[llvm] [RISCV] 'Zalrsc' may permit non-base instructions (PR #165042)

[via llvm-commits]

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@@ -1906,6 +1906,22 @@ def FeatureForcedAtomics : SubtargetFeature<
 def HasAtomicLdSt
     : Predicate<"Subtarget->hasStdExtZalrsc() || Subtarget->hasForcedAtomics()">;
 
+// The RISC-V Unprivileged Architecture defines _constrained_ LR/SC loops:
+//   The dynamic code executed between the LR and SC instructions can only
+//   contain instructions from the base ''I'' instruction set, excluding loads,
+//   stores, backward jumps, taken backward branches, JALR, FENCE, and SYSTEM
+//   instructions. Compressed forms of the aforementioned ''I'' instructions in
+//   the Zca and Zcb extensions are also permitted.
+// LR/SC loops that do not adhere to the above are _unconstrained_ LR/SC loops,
+// and success is implementation specific. For implementations which know that
+// non-base instructions (such as the ''B'' extension) will not violate any
+// forward progress guarantees, using these instructions to reduce the LR/SC
+// sequence length is desirable.
+def FeaturePermissiveZalrsc
+    : SubtargetFeature<
+          "permissive-zalrsc", "HasPermissiveZalrsc", "true",
+          "Implementation permits non-base instructions between LR/SC pairs">;
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slachowsky wrote:

It is now implied.

https://github.com/llvm/llvm-project/pull/165042