Fix float add/mul reduction codegen

The accumulator is now respected for unordered reductions.

src/librustc_codegen_llvm/common.rs

@@ -249,6 +249,10 @@ impl ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         self.const_uint(self.type_i8(), i as u64)
     }
 
+    fn const_real(&self, t: &'ll Type, val: f64) -> &'ll Value {
+        unsafe { llvm::LLVMConstReal(t, val) }
+    }
+
     fn const_struct(
         &self,
         elts: &[&'ll Value],

src/librustc_codegen_llvm/intrinsic.rs

@@ -1663,9 +1663,10 @@ fn generic_simd_intrinsic(
                             acc
                         } else {
                             // unordered arithmetic reductions do not:
+                            let identity_acc = if $name.contains("mul") { 1.0 } else { 0.0 };
                             match f.bit_width() {
-                                32 => bx.const_undef(bx.type_f32()),
+                                32 => bx.const_real(bx.type_f32(), identity_acc),
-                                64 => bx.const_undef(bx.type_f64()),
+                                64 => bx.const_real(bx.type_f64(), identity_acc),
                                 v => {
                                     return_error!(r#"
 unsupported {} from `{}` with element `{}` of size `{}` to `{}`"#,

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -715,6 +715,7 @@ extern "C" {
     // Operations on scalar constants
     pub fn LLVMConstInt(IntTy: &Type, N: c_ulonglong, SignExtend: Bool) -> &Value;
     pub fn LLVMConstIntOfArbitraryPrecision(IntTy: &Type, Wn: c_uint, Ws: *const u64) -> &Value;
+    pub fn LLVMConstReal(RealTy: &Type, N: f64) -> &Value;
     pub fn LLVMConstIntGetZExtValue(ConstantVal: &Value) -> c_ulonglong;
     pub fn LLVMRustConstInt128Get(ConstantVal: &Value, SExt: bool,
                                   high: &mut u64, low: &mut u64) -> bool;

src/librustc_codegen_ssa/traits/consts.rs

@@ -17,6 +17,7 @@ pub trait ConstMethods<'tcx>: BackendTypes {
     fn const_u64(&self, i: u64) -> Self::Value;
     fn const_usize(&self, i: u64) -> Self::Value;
     fn const_u8(&self, i: u8) -> Self::Value;
+    fn const_real(&self, t: Self::Type, val: f64) -> Self::Value;
 
     fn const_struct(&self, elts: &[Self::Value], packed: bool) -> Self::Value;