[llvm] [Flang-rt][build] fixes building warnings under gcc: (PR #173955)

[Eugene Epshteyn via llvm-commits]

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@@ -452,7 +452,13 @@ float RTNAME(Rand)(int *i, const char *sourceFile, int line) {
   if (radix == 2) {
     mask = ~(unsigned)0u << (32 - digits + 1);
   } else if (radix == 16) {
-    mask = ~(unsigned)0u << ((8 - digits) * 4 + 1);
+    int shift_val = ((8 - digits) * 4 + 1);
+    if (shift_val < 0) {
+      Terminator terminator{sourceFile, line};
+      terminator.Crash("Radix 16: negative shift for mask. digits value maybe invalid.");
+    } else {
+      mask = ~(unsigned)0u << shift_val;
+    }
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eugeneepshteyn wrote:

`digits` is known at compile time, thus at most this should be `static_assert`.

Also, the value of `digits` comes from system headers, and if the system headers give us wrong values, then we have bigger problems than building flang...

In addition, I'm curious why gcc had problem with `8 - digits` and not `32 - digits`.

https://github.com/llvm/llvm-project/pull/173955