[libcxx-commits] [libcxx] linear_congruential_engine: Fixes for __lce_alg_picker (PR #81080)

[Jonathan Wakely via libcxx-commits]

================
@@ -23,47 +23,62 @@ int main(int, char**)
     typedef unsigned long long T;
 
     // m might overflow, but the overflow is OK so it shouldn't use schrage's algorithm
-    typedef std::linear_congruential_engine<T, 25214903917ull, 1, (1ull<<48)> E1;
+    typedef std::linear_congruential_engine<T, 25214903917ull, 1, (1ull << 48)> E1;
     E1 e1;
     // make sure the right algorithm was used
     assert(e1() == 25214903918);
     assert(e1() == 205774354444503);
     assert(e1() == 158051849450892);
     // make sure result is in bounds
-    assert(e1() < (1ull<<48));
-    assert(e1() < (1ull<<48));
-    assert(e1() < (1ull<<48));
-    assert(e1() < (1ull<<48));
-    assert(e1() < (1ull<<48));
+    assert(e1() < (1ull << 48));
+    assert(e1() < (1ull << 48));
+    assert(e1() < (1ull << 48));
+    assert(e1() < (1ull << 48));
+    assert(e1() < (1ull << 48));
 
     // m might overflow. The overflow is not OK and result will be in bounds
     // so we should use shrage's algorithm
-    typedef std::linear_congruential_engine<T, (1ull<<2), 0, (1ull<<63) + 1> E2;
+    typedef std::linear_congruential_engine<T, (1ull << 2), 0, (1ull << 63) + 1> E2;
     E2 e2;
     // make sure shrage's algorithm is used (it would be 0s otherwise)
     assert(e2() == 4);
     assert(e2() == 16);
     assert(e2() == 64);
     // make sure result is in bounds
-    assert(e2() < (1ull<<48) + 1);
-    assert(e2() < (1ull<<48) + 1);
-    assert(e2() < (1ull<<48) + 1);
-    assert(e2() < (1ull<<48) + 1);
-    assert(e2() < (1ull<<48) + 1);
+    assert(e2() < (1ull << 48) + 1);
+    assert(e2() < (1ull << 48) + 1);
+    assert(e2() < (1ull << 48) + 1);
+    assert(e2() < (1ull << 48) + 1);
+    assert(e2() < (1ull << 48) + 1);
 
-    // m will not overflow so we should not use shrage's algorithm
-    typedef std::linear_congruential_engine<T, 1ull, 1, (1ull<<48)> E3;
+    // m might overflow. The overflow is not OK and result will be in bounds
+    // so we should use shrage's algorithm. m is even
+    typedef std::linear_congruential_engine<T, 0x18000001ull, 0x12347ull, (3ull << 56)> E3;
     E3 e3;
+    // make sure shrage's algorithm is used (it would be 0s otherwise)
----------------
jwakely wrote:

```suggestion
    // so we should use Schrage's algorithm. m is even
    typedef std::linear_congruential_engine<T, 0x18000001ull, 0x12347ull, (3ull << 56)> E3;
    E3 e3;
    // make sure Schrage's algorithm is used (it would be 0s otherwise)
```

https://github.com/llvm/llvm-project/pull/81080