[llvm] [RISCV] Initial ISel support for the experimental zacas extension (PR #67918)

[Craig Topper via llvm-commits]

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@@ -621,7 +621,10 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   }
 
   if (Subtarget.hasStdExtA()) {
-    setMaxAtomicSizeInBitsSupported(Subtarget.getXLen());
+    if (Subtarget.hasStdExtZacas())
+      setMaxAtomicSizeInBitsSupported(Subtarget.getXLen() * 2);
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topperc wrote:

This change affects the behavior of atomicrmw and atomic load/store for 2*XLen types. I'm working my own version of this patch that will address this.

https://github.com/llvm/llvm-project/pull/67918