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)?

Solution 1

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 = (bar *) 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.

Solution 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 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.

Solution 3

Here is a link to study conducted in 2004: http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1124&context=ecetr concerning, among others, strict aliasing impact on code performance. Figure 2.5 shows relative improvement of 3% to 10%.

Researchers' explanation of performance degradation:

From inspecting the assembly code, we found that the degradation is an effect of the register allocation algorithm. GCC implements a graph coloring register allocator[2, 3]. With strict aliasing, the live ranges of the variables become longer, leading to high register pressure and ‘ spilling. With more conservative aliasing, the same variables incur memory transfers at the end of their (shorter) live ranges as well.

[2] Peter Bergner, Peter Dahl, David Engebretsen, and Matthew T. O’Keefe. Spill code minimization via interference region spilling. In SIGPLAN Conference on Programming Language Design and Implementation, pages 287–295, 1997.

[3] Preston Briggs, Keith D. Cooper, and Linda Torczon. Improvements to graph coloring register allocation. ACM Transactions on Programming Languages and Systems, 16(3):428–455, May 1994.