Rollup merge of #52391 - Amanieu:volatile_unaligned, r=alexcrichton

Add unaligned volatile intrinsics

Surprisingly enough, it turns out that unaligned volatile loads are actually useful for certain (very niche) types of lock-free code. I included unaligned volatile stores for completeness, but I currently do not know of any use cases for them.

These are only exposed as intrinsics for now. If they turn out to be useful in practice, we can work towards stabilizing them.

r? @alexcrichton

src/librustc_codegen_llvm/builder.rs

@@ -54,6 +54,7 @@ bitflags! {
     pub struct MemFlags: u8 {
         const VOLATILE = 1 << 0;
         const NONTEMPORAL = 1 << 1;
+        const UNALIGNED = 1 << 2;
     }
 }
 
@@ -602,7 +603,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let ptr = self.check_store(val, ptr);
         unsafe {
             let store = llvm::LLVMBuildStore(self.llbuilder, val, ptr);
-            llvm::LLVMSetAlignment(store, align.abi() as c_uint);
+            let align = if flags.contains(MemFlags::UNALIGNED) {
+                1
+            } else {
+                align.abi() as c_uint
+            };
+            llvm::LLVMSetAlignment(store, align);
             if flags.contains(MemFlags::VOLATILE) {
                 llvm::LLVMSetVolatile(store, llvm::True);
             }