jit

On .NET 10 a plain for loop over span[i] bounded by span.Length compiles to a bounds-check-free, strength-reduced, down-counting loop. A hand-written Unsafe.Add(ref r, i) loop with an int index keeps a sign extension and an up-counting compare, and measured 2-5% slower. Benchmarks and JIT disassembly for arm64 and x64 on .NET 10.0.10.

A transparent struct is one the ABI treats exactly like its only field. .NET 11 has no such guarantee: a struct wrapping a double is passed in RCX on Windows x64 but in XMM0 on Linux and macOS. Here is the disassembly from the .NET 11 JIT on three ABIs, the P/Invoke bugs it causes, and how to write wrapper types that are safe at every boundary.

Stephen Toub's Performance Improvements in .NET 11 post shows generic virtual method calls dropping from 6.7 ns and 24 bytes to 1.8 ns and zero allocation. Three RyuJIT pull requests unlocked inlining on the dispatch that used to be the most opaque one in .NET.

W^X (write xor execute) is the rule that no memory page is writable and executable at the same time. In .NET it is the DOTNET_EnableWriteXorExecute knob, on by default since .NET 7, and it exists entirely for the JIT. Native AOT never reads it. Here is how the runtime implements it, what it costs, and when turning it off is a legitimate fix.

PGO (profile-guided optimization) lets the .NET JIT specialize hot code for the types and branches your workload actually hits. Dynamic PGO has been on by default since .NET 8, so on .NET 8 and later you do not need to opt in. Here is what it does, how to see its effect, and the rare cases where you touch the knob.

Tiered compilation lets the .NET JIT compile every method twice: once fast and unoptimized to get your app running, then again with full optimizations once the runtime knows a method is hot. Here is how tier 0, tier 1, on-stack replacement, and Dynamic PGO fit together, and how to observe and tune them.

.NET 11 Preview 3 teaches RyuJIT to eliminate redundant bounds checks on consecutive index-from-end access and on i + constant < length patterns, cutting branch pressure in tight loops.