gcc differences between -O3 vs -Ofast optimizations

Question

I was just reading through the gcc manual to find out the difference between -O3 and -Ofast.

For -O3

-O3

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:
-fgcse-after-reload 
-fipa-cp-clone
-floop-interchange 
-floop-unroll-and-jam 
-fpeel-loops 
-fpredictive-commoning 
-fsplit-paths 
-ftree-loop-distribute-patterns 
-ftree-loop-distribution 
-ftree-loop-vectorize 
-ftree-partial-pre 
-ftree-slp-vectorize 
-funswitch-loops 
-fvect-cost-model 
-fversion-loops-for-strides

While of -Ofast

-Ofast

Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for
all standard-compliant programs. It turns on -ffast-math,
-fallow-store-data-races and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens

Therefore I was wondering, if maybe -Ofast is for some reason less safe than -O3, and therefore I should stick to -O3 most of the times then.

Can you clarify the "practical difference" when using them? And if -Ofast is actually safe?

Answer 1

Ofast enables optimizations which violate requirements of C standard for floating-point semantics. In particular under -Ofast (aka -ffast-math) it'll freely reorder float computations (which is prohibited by default because in general a + (b + c) != (a + b) + c != (a + c) + b for floats).

Whether -Ofast is safe in particular case depends on the algorithm but usually most non-scientific applications work fine with it. For example most game engines are built with -ffast-math.

EDIT:

Since 2021 -Ofast in GCC also enables -fno-semantic-interposition which is described in detail here.

• Bart: And nowadays also -fallow-store-data-races gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html