问题
Consider the following C statements:
unsigned long x = 1;
float a = -x;
double b = -x;
I would expect the unary minus term to yield an unsigned long value equal to ULONG_MAX and a and b to be set to single and double precision representations of ULONG_MAX, respectively.
This is the result I obtain with gcc 4.4.7 on 32-bit Linux and with the Intel and PGI compilers on 64-bit Linux. With gcc (tested versions 4.4.7, 4.7.2 and 4.8.0, both with -O0 and -O2) on 64-bit Linux, however, the double variable b has the expected value, but float a becomes equal to -1 instead.
By contrast, the following statements will set both a and b to floating point representations of ULONG_MAX on all compilers and systems I tested:
unsigned long x = 1;
unsigned long y = -x;
float a = y;
double b = y;
If I use unsigned int instead of unsigned long, I get the expected result on all systems, too.
Is this some kind of undefined behaviour or a compiler error?
回答1:
This is due to a bug in GCC -- the type conversion happens before the negation.
The issue seems to be have been around for a while. Bug 55771 - Negation and type conversion incorrectly exchanged
In your second example, the negation happens before the type conversion. As such, you see expected results.
回答2:
What you describe is a compiler bug.
(and there is no undefined behavior in the program below to excuse the compiler)
unsigned long x = 1;
float a = -x;
double b = -x;
来源：https://stackoverflow.com/questions/17611799/combination-of-unary-minus-and-float-conversion