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

[Craig Topper via llvm-commits]

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@@ -295,6 +295,28 @@ multiclass PseudoCmpXchgPat<string Op, Pseudo CmpXchgInst,
             (CmpXchgInst GPR:$addr, GPR:$cmp, GPR:$new, 7)>;
 }
 
+let Predicates = [HasStdExtZacas] in {
+class PseudoAMOCAS<RegisterClass RC = GPR>
+    : Pseudo<(outs RC:$res),
+             (ins GPR:$addr, RC:$cmpval, RC:$newval, ixlenimm:$ordering), []> {
+  let Constraints = "@earlyclobber $res, $res = $cmpval";
+  let mayLoad = 1;
+  let mayStore = 1;
+  let hasSideEffects = 0;
+}
+def PseudoAMOCAS_W: PseudoAMOCAS;
+defm : PseudoCmpXchgPat<"atomic_cmp_swap_32", PseudoAMOCAS_W>;
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topperc wrote:

Can we directly select PseudoAMOCAS_W and PseudoAMOCAS_D_RV64 without going through a pseudoinstruction? Similar to `AMOPat`?

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