[PATCH] D143708: [RISCV] Add emulated TLS supported

[Vitaly Cheptsov via Phabricator via llvm-commits]

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As discussed earlier in the GitHub issue <https://github.com/llvm/llvm-project/issues/59500>, currently LLVM generates invalid code when emulated TLS is used. There were attempts to resolve this previously (D102527 <https://reviews.llvm.org/D102527>), but they were not merged because the component owners raised concerns about emulated TLS efficiency.

The current state of the art is that:

- OpenBSD team, which raised the initial issue, simply has patches downstream <https://github.com/openbsd/src/blob/a0747c9/gnu/llvm/llvm/lib/Target/RISCV/RISCVISelLowering.cpp#L2850-L2852>.
- Our team, which raised the GH issue, has patches downstream as well. We also do not use `malloc` or any dynamic allocations <https://github.com/llvm/llvm-project/issues/59500#issuecomment-1349046835> with emulated TLS, so the concerns raised in the original issue does not apply to us.
- GCC compatibility is broken, because GCC supports emulated TLS.
- RISC-V is the only architecture in LLVM that does not support emulated TLS, and work is being done to at least warn the users about it (D143619 <https://reviews.llvm.org/D143619>).

With all these in mind I believe it is important to address the consumers' needs especially given that there is little to no maintenance downsides.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D143708

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp


Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -4569,6 +4569,9 @@
 
 SDValue RISCVTargetLowering::lowerGlobalTLSAddress(SDValue Op,
                                                    SelectionDAG &DAG) const {
+  GlobalAddressSDNode *GA = cast<GlobalAddressSDNode>(Op);
+  if (DAG.getTarget().useEmulatedTLS())
+    return LowerToTLSEmulatedModel(GA, DAG);
   GlobalAddressSDNode *N = cast<GlobalAddressSDNode>(Op);
   assert(N->getOffset() == 0 && "unexpected offset in global node");
 


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