digitalmars.D - Function calls overhead
- AlbertG (83/83) Jun 23 Hi everyone,
- Richard (Rikki) Andrew Cattermole (7/7) Jun 23 Performance of builds that are not optimized by ldc or gdc are
- Resee (5/12) Jun 24 It's time to ship Phobos/DRuntime as a source, instead of a
Hi everyone, I am currently working on the templatization of the casting hooks in DMD. The goal of runtime hooks templatization was to improve the runtime performance by moving the type chekcs to compile time. This has worked great for array-related hooks, but since most of the casting hooks require runtime type information (e.g. for performing dynamic casts), the potential gains are much more limited. However, it still makes sense to templatize them for consistency and maintainability. To assess the performance impact of templatization, I have run some benchmarks. One that caught my attention was the one for `_d_class_cast`. The benchmarked code is as follows: ```d class A {} class B : A {} class C : B {} A ac = new C(); for (auto cnt = 0; cnt < 256; ++cnt) { B b = cast(B) ac; } ``` I have measured the time it takes to run this code **100_000** times, and the results are the following: - Template vs Non-template - raw: ```bash ============================================================ Testing non-template hook 256 iterations 100000 runs: average time = 103.8ms; std dev = 1.16619 ============================================================ Testing template hook 256 iterations 100000 runs: average time = 172.3ms; std dev = 0.9 ============================================================ libdruntime.a size: old=16103056 B / new=16297864 B => 1.21% change libphobos2.a size: old=55382078 B / new=55637346 B => 0.46% change libphobos2.so size: old=8586016 B / new=8606224 B => 0.24% change ``` - Template vs Non-template - with `_d_class_cast` inlined, so without the overhead of the wrapper function: ```bash ============================================================ Testing non-template hook 256 iterations 100000 runs: average time = 101.6ms; std dev = 0.663325 ============================================================ Testing template hook 256 iterations 100000 runs: average time = 138.6ms; std dev = 1.2 ============================================================ libdruntime.a size: old=16103056 B / new=16231972 B => 0.80% change libphobos2.a size: old=55382078 B / new=55552132 B => 0.31% change libphobos2.so size: old=8586016 B / new=8599512 B => 0.16% change ``` - Template vs Non-template - with both `_d_class_cast` and `_d_class_cast_impl` inlined, so no extra function calls at all: ```bash ============================================================ Testing non-template hook 256 iterations 100000 runs: average time = 103.5ms; std dev = 1.74643 ============================================================ Testing template hook 256 iterations 100000 runs: average time = 96.9ms; std dev = 1.44568 ============================================================ libdruntime.a size: old=16103056 B / new=16232328 B => 0.80% change libphobos2.a size: old=55382078 B / new=55553686 B => 0.31% change libphobos2.so size: old=8586016 B / new=8603512 B => 0.20% change ``` As you can see, there is quite a significant overhead when extra function calls are involved. The size increase is also slightly lower when inlining is applied, but this might be particular to druntime/phobos as they may use casts between different types without too much overlapping that would benefit from no inlining. Overall, my suggestion is to perform total inlining of casting functions in the main `_d_cast` hook in order to minimize the overhead, at the cost of a possible increase in binary size. What are your thoughts on this? Do you have any better ideas?
Jun 23
Performance of builds that are not optimized by ldc or gdc are irrelevant to concerns on performance. These builds will always be slow, additional overhead of some hooks are the least of it. Usually for such builds the concern is more for debuggability, so keeping them as separate symbols is to the benefit of the user, rather than an issue.
Jun 23
On Monday, 23 June 2025 at 18:29:59 UTC, Richard (Rikki) Andrew Cattermole wrote:Performance of builds that are not optimized by ldc or gdc are irrelevant to concerns on performance. These builds will always be slow, additional overhead of some hooks are the least of it. Usually for such builds the concern is more for debuggability, so keeping them as separate symbols is to the benefit of the user, rather than an issue.It's time to ship Phobos/DRuntime as a source, instead of a compiled binary. It'll improve both debug-ability and performance.
Jun 24