more precise check for negation

Author: alexreinking

src/IROperator.cpp

@@ -609,10 +609,12 @@ Expr lossless_negate(const Expr &x) {
     } else if (const FloatImm *f = x.as<FloatImm>()) {
         return FloatImm::make(f->type, -f->value);
     } else if (const Cast *c = x.as<Cast>()) {
-        // Only safe to negate through a cast when the inner type is signed.
+        // Only safe to negate through a cast when the inner type is not unsigned.
         // For unsigned inner types, negation wraps modularly (e.g., -uint8(65)
         // = uint8(191)), so cast(outer, -inner) != -cast(outer, inner).
-        if (c->value.type().is_int()) {
+        // Signed integers and floats are fine: signed negation is checked for
+        // overflow below via lossless_cast, and float negation is exact.
+        if (!c->value.type().is_uint()) {
             Expr value = lossless_negate(c->value);
             if (value.defined()) {
                 // This logic is only sound if we know the cast can't overflow.