Editing Instruction scheduling (section)

== Compiler examples ==
The [[GNU Compiler Collection]] is one compiler known to perform instruction scheduling, using the {{code|-march}} (both instruction set and scheduling) or {{code|-mtune}} (only scheduling) flags. It uses descriptions of instruction latencies and what instructions can be run in parallel (or equivalently, which "port" each use) for each microarchitecture to perform the task. This feature is available to almost all architectures that GCC supports.<ref>{{cite web |title=x86 Options |url=https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html |website=Using the GNU Compiler Collection (GCC)}}</ref>

Until version 12.0.0, the instruction scheduling in [[LLVM]]/Clang could only accept a {{code|-march}} (called {{code|target-cpu}} in LLVM parlance) switch for both instruction set and scheduling. Version 12 adds support for {{code|-mtune}} ({{code|tune-cpu}}) for x86 only.<ref>{{cite web |title=⚙ D85384 [X86] Add basic support for -mtune command line option in clang |url=http://reviews.llvm.org/D85384 |website=reviews.llvm.org}}</ref>

Sources of information on latency and port usage include:
* GCC and LLVM;
* [[Agner Fog]], who compiles extensive data for the [[x86 architecture]];<ref name="optimize">{{cite web |title=Software optimization resources. C++ and assembly. Windows, Linux, BSD, Mac OS X |url=https://www.agner.org/optimize/ |website=Agner Fog}}</ref>
* InstLatx64, which uses [[AIDA64]] to collect data on x86 CPUs.<ref>{{cite web |title=x86, x64 Instruction Latency, Memory Latency and CPUID dumps |url=http://instlatx64.atw.hu/ |website=instlatx64.atw.hu}} See also the "Comments" link on the page.</ref>

LLVM's {{code|llvm-exegesis}} should be usable on all machines, especially to gather information on non-x86 ones.<ref>{{cite web |title=llvm-exegesis - LLVM Machine Instruction Benchmark |url=https://llvm.org/docs/CommandGuide/llvm-exegesis.html |website=LLVM 12 Documentation}}</ref>