https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88153
Daniel Fruzynski <bugzi...@poradnik-webmastera.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|INVALID |---
--- Comment #4 from Daniel Fruzynski <bugzi...@poradnik-webmastera.com> ---
I checked man page for errno and it has following sencence:
"Valid error numbers are all nonzero; errno is never set to zero by any system
call or library function."
This means that code like mine from Comment 0 should do the trick: it checks
for negative values for all processed values, stores status in temporary
variable, and calls sqrt(-1) once at the end if one of these values was
negative.
I have created small benchmark:
[code]
#include <benchmark/benchmark.h>
#include <math.h>
#include <emmintrin.h>
#define SIZE 160
double src[SIZE];
double dest[SIZE];
static void BM_sqrt(benchmark::State& state)
{
for (auto _ : state)
{
for (int n = 0; n < SIZE; ++n)
dest[n] = sqrt(src[n]);
benchmark::ClobberMemory();
}
}
// Register the function as a benchmark
BENCHMARK(BM_sqrt);
static void BM_sse_sqrt_errno(benchmark::State& state)
{
for (auto _ : state)
{
int m = 0;
for (int n = 0; n < SIZE; n += 2)
{
__m128d v = _mm_load_pd(&src[n]);
__m128d vs = _mm_sqrt_pd(v);
__m128d vn = _mm_cmplt_pd(v, _mm_setzero_pd());
m |= _mm_movemask_pd(vn);
_mm_store_pd(&dest[n], vs);
}
if (m)
sqrt(-1.0);
benchmark::ClobberMemory();
}
}
// Register the function as a benchmark
BENCHMARK(BM_sse_sqrt_errno);
static void BM_sse_sqrt(benchmark::State& state)
{
for (auto _ : state)
{
for (int n = 0; n < SIZE; n += 2)
{
__m128d v = _mm_load_pd(&src[n]);
__m128d vs = _mm_sqrt_pd(v);
_mm_store_pd(&dest[n], vs);
}
benchmark::ClobberMemory();
}
}
// Register the function as a benchmark
BENCHMARK(BM_sse_sqrt);
BENCHMARK_MAIN();
[/code]
This code was compiled using gcc 4.8.5, with following options:
g++ -std=c++11 -o test test.cc -O3 -I/benchmark/include/ -L/benchmark/lib/
-lbenchmark
Results for SIZE = 16 (loops unrolled):
---------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------
BM_sqrt 86 ns 86 ns 7188074
BM_sse_sqrt_errno 15 ns 15 ns 48084834
BM_sse_sqrt 15 ns 15 ns 47797778
Results for SIZE = 160 (loops not unrolled):
---------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------
BM_sqrt 995 ns 995 ns 839866
BM_sse_sqrt_errno 156 ns 156 ns 4348870
BM_sse_sqrt 144 ns 144 ns 4549107
As you can see, results for BM_sse_sqrt_errno are much better than BM_sqrt and
close to BM_sse_sqrt. If optimization implemented in BM_sse_sqrt_errno
satisfies error handling requirements for sqrt(), it is definitely worth
implementing in gcc.
