Performance impact of -fno-strict-aliasing

Question

Is there any study or set of benchmarks showing the performance degradation due to specifying -fno-strict-aliasing in GCC (or equivalent in other compilers)?

Answer 1

Have you already checked the strict aliasing question here?
(there is also an Ada reference here)

Answer 2

What I can tell you from experience (having tested this with a large project on PS3, PowerPC being an architecture that due to it's many registers can actually benefit from SA quite well) is that the optimizations you're going to see are generally going to be very local (scope wise) and small. On a 20MB executable it scraped off maybe 80kb of the .text section (= code) and this was all in small scopes & loops.

This option has can make your generated code a bit more lightweight and optimized than it is right now (think in the 1 to 5 percent range), but do not expect any big results. Hence, the effect of using -fno-strict-aliasing is probably not going to be a big influence on your performance, at all. That said, having code that requires -fno-strict-aliasing is a suboptimal situation at best.

Answer 3

It will vary a lot from compiler to compiler, as different compilers implement it with different levels of aggression. GCC is fairly aggressive about it: enabling strict aliasing will cause it to think that pointers that are "obviously" equivalent to a human (as in, foo* a; bar * b; b = (foo*)a;) cannot alias, which allows for some very aggressive transformations, but can obviously break non-carefully written code. Apple's GCC disables strict aliasing by default for this reason.

LLVM, by contrast, does not even have strict aliasing, and, while it is planned, the developers have said that they plan to implement it as a fall-back case when nothing else can judge equivalence. In the above example, it would still judge a and b equivalent. It would only use type-based aliasing if it could not determine their relationship in any other way.

In my experience, the performance impact of strict aliasing mostly has to do with loop invariant code motion, where type information can be used to prove that in-loop loads can't alias the array being iterated over, allowing them to be pulled out of the loop. YMMV.