[llvm] [SPARC][IAS] Make 64-bit instructions available in 32-bit mode on V9 (PR #187534)

[Sergei Barannikov via llvm-commits]

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@@ -20,6 +20,38 @@ include "llvm/TableGen/SearchableTable.td"
 // SPARC Subtarget features.
 //
 
+// Some words about SPARC ISA versions and feature levels:
+// - V7: The first public version, used in some early Sun 4 models. Lacks integer
+//       multiply and divide in hardware, and doesn't have a formal memory model,
+//       though in practice the few multiprocessor machines that do exist
+//       are sequentially consistent.
+// - V8: Adds hardware multiply and divide, and formally defines TSO and PSO
+//       memory models. Most 32-bit chips currently used are of this version.
+// - V9: Adds 64-bit capability and RMO memory model, aside from new instructions.
+//       Note that unlike most other architectures, instructions that operate
+//       on 64-bit registers (e.g casx) are still available even in 32-bit ILP32
+//       mode - the G/O registers are always 64-bit regardless of runtime bitness.
+//       To let compilers take advantage of this feature, Sun defined an extension
+//       to the 32-bit ABI called V8+. It's still possible, though, to compile for
+//       V9 ISA level while not emitting any of the 64-bit instructions, that is
+//       why -mv8plus is defined separately from the usual -mcpu flags.
+//       (OTOH, the combination of `-mcpu=v8 -mv8plus` is effectively a no-op)
+//
+// As for memory models, the ordering from weakest to strongest is as follows:
+// (weaker) RMO -> PSO -> TSO -> SC (stronger)
+// Those are defined in such a way that code compiled for a weaker model will
+// run fine on processors that only implements a stronger model.
+// This is the route that we take in LLVM; compile for RMO when targeting v9,
+// and PSO when targeting v7/v8.
+//
+// As for triples, Linux/BSD and Solaris follows different conventions:
+// Linux/BSD name | Solaris name | Meaning
+// ---------------|--------------|-------------
+// sparc-*-*      | sparc-*-*    | 32-bit SPARC
+// sparc64-*-*    | sparcv9-*-*  | 64-bit SPARC
----------------
s-barannikov wrote:

Should the meaning be `32-/64-bit ABI` instead?
I guess I can run a program compiled with --target=sparc on a 64-bit SPARC... or no?


https://github.com/llvm/llvm-project/pull/187534