[clang] [Sema][AArch64] Emit error for mismatched VLs on streaming mode transitions (PR #159131)

[Mariya Podchishchaeva via cfe-commits]

================
@@ -3768,12 +3770,26 @@ void Sema::checkCall(NamedDecl *FDecl, const 
FunctionProtoType *Proto,
       if (!IsCalleeStreamingCompatible &&
           (CallerFnType == SemaARM::ArmStreamingCompatible ||
            ((CallerFnType == SemaARM::ArmStreaming) ^ IsCalleeStreaming))) {
-        if (IsScalableArg)
-          Diag(Loc, diag::warn_sme_streaming_pass_return_vl_to_non_streaming)
-              << /*IsArg=*/true;
-        if (IsScalableRet)
-          Diag(Loc, diag::warn_sme_streaming_pass_return_vl_to_non_streaming)
-              << /*IsArg=*/false;
+        const LangOptions &LO = getLangOpts();
+        auto VL = LO.VScaleMin * 128;
+        auto SVL = LO.VScaleStreamingMin * 128;
+
+        if (IsScalableArg) {
+          if (VL && SVL && VL != SVL)
+            Diag(Loc, diag::err_sme_streaming_transition_vl_mismatch)
+                << /*IsArg=*/true << VL << SVL;
+          else
+            Diag(Loc, diag::warn_sme_streaming_pass_return_vl_to_non_streaming)
+                << /*IsArg=*/true;
+        }
+        if (IsScalableRet) {
+          if (VL && SVL && VL != SVL)
+            Diag(Loc, diag::err_sme_streaming_transition_vl_mismatch)
+                << /*IsArg=*/false << VL << SVL;
+          else
+            Diag(Loc, diag::warn_sme_streaming_pass_return_vl_to_non_streaming)
+                << /*IsArg=*/false;
+        }
----------------
Fznamznon wrote:

I wonder if we can just merge this like that. I do realize it won't emit an 
error for both ret and arg cases if they are not mutually exclusive. Otherwise 
wrapping with a lambda probably makes sense
```suggestion
        if (IsScalableArg || IsScalableRet) {
          if (VL && SVL && VL != SVL)
            Diag(Loc, diag::err_sme_streaming_transition_vl_mismatch)
                << IsScalableArg << VL << SVL;
          else
            Diag(Loc, diag::warn_sme_streaming_pass_return_vl_to_non_streaming)
                << IsScalableArg;
        }
```

https://github.com/llvm/llvm-project/pull/159131
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