How to force GCC to assume that a floating-point expression is non-negative?

Question

There are cases where you know that a certain floating-point expression will always be non-negative. For example, when computing the length of a vector, one does sqrt(

Answer 1

Without any global options, here is a (low-overhead, but not free) way to get a square root with no branch:

#include <immintrin.h>

float test(float x)
{
    return _mm_cvtss_f32(_mm_sqrt_ss(_mm_set1_ps(x * x)));
}

(on godbolt)

As usual, Clang is smart about its shuffles. GCC and MSVC lag behind in that area, and don't manage to avoid the broadcast. MSVC is doing some mysterious moves as well..

There are other ways to turn a float into an __m128, for example _mm_set_ss. For Clang that makes no difference, for GCC that makes the code a little bigger and worse (including a movss reg, reg which counts as a shuffle on Intel, so it doesn't even save on shuffles).

Answer 2

Pass the option -fno-math-errno to gcc. This fixes the problem without making your code unportable or leaving the realm of ISO/IEC 9899:2011 (C11).

What this option does is not attempting to set errno when a math library function fails:

       -fno-math-errno
           Do not set "errno" after calling math functions that are executed
           with a single instruction, e.g., "sqrt".  A program that relies on
           IEEE exceptions for math error handling may want to use this flag
           for speed while maintaining IEEE arithmetic compatibility.

           This option is not turned on by any -O option since it can result
           in incorrect output for programs that depend on an exact
           implementation of IEEE or ISO rules/specifications for math
           functions. It may, however, yield faster code for programs that do
           not require the guarantees of these specifications.

           The default is -fmath-errno.

           On Darwin systems, the math library never sets "errno".  There is
           therefore no reason for the compiler to consider the possibility
           that it might, and -fno-math-errno is the default.

Given that you don't seem to be particularly interested in math routines setting errno, this seems like a good solution.

Answer 3

After about a week, I asked on the matter on GCC Bugzilla & they've provided a solution which is the closest to what I had in mind

float test (float x)
{
    float y = x*x;
    if (std::isless(y, 0.f))
        __builtin_unreachable();
    return std::sqrt(y);
}

that compiles to the following assembly:

test(float):
    mulss   xmm0, xmm0
    sqrtss  xmm0, xmm0
    ret

I'm still not quite sure what exactly happens here, though.

Answer 4

You can write assert(x*x >= 0.f) as a compile-time promise instead of a runtime check as follows in GNU C:

#include <cmath>

float test1 (float x)
{
    float tmp = x*x;
    if (!(tmp >= 0.0f)) 
        __builtin_unreachable();    
    return std::sqrt(tmp);
}

(related: What optimizations does __builtin_unreachable facilitate? You could also wrap if(!x)__builtin_unreachable() in a macro and call it promise() or something.)

But gcc doesn't know how to take advantage of that promise that tmp is non-NaN and non-negative. We still get (Godbolt) the same canned asm sequence that checks for x>=0 and otherwise calls sqrtf to set errno. Presumably that expansion into a compare-and-branch happens after other optimization passes, so it doesn't help for the compiler to know more.

This is a missed-optimization in the logic that speculatively inlines sqrt when -fmath-errno is enabled (on by default unfortunately).

What you want instead is -fno-math-errno, which is safe globally

This is 100% safe if you don't rely on math functions ever setting errno. Nobody wants that, that's what NaN propagation and/or sticky flags that record masked FP exceptions are for. e.g. C99/C++11 fenv access via #pragma STDC FENV_ACCESS ON and then functions like fetestexcept(). See the example in feclearexcept which shows using it to detect division by zero.

The FP environment is part of thread context while errno is global.

Support for this obsolete misfeature is not free; you should just turn it off unless you have old code that was written to use it. Don't use it in new code: use fenv. Ideally support for -fmath-errno would be as cheap as possible but the rarity of anyone actually using __builtin_unreachable() or other things to rule out a NaN input presumably made it not worth developer's time to implement the optimization. Still, you could report a missed-optimization bug if you wanted.

Real-world FPU hardware does in fact have these sticky flags that stay set until cleared, e.g. x86's mxcsr status/control register for SSE/AVX math, or hardware FPUs in other ISAs. On hardware where the FPU can detect exceptions, a quality C++ implementation will support stuff like fetestexcept(). And if not, then math-errno probably doesn't work either.

errno for math was an old obsolete design that C / C++ is still stuck with by default, and is now widely considered a bad idea. It makes it harder for compilers to inline math functions efficiently. Or maybe we're not as stuck with it as I thought: Why errno is not set to EDOM even sqrt takes out of domain arguement? explains that setting errno in math functions is optional in ISO C11, and an implementation can indicate whether they do it or not. Presumably in C++ as well.

It's a big mistake to lump -fno-math-errno in with value-changing optimizations like -ffast-math or -ffinite-math-only. You should strongly consider enabling it globally, or at least for the whole file containing this function.

float test2 (float x)
{
    return std::sqrt(x*x);
}

# g++ -fno-math-errno -std=gnu++17 -O3
test2(float):   # and test1 is the same
        mulss   xmm0, xmm0
        sqrtss  xmm0, xmm0
        ret

---

You might as well use -fno-trapping-math as well, if you aren't ever going to unmask any FP exceptions with feenableexcept(). (Although that option isn't required for this optimization, it's only the errno-setting crap that's a problem here.).

-fno-trapping-math doesn't assume no-NaN or anything, it only assumes that FP exceptions like Invalid or Inexact won't ever actually invoke a signal handler instead of producing NaN or a rounded result. -ftrapping-math is the default but it's broken and "never worked" according to GCC dev Marc Glisse. (Even with it on, GCC does some optimizations which can change the number of exceptions that would be raised from zero to non-zero or vice versa. And it blocks some safe optimizations). But unfortunately, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54192 (make it off by default) is still open.

If you actually ever did unmask exceptions, it might be better to have -ftrapping-math, but again it's very rare that you'd ever want that instead of just checking flags after some math operations, or checking for NaN. And it doesn't actually preserve exact exception semantics anyway.

See SIMD for float threshold operation for a case where -fno-trapping-math incorrectly blocks a safe optimization. (Even after hoisting a potentially-trapping operation so the C does it unconditionally, gcc makes non-vectorized asm that does it conditionally! So not only does it block vectorization, it changes the exception semantics vs. the C abstract machine.)