From 62581b8f0d92304914310640a91dc6f919502daf Mon Sep 17 00:00:00 2001
From: Oliver Schneider <github35764891676564198441@oli-obk.de>
Date: Sun, 22 Jul 2018 12:15:04 +0200
Subject: [PATCH] Explain promoted extraction for simd shuffle

---
 src/librustc_codegen_llvm/mir/block.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/librustc_codegen_llvm/mir/block.rs b/src/librustc_codegen_llvm/mir/block.rs
index 92313d69810a..dc4b9e0ae99d 100644
--- a/src/librustc_codegen_llvm/mir/block.rs
+++ b/src/librustc_codegen_llvm/mir/block.rs
@@ -507,6 +507,9 @@ impl<'a, 'tcx> FunctionCx<'a, 'tcx> {
                         // promotes any complex rvalues to constants.
                         if i == 2 && intrinsic.unwrap().starts_with("simd_shuffle") {
                             match *arg {
+                                // The shuffle array argument is usually not an explicit constant,
+                                // but specified directly in the code. This means it gets promoted
+                                // and we can then extract the value by evaluating the promoted.
                                 mir::Operand::Copy(mir::Place::Promoted(box(index, ty))) |
                                 mir::Operand::Move(mir::Place::Promoted(box(index, ty))) => {
                                     let param_env = ty::ParamEnv::reveal_all();