i32)
   local.get $0
   i64.const 86399999
   i64.const 0
@@ -5413,19 +5412,19 @@
   i32.add
   i32.const 3
   i32.or
-  local.tee $1
+  local.tee $2
   i32.const 146096
   i32.const 0
-  local.get $1
+  local.get $2
   i32.const 0
   i32.lt_s
   select
   i32.sub
   i32.const 146097
   i32.div_s
-  local.set $2
-  local.get $1
+  local.set $1
   local.get $2
+  local.get $1
   i32.const 146097
   i32.mul
   i32.sub
@@ -5438,49 +5437,41 @@
   i32.wrap_i64
   i32.const 11758980
   i32.div_u
-  local.tee $4
+  local.tee $2
   i32.const 2141
   i32.mul
   i32.const 197913
   i32.add
-  local.set $3
-  local.get $0
-  i64.const 32
-  i64.shr_u
-  i32.wrap_i64
-  local.get $2
-  i32.const 100
-  i32.mul
-  i32.add
-  local.set $1
-  local.get $3
-  i32.const 16
-  i32.shr_u
-  local.set $2
-  local.get $3
+  local.tee $3
   i32.const 65535
   i32.and
   i32.const 2141
   i32.div_u
   i32.const 1
   i32.add
   global.set $~lib/date/_day
-  local.get $4
+  local.get $3
+  i32.const 16
+  i32.shr_u
+  i32.const 12
+  i32.const 0
+  local.get $2
   i32.const 306
   i32.ge_u
-  if
-   local.get $2
-   i32.const 12
-   i32.sub
-   local.set $2
-   local.get $1
-   i32.const 1
-   i32.add
-   local.set $1
-  end
-  local.get $2
+  local.tee $2
+  select
+  i32.sub
   global.set $~lib/date/_month
+  local.get $0
+  i64.const 32
+  i64.shr_u
+  i32.wrap_i64
   local.get $1
+  i32.const 100
+  i32.mul
+  i32.add
+  local.get $2
+  i32.add
  )
  (func $~lib/date/Date#toTimeString (param $0 i32) (result i32)
   (local $1 i32)