Add SafeAbs to avoid UB when casting INT_MIN.

Author: smcallis

src/s2/util/math/exactfloat/exactfloat.cc

@@ -64,15 +64,17 @@ ExactFloat::ExactFloat(double v) {
   }
 }
 
+// Calculates abs(v) without UB.  SafeAbs(INT_MIN) == INT_MIN.
+// Generates the same code as std::abs().
+// https://godbolt.org/z/eT6KW1zGb
+int SafeAbs(int v) {
+  return v < 0 ? -static_cast<unsigned>(v) : v;
+}
+
 ExactFloat::ExactFloat(int v) {
   sign_ = (v >= 0) ? 1 : -1;
   bn_exp_ = 0;
-
-  if (v == std::numeric_limits<int>::min()) {
-    bn_ = Bignum(static_cast<unsigned>(v));
-  } else {
-    bn_ = Bignum(abs(v));
-  }
+  bn_ = Bignum(static_cast<unsigned>(SafeAbs(v)));
   Canonicalize();
 }