[clang] [ARM][Driver] Ensure NEON is enabled and disabled correctly (PR #137595)

[via cfe-commits]

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@@ -781,6 +781,30 @@ llvm::ARM::FPUKind arm::getARMTargetFeatures(const Driver &D,
   if (FPUKind == llvm::ARM::FK_FPV5_D16 || FPUKind == llvm::ARM::FK_FPV5_SP_D16)
     Features.push_back("-mve.fp");
 
+  // If SIMD has been disabled and the selected FPU support NEON, then features
+  // that rely on NEON Instructions should also be disabled. Cases where NEON
+  // needs activating to support another feature is handled below with the
+  // crypto feature.
+  bool HasSimd = false;
+  const auto ItSimd =
+      llvm::find_if(llvm::reverse(Features),
+                    [](const StringRef F) { return F.contains("neon"); });
+  const bool FoundSimd = ItSimd != Features.rend();
+  const bool FPUSupportsNeon = (llvm::ARM::FPUNames[FPUKind].NeonSupport ==
+                                llvm::ARM::NeonSupportLevel::Neon) ||
+                               (llvm::ARM::FPUNames[FPUKind].NeonSupport ==
+                                llvm::ARM::NeonSupportLevel::Crypto);
+  if (FoundSimd)
+    HasSimd = ItSimd->take_front() == "+";
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sivan-shani wrote:

The function that should be used here is `starts_with("+")` and not `take_front()`
Even though the existing code below:
`HasSHA2 = ItSHA2->take_front() == "+";` already use `take_front()`. I think it might be better to change all, including existing one, to `starts_with`

https://github.com/llvm/llvm-project/pull/137595