[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:

Hmm, yeah, ideally we'd want to do that but I don't think we can do that easily?
Even if I turn off all the errors, stuff like this, for example:
```
register unsigned long r asm("l0");
void set(unsigned long x) { r = x; }
```
Becomes a nop function on clang unless `l0` is reserved.
Other LLVM backends also do this (and differ from GCC behavior) so I don't think the issue is localized to ours.

So I believe it is better for us to throw an error instead of silently miscompiling code?

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