[clang] [SPARC][clang] Add -m(no-)v8plus flags handling (PR #98713)

[via cfe-commits]

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@@ -179,6 +179,13 @@ void sparc::getSparcTargetFeatures(const Driver &D, const ArgList &Args,
       Features.push_back("-hard-quad-float");
   }
 
+  if (Arg *A = Args.getLastArg(options::OPT_mv8plus, options::OPT_mno_v8plus)) {
+    if (A->getOption().matches(options::OPT_mv8plus))
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koachan wrote:

> AFAIK it is an ABI affecting flag, although I don't know if it changes anything except for ELF header's e_machine field compared to 32-bit V8/V9. It can't safely be ignored if we claim support for V8+. If we don't, and V8+ is otherwise compatible with 32-bit ABI, I think ignoring it and generating instructions should be fine, probably with a warning.

AFAICT the main ABI change is widening the G and O registers to 64-bits, which is compatible with legacy ABI:
- The kernel handles saving and restoring those registers during context switches, so it is out of scope for us (aside from setting the correct ELF e_machine value so the kernel knows how much state to handle);
- The (non-reserved) G registers are volatile anyway, so any code should not make assumptions about its value past a call or return;
- The O registers are widened too, however, during a call, the callee might issue a `save` (which will result in the values being rotated - and truncated - into the I registers), so 64-bit arguments need to be split into 2x32-bit pair, just as in legacy ABI; and
- Similarly, the callee might put its return value in the I registers before issuing a `restore`, so the caller will receive 64-bit returns as 2x32-bit pairs, just as in legacy ABI.

> It could be helpful to have a draft that adds some V8+ support to the backend and the corresponding feature bits.

Backend bits are in #101367 (incl. ELF e_machine type setting), that should be merged first before this PR.

https://github.com/llvm/llvm-project/pull/98713