[llvm] [clang] [SPARC] Support reserving arbitrary general purpose registers (PR #74927)

[via cfe-commits]

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@@ -1125,6 +1130,10 @@ Register SparcTargetLowering::getRegisterByName(const char* RegName, LLT VT,
     .Case("g4", SP::G4).Case("g5", SP::G5).Case("g6", SP::G6).Case("g7", SP::G7)
     .Default(0);
 
+  const SparcRegisterInfo *TRI = Subtarget->getRegisterInfo();
+  if (!TRI->isReservedReg(MF, Reg))
+    Reg = 0;
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koachan wrote:

> Because of that, allocatable registers are not supported.

Yeah, this seems to be the main difference between them, from what I understand.
GCC does warn you about it, but it will still allow you to read/write allocatable registers. LLVM, on the other hand, will flat out refuse to do that (either by hard-crashing or turning the read/write_register intrinsics into nops).

And yes, this should be enough for the Linux kernel, methinks.

- - - - -

Also, re:testing, how should I test this, given that the expected failure mode is a hard crash?
I don't know if there's any way to tell the test framework that a hard crash is the intended result.

https://github.com/llvm/llvm-project/pull/74927