[PATCH] D67602: GlobalISel: Handle llvm.read_register

[Jessica Paquette via Phabricator via llvm-commits]

paquette added inline comments.


================
Comment at: lib/CodeGen/GlobalISel/IRTranslator.cpp:1529
+    EVT VT = TLI->getValueType(*DL, CI.getType());
+    Register Reg = TLI->getRegisterByName(RegStr->getString().data(), VT, *MF);
+    if (!Reg.isValid())
----------------
rengolin wrote:
> paquette wrote:
> > `TLI->getRegisterByName` can't handle sysregs on the AArch64 side, since they're modelled as immediate operands. So, this approach can't work for AArch64.
> > 
> > (see llvm/test/CodeGen/AArch64/special-reg.ll for an example)
> By design, they can only get the stack pointer, I think on any target. It should be an assembler error otherwise.
I don't think that's true in AArch64. E.g.:

https://reviews.llvm.org/D53115

The test added there uses read/write_register to access other special registers. This change made the test added there fail.

There's also the special-reg.ll test I mentioned above which has stuff like this:

```
%reg = call i64 @llvm.read_register.i64(metadata !1)
...
!1 = !{!"daif"}
```

Which is supposed to produce this (which does not involve SP):

```
mrs x0, DAIF
```

So I don't think that the stack pointer is the only thing we have to think about here. (Unless I'm seriously misunderstanding something, which is entirely possible. :P)



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67602/new/

https://reviews.llvm.org/D67602