[clang] [clang] Enable constexpr handling for __builtin_elementwise_fma (PR #152919)

[Chaitanya Koparkar via cfe-commits]

================
@@ -2320,6 +2330,65 @@ static bool interp__builtin_elementwise_sat(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_elementwise_fma(InterpState &S, CodePtr OpPC,
+                                            const CallExpr *Call) {
+  assert(Call->getNumArgs() == 3);
+
+  llvm::RoundingMode RM = getRoundingMode(S, Call);
+
+  const QualType Arg1Type = Call->getArg(0)->getType();
+  const QualType Arg2Type = Call->getArg(1)->getType();
+  const QualType Arg3Type = Call->getArg(2)->getType();
+
+  // Non-vector floating point types.
+  if (!Arg1Type->isVectorType()) {
+    assert(!Arg2Type->isVectorType());
+    assert(!Arg3Type->isVectorType());
+
+    const Floating &Z = S.Stk.pop<Floating>();
+    const Floating &Y = S.Stk.pop<Floating>();
+    const Floating &X = S.Stk.pop<Floating>();
+
+    APFloat F = X.getAPFloat();
+    F.fusedMultiplyAdd(Y.getAPFloat(), Z.getAPFloat(), RM);
+    Floating Result = S.allocFloat(X.getSemantics());
+    Result.copy(F);
+    S.Stk.push<Floating>(Result);
+    return true;
+  }
+
+  // Vector type.
+  assert(Arg1Type->isVectorType() &&
+         Arg2Type->isVectorType() &&
+         Arg3Type->isVectorType());
----------------
ckoparkar wrote:

`clang-format` would rather write this as:

```diff
-  assert(Arg1Type->isVectorType() &&
-         Arg2Type->isVectorType() &&
+  assert(Arg1Type->isVectorType() && Arg2Type->isVectorType() &&
          Arg3Type->isVectorType());
```

I like it less but I can change it if required.

https://github.com/llvm/llvm-project/pull/152919