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

[Sergei Barannikov 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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s-barannikov wrote:

> 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?

Reserving a register assigned to a global variable makes sense. Not sure about local variables.
llvm language reference manual says this about llvm.read_register / llvm.write_register intrinsics:

> The compiler doesn’t check for register availability or use of the used register in surrounding code, including inline assembly. Because of that, allocatable registers are not supported.
> 
> Warning: So far it only works with the stack pointer on selected architectures (ARM, AArch64, PowerPC and x86_64). Significant amount of work is needed to support other registers and even more so, allocatable registers.

That said, I think we can go with the current implementation as long as it satisfies the need of the targeted software (Linux kernel I presume).


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