multf3 has incorrect rounding of subnormals · Issue #91840 · llvm/llvm-project (original) (raw)

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;

}

I am testing on aarch64. Using the built-in 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:

$ 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):

$ 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.

I am not sure why this appears to be aarch64 only. reproduced on x86. See some more background at rust-lang/compiler-builtins#607.