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std.rand.float: simplify leading zero calculations

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This saves a `bitwise or` operation in the common case and
removes the (slightly magic) mask constants.
This commit is contained in:
Erik Arvstedt
2022-05-12 16:00:35 +02:00
parent 1d5ea10bee
commit 23ef7a8060
1 changed files with 6 additions and 5 deletions
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11
lib/std/rand.zig
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@@ -256,12 +256,12 @@ pub const Random = struct {
// If all 41 bits are zero, generate additional random bits, until a
// set bit is found, or 126 bits have been generated.
const rand = r.int(u64);
var rand_lz = @clz(u64, rand | 0x7FFFFF);
if (rand_lz == 41) {
var rand_lz = @clz(u64, rand);
if (rand_lz >= 41) {
// TODO: when #5177 or #489 is implemented,
// tell the compiler it is unlikely (1/2^41) to reach this point.
// (Same for the if branch and the f64 calculations below.)
rand_lz += @clz(u64, r.int(u64));
rand_lz = 41 + @clz(u64, r.int(u64));
if (rand_lz == 41 + 64) {
// It is astronomically unlikely to reach this point.
rand_lz += @clz(u32, r.int(u32) | 0x7FF);
@@ -276,8 +276,9 @@ pub const Random = struct {
// If all 12 bits are zero, generate additional random bits, until a
// set bit is found, or 1022 bits have been generated.
const rand = r.int(u64);
var rand_lz: u64 = @clz(u64, rand | 0xFFFFFFFFFFFFF);
if (rand_lz == 12) {
var rand_lz: u64 = @clz(u64, rand);
if (rand_lz >= 12) {
rand_lz = 12;
while (true) {
// It is astronomically unlikely for this loop to execute more than once.
const addl_rand_lz = @clz(u64, r.int(u64));