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

[Sander de Smalen via cfe-commits]

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@@ -3771,25 +3771,25 @@ void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
           (CallerFnType == SemaARM::ArmStreamingCompatible ||
            ((CallerFnType == SemaARM::ArmStreaming) ^ IsCalleeStreaming))) {
         const LangOptions &LO = getLangOpts();
-        auto VL = LO.VScaleMin * 128;
-        auto SVL = LO.VScaleStreamingMin * 128;
+        unsigned VL = LO.VScaleMin * 128;
+        unsigned 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)
+        bool IsVLError = CallerFnType != SemaARM::ArmStreamingCompatible &&
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sdesmalen-arm wrote:

I think it's worth making a special warning diagnostic for the streaming-compatible -> streaming case, and the streaming-compatible -> non-streaming case when we know that VL != SVL, to make it clear that passing a VL-dependent argument may be undefined behaviour if the machine is [not] in streaming mode when executing the call.

(This is subtly different from the current warning diagnostic which says that passing a VL-dependent argument may be undefined behaviour if the vector lengths don't match)

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