[flang] Allow all OSes in fir::CodeGenSpecifics::get

[MaskRay]

Summary

This allows all ELF operating systems to use target specifics tuned for Linux,
since they use mostly the same ABIs. If some triples are to excluded, it's
better done at the driver layer.

[schweitz]

flang/lib/Optimizer/CodeGen/Target.cpp

348–349

Noting that the project may want the flexibility to create distinctions (or react to distinctions) between platforms at the OS level at some point. This code is largely focused on complex at the moment, but that may expand to other cases. Consider, for example, supporting different descriptor formats, different runtimes, etc.

[schweitz]

For what it is worth, flang was originally written to be strict rather than permissive. Checks are made to encode things like "this configuration has been tested" rather than just letting everything fall through and sifting through the fallout after the fact. Broadly speaking, the intention was for the compiler to produce known correct output/code when it could. If something was untested, unsupported, unimplemented, etc. however, the compiler was to give a reasonable error message rather than a possibly false impression that everything might have been ok or not.

[MaskRay]

In D135100#3835299, @schweitz wrote:

For what it is worth, flang was originally written to be strict rather than permissive. Checks are made to encode things like "this configuration has been tested" rather than just letting everything fall through and sifting through the fallout after the fact. Broadly speaking, the intention was for the compiler to produce known correct output/code when it could. If something was untested, unsupported, unimplemented, etc. however, the compiler was to give a reasonable error message rather than a possibly false impression that everything might have been ok or not.

The strictness is usually done in the driver, not in codegen.
This file has been touched by many groups adding various not-Linux systems. I think it should be open for all ELF OSes if an arch supports Linux.

Technically I can use a macOS/Windows/ELF dispatch but macOS/Windows aren't supported on many architectures anyway (LLVM doesn't allow) so I don't know whether that is useful.

[emaste]

Checks are made to encode things like "this configuration has been tested"

This unfortunately introduces hurdles in the path of testing additional configurations.

I wouldn't object to a warning along the lines of "This OS/CPU architecture combination has not been extensively tested" but disallowing it completely in codegen makes it harder to transition to tested and supported.

[schweitz]

I wasn't objecting. I was informing. (Flang did not even have a driver for a long, long time.)

[MaskRay]

Then wish I can get a stamp :)

[emaste]

LGTM for FreeBSD

[MaskRay]

I'll wait a bit and push this change.

[MaskRay]

flang/lib/Optimizer/CodeGen/Target.cpp

348–349

At that point, make the check at the driver, not in codegen:)

flang/lib/Optimizer/CodeGen/Target.cpp

 // TODO: Add other targets to this file as needed.
 std::unique_ptr<fir::CodeGenSpecifics>
 fir::CodeGenSpecifics::get(mlir::MLIRContext *ctx, llvm::Triple &&trp,
                            KindMapping &&kindMap) {
   switch (trp.getArch()) {
   default:
     break;
   case llvm::Triple::ArchType::x86:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-    case llvm::Triple::OSType::Darwin:
-    case llvm::Triple::OSType::MacOSX:
-    case llvm::Triple::OSType::Solaris:
-    case llvm::Triple::OSType::Win32:
-      return std::make_unique<TargetI386>(ctx, std::move(trp),
-                                          std::move(kindMap));
-    }
-    break;
   case llvm::Triple::ArchType::x86_64:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-    case llvm::Triple::OSType::Darwin:
-    case llvm::Triple::OSType::MacOSX:
-    case llvm::Triple::OSType::Solaris:
-    case llvm::Triple::OSType::Win32:
-      return std::make_unique<TargetX86_64>(ctx, std::move(trp),
-                                            std::move(kindMap));
-    }
-    break;
   case llvm::Triple::ArchType::aarch64:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-    case llvm::Triple::OSType::Darwin:
-    case llvm::Triple::OSType::MacOSX:
-    case llvm::Triple::OSType::Win32:
-      return std::make_unique<TargetAArch64>(ctx, std::move(trp),
-                                             std::move(kindMap));
-    }
-    break;
   case llvm::Triple::ArchType::ppc64le:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-      return std::make_unique<TargetPPC64le>(ctx, std::move(trp),
-                                             std::move(kindMap));
-    }
-    break;
   case llvm::Triple::ArchType::sparc:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-    case llvm::Triple::OSType::Solaris:
-      return std::make_unique<TargetSparc>(ctx, std::move(trp),
-                                           std::move(kindMap));
-    }
-    break;
   case llvm::Triple::ArchType::sparcv9:
-    switch (trp.getOS()) {
-    default:
-      break;
-    case llvm::Triple::OSType::Linux:
-    case llvm::Triple::OSType::Solaris:
-      return std::make_unique<TargetSparcV9>(ctx, std::move(trp),
-                                             std::move(kindMap));
-    }
-    break;
-  }
   TODO(mlir::UnknownLoc::get(ctx), "target not implemented");
 }