Gabriel Ravier <gabrav...@gmail.com> wrote:
Independent from the defunct flow analysis in the presence of NaNs, my
example demonstrates another minor deficiency: know thy instruction set!
See the comments in the assembly below.
> On 8/13/21 8:58 PM, Stefan Kanthak wrote:
>> Hi,
>>
>> compile the following naive implementation of nextafter() for AMD64:
>>
>> JFTR: ignore the aliasing casts, they don't matter here!
>>
>> $ cat repro.c
>> double nextafter(double from, double to)
>> {
>> if (to != to)
>> return to; // to is NAN
>>
>> if (from != from)
>> return from; // from is NAN
>>
>> if (from == to) // neither from nor to can be NAN here!
>> return to;
>>
>> if (from == 0.0) // dito!
>> return to < 0.0 ? -0x1.0p-1074 : 0x1.0p-1074;
>>
>> unsigned long long ull = *(unsigned long long *) &from;
>>
>> if ((from < to) == (from < 0.0))
>> ull--;
>> else
>> ull++;
>>
>> return *(double *) &ull;
>> }
>> $ gcc -m64 -o- -O3 -S repro.c
>> ...
>> nextafter:
[...]
>> .L4:
>> comisd %xmm0, %xmm1 # sets CF if first comparand < second
>> comparand
>> movq %xmm0, %rdx
>> leaq -1(%rdx), %rax # superfluous
>> seta %r8b # sbb %rcx, %rcx
>> comisd %xmm0, %xmm2
>> seta %cl # sbb %rax, %rax
>> addq $1, %rdx # xor %rcx, %rax
>> cmpb %cl, %r8b # or $1, %rax
>> cmovne %rdx, %rax # add %rdx, %rax
>> movq %rax, %xmm0
Stefan
