| Issue |
165989
|
| Summary |
`const`/`constexpr` INFINITY can cause clang to make a different result from `inf/inf` or `inf-inf``
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
wind0204
|
_Title_.
The usual result from `inf/inf` or `inf-inf` is a negative NaN, but when I use a `const` infinity clang calculates it in compile-time and saves a positive NaN for the program
`Ubuntu GCC 13.3.0` didn't have the issue but `Ubuntu clang 18` and `Ubuntu clang 20` showed the symptom.
The CPU I use is Haswell.
I think you can reproduce the symptom with the code below ( `clang minimal.c -std=gnu23 -lm` )
```c
#include <stdio.h>
#include <stdint.h>
#include <math.h>
/*
* \brief for little endian systems
*/
union ieee754_binary32 {
struct {
uint32_t fraction: 23;
uint16_t exponent: 8;
uint8_t sign: 1;
};
float whole;
};
int main(int argc, char * argv[])
{
float inf = INFINITY;
float nan = NAN;
#if __clang_major__ <= 18
const float cinf = INFINITY;
const float cnan = NAN;
#else
constexpr float cinf = INFINITY;
constexpr float cnan = NAN;
#endif
fprintf( stdout, "inf - inf = %f\n", (inf - inf) );
fprintf( stdout, "const_inf - const_inf = %f\n", (cinf - cinf) );
union ieee754_binary32 inspector = {.whole=(cinf - cinf)};
fprintf( stdout, "inspector = {.whole=(const_inf - const_inf)}; inspector.sign = %u\n", inspector.sign );
fprintf( stdout, "inf / inf = %f\n", (inf / inf) );
inspector.whole = (inf / inf);
fprintf( stdout, "inspector = {.whole=(inf / inf)}; inspector.sign = %u\n", inspector.sign );
fprintf( stdout, "const_inf / const_inf = %f\n", (cinf / cinf) );
inspector.whole = (cinf / cinf);
fprintf( stdout, "inspector = {.whole=(const_inf / const_inf)}; inspector.sign = %u\n", inspector.sign );
return 0;
}
```
The output from clang-built binaries:
```c
inf - inf = -nan
const_inf - const_inf = nan
inspector = {.whole=(const_inf - const_inf)}; inspector.sign = 0
inf / inf = -nan
inspector = {.whole=(inf / inf)}; inspector.sign = 1
const_inf / const_inf = nan
inspector = {.whole=(const_inf / const_inf)}; inspector.sign = 0
```
The output from a GCC-built binary
```c
inf - inf = -nan
const_inf - const_inf = -nan
inspector = {.whole=(const_inf - const_inf)}; inspector.sign = 1
inf / inf = -nan
inspector = {.whole=(inf / inf)}; inspector.sign = 1
const_inf / const_inf = -nan
inspector = {.whole=(const_inf / const_inf)}; inspector.sign = 1
```
Thank you so much!
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