| Issue |
91840
|
| Summary |
`multf3` has incorrect rounding on `aarch64` linux
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tgross35
|
Testing with the following program:
```
#define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#if defined(__clang__) && (defined(__i386) || defined(__x86_64))
#define _Float128 __float128
#endif
typedef struct {
uint64_t lower, upper;
} u128;
void f128_print(_Float128 val) {
u128 ival = *((u128 *)(&val));
#ifndef __clang__
char buf[1024];
strfromf128(buf, sizeof(buf), "%.32g", val);
printf("%#018" PRIx64 "%016" PRIx64 " %s\n", ival.upper, ival.lower, buf);
#else
printf("%#018" PRIx64 "%016" PRIx64 " %lf\n", ival.upper, ival.lower, (double)val);
#endif
}
_Float128 new_f128(uint64_t upper, uint64_t lower) {
u128 val;
val.lower = lower;
val.upper = upper;
return *((_Float128 *)(&val));
}
int main() {
_Float128 a = new_f128(0x00007fffffffffff, 0xffffffffffffffff);
_Float128 b = new_f128(0x40007fffffffffff, 0xffffffffffffffff);
f128_print(a);
f128_print(b);
_Float128 c = a * b;
f128_print(c);
return 0;
}
```
Using the builtin version of Clang that links against system libraries gets the multf3 symbol from either `/usr/lib/gcc/aarch64-linux-gnu/11/libgcc.a` or `/usr/lib/llvm-14/lib/clang/14.0.0/lib/linux/libclang_rt.builtins-aarch64.a`. This is the output, which is correct:
```sh
$ clang f128_demo.c -o f128_demo
$ ./f128_demo
0x00007fffffffffffffffffffffffffff 0.000000
0x40007fffffffffffffffffffffffffff 3.000000
0x00017ffffffffffffffffffffffffffc 0.000000
```
However, building with the current `multf3.c` produces an incorrect value (note the ending `b`):
```sh
$ clang f128_demo.c llvm-project/compiler-rt/lib/builtins/multf3.c -o f128_demo_current_rt
$ ./f128_demo_current_rt
0x00007fffffffffffffffffffffffffff 0.000000
0x40007fffffffffffffffffffffffffff 3.000000
0x00017ffffffffffffffffffffffffffb 0.000000
```
This was just tested in a ubuntu docker container which has clang 14.0.0 and gcc 11.4.0
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