[Docs][llvm-exegesis] Specify supported platforms and architectures

[aidengrossman]

Summary

Currently, there is no documentation on what platforms and architectures
llvm-exegesis is supported on. This patch adds in user-facing
documentation in the CommandGuide about what architectures are supported
as well as developer facing documentation detailing the technical
reasons for why certain platforms are supported and some aren't.

This is a follow-up after discussion in
https://discourse.llvm.org/t/clarification-on-platform-support-for-llvm-exegesis/70206.

[aidengrossman]

@jsji @atanasyan @john.brawn In case they have any suggestions on what more specificity in currently supported architectures.

[aidengrossman]

Fix poorly resolved merge conflict that resulted in duplicate section in llvm-exegesis.rst.

[kpdev42]

As far as I know, all powerpc(64)?(le)?-linux-gnu triples are reported as unsupported except for powerpc64le-..., so PowerPC support seems to be not unconditional.

Considering calling conventions, some parts of it are probably hardcoded in snippet generators and repetitors and some are queried from LLVM subtarget info. The benchmark function accepts char *Memory argument that can be unused if snippet consists of only non memory related instructions. If loop repetitor is used, it may hardcode loop counter register that may interfere with unexpected calling convetions. On the other hand, prologue and epilogue are emitted by generic LLVM machinery and some ABI-specific information can be queried from it (see getFunctionReservedRegs from Assembler.cpp for example). Thus chances are that snippets generated for non-memory instructions with "duplicate" repetitor might be runnable even with non-SysV calling conventions.

llvm/docs/CommandGuide/llvm-exegesis.rst

36

[nit] Missing space between "ARM" and "("

[aidengrossman]

Update.

[aidengrossman]

Right. Some of it will work but a lot of it won't work and will probably crash. All of the parts that I'm aware of call into the ExegesisTarget implementations to get specific registers to use. In the case of getScratchMemoryRegister, for the x86 implementation, it does seem like Windows is supported as the function accepts a triple and changes the register depending upon the platform, so it seems like the ABI differences are better accounted for than I thought, and the the other snippet repetitor implementations, specifically the loop one, seem to take this into account when choosing registers.

[RKSimon]

llvm/docs/CommandGuide/llvm-exegesis.rst

38

Do we need to make it clear that analysis mode DOES work on any platform - just the capture modes have limitations?

llvm/docs/CommandGuide/llvm-exegesis.rst

 to the standard output.
 
 The main goal of this tool is to automatically (in)validate the LLVM's TableDef
 scheduling models. To that end, we also provide analysis of the results.
 
 :program:`llvm-exegesis` can also benchmark arbitrary user-provided code
 snippets.
 
+SUPPORTED PLATFORMS
+-------------------
+
+:program:`llvm-exegesis` currently only supports X86 (64-bit only), ARM(AArch64
+only), MIPS, and PowerPC on Linux. Not all functionality is guaranteed to work
+on every platform.
+
 SNIPPET ANNOTATIONS
 -------------------
 
 :program:`llvm-exegesis` supports benchmarking arbitrary snippets of assembly.
 However, benchmarking these snippets often requires some setup so that they
 can execute properly. :program:`llvm-exegesis` has two annotations and some
 additional utilities to help with setup so that snippets can be benchmarked
 properly.

llvm/tools/llvm-exegesis/README.md

+# llvm-exegesis
+
+`llvm-exegesis` is a benchmarking tool that accepts or assembles a snippet and
+can measure characteristics of that snippet by executing it while keeping track
+of performance counters.
+
+### Currently Supported Platforms
+
+`llvm-exegesis` is quite platform-dependent and currently only supports a couple
+platform configurations.
+
+##### Currently Supported Operating Systems
+
+Currently, `llvm-exegesis` only supports Linux. This is mainly due to a
+dependency on the Linux perf subsystem for reading performance counters.
+In addition, `llvm-exegesis` currently only implements the System V
+calling convention internally (used to pass data from the executing process
+to the snippet), further limiting platform independence.
+
+##### Currently Supported Architectures
+
+Currently, `llvm-exegesis` supports the following architectures:
+
+* x86
+  * 64-bit only due to this being the only implemented calling convention
+    in `llvm-exegesis` currently.
+* ARM
+  * AArch64 only
+* MIPS
+* PowerPC
+
+Note that not all functionality is guaranteed to work on all architectures.