On 7/8/21 8:08 PM, LIU Hao via Gcc-help wrote:
在 7/8/21 8:05 PM, johnsfine--- via Gcc-help 写道:
This is not the forum for such a discussion. But I want to make
people reading this aware that many expert C and C++ programmers
(likely a majority) consider that advice to avoid unsigned types to be
horrible advice.
I advise people to avoid signed types and I do so myself. If an
integer value won't be negative, it shouldn't be signed. That makes
your intent clearer to anyone reading your code, and (especially in
x86-64) lets the compiler generate smaller and faster code.
That makes no sense. Would you prefer unsigned integers to signed ones,
for something that can only be one of {1,2,3,4}? Just because something
can't be negative, does not mean it should be unsigned.
There are benefits to making that explicit by choosing an unsigned
type: the result of converting a narrower unsigned type to a wider
unsigned type is unchanged. The result of converting it to a wider
signed type may change. This has an impact on value range propagation
which in turn can influence other optimizations as well as warnings.
Choosing an unsigned type has liabilities as well. Because unsigned
arithmetic wraps around zero whereas signed arithmetic does not (it
overflows which is undefined), its results are less constrained than
those of signed arithmetic.
Martin
Conversion between `uint64_t` and `double` is much slower than `int64_t`
on some platforms, e.g. x86, so 'smaller and faster code' is also
groundless [1].
A signed integer is a value that denotes the number or amount of
something, such as bytes, characters, files, seconds, kilometers, or
even dollars. An unsigned integer is merely a fixed-sized collection of
bits. That's the only fundamental difference.
[1] https://gcc.godbolt.org/z/6YY61rhGe
