[llvm] [AArch64] Consider runtime mode when deciding to use SVE for fixed-length vectors. (PR #96081)

[Paul Walker via llvm-commits]

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@@ -1528,14 +1528,24 @@ AArch64TargetLowering::AArch64TargetLowering(const TargetMachine &TM,
       }
     }
 
-    // NEON doesn't support masked loads/stores/gathers/scatters, but SVE does
-    for (auto VT : {MVT::v4f16, MVT::v8f16, MVT::v2f32, MVT::v4f32, MVT::v1f64,
-                    MVT::v2f64, MVT::v8i8, MVT::v16i8, MVT::v4i16, MVT::v8i16,
-                    MVT::v2i32, MVT::v4i32, MVT::v1i64, MVT::v2i64}) {
+    // NEON doesn't support masked loads/stores, but SME and SVE do.
+    for (auto VT :
+         {MVT::v4f16, MVT::v8f16, MVT::v2f32, MVT::v4f32, MVT::v1f64,
+          MVT::v2f64, MVT::v8i8, MVT::v16i8, MVT::v4i16, MVT::v8i16,
+          MVT::v2i32, MVT::v4i32, MVT::v1i64, MVT::v2i64}) {
       setOperationAction(ISD::MLOAD, VT, Custom);
       setOperationAction(ISD::MSTORE, VT, Custom);
-      setOperationAction(ISD::MGATHER, VT, Custom);
-      setOperationAction(ISD::MSCATTER, VT, Custom);
+    }
+
+    // NEON doesn't support masked gathers/scatters, but SVE does.
+    if (Subtarget->isSVEAvailable()) {
+      for (auto VT :
+           {MVT::v4f16, MVT::v8f16, MVT::v2f32, MVT::v4f32, MVT::v1f64,
+            MVT::v2f64, MVT::v8i8, MVT::v16i8, MVT::v4i16, MVT::v8i16,
+            MVT::v2i32, MVT::v4i32, MVT::v1i64, MVT::v2i64}) {
+        setOperationAction(ISD::MGATHER, VT, Custom);
+        setOperationAction(ISD::MSCATTER, VT, Custom);
+      }
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paulwalker-arm wrote:

It would be better to break this out rather than nesting incompatible feature checks.  When navigating the code people might bypass the whole `isSVEorStreamingSVEAvailable` block as not relevant and subsequently miss what they're looking for.  

https://github.com/llvm/llvm-project/pull/96081