Auto merge of #40385 - arielb1:packed-again, r=eddyb

emit !align attributes on stores of operand pairs

This avoids another case of missing-align UB. cc #40373

r? @eddyb

src/librustc_trans/mir/block.rs

@@ -833,8 +833,21 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
             self.trans_lvalue(bcx, dest)
         };
         if fn_ret_ty.is_indirect() {
-            llargs.push(dest.llval);
+            match dest.alignment {
-            ReturnDest::Nothing
+                Alignment::AbiAligned => {
+                    llargs.push(dest.llval);
+                    ReturnDest::Nothing
+                },
+                Alignment::Packed => {
+                    // Currently, MIR code generation does not create calls
+                    // that store directly to fields of packed structs (in
+                    // fact, the calls it creates write only to temps),
+                    //
+                    // If someone changes that, please update this code path
+                    // to create a temporary.
+                    span_bug!(self.mir.span, "can't directly store to unaligned value");
+                }
+            }
         } else {
             ReturnDest::Store(dest.llval)
         }

src/librustc_trans/mir/operand.rs

@@ -268,10 +268,17 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                 bcx.store(base::from_immediate(bcx, s), lldest, align);
             }
             OperandValue::Pair(a, b) => {
+                let f_align = match *bcx.ccx.layout_of(operand.ty) {
+                    Layout::Univariant { ref variant, .. } if variant.packed => {
+                        Some(1)
+                    }
+                    _ => align
+                };
+
                 let a = base::from_immediate(bcx, a);
                 let b = base::from_immediate(bcx, b);
-                bcx.store(a, bcx.struct_gep(lldest, 0), align);
+                bcx.store(a, bcx.struct_gep(lldest, 0), f_align);
-                bcx.store(b, bcx.struct_gep(lldest, 1), align);
+                bcx.store(b, bcx.struct_gep(lldest, 1), f_align);
             }
         }
     }

src/test/codegen/packed.rs

@@ -27,3 +27,36 @@ pub fn write_pkd(pkd: &mut Packed) -> u32 {
     pkd.data = 42;
     result
 }
+
+pub struct Array([i32; 8]);
+#[repr(packed)]
+pub struct BigPacked {
+    dealign: u8,
+    data: Array
+}
+
+// CHECK-LABEL: @call_pkd
+#[no_mangle]
+pub fn call_pkd(f: fn() -> Array) -> BigPacked {
+// CHECK: [[ALLOCA:%[_a-z0-9]+]] = alloca %Array
+// CHECK: call void %{{.*}}(%Array* noalias nocapture sret dereferenceable(32) [[ALLOCA]])
+// CHECK: call void @llvm.memcpy.{{.*}}(i8* %{{.*}}, i8* %{{.*}}, i{{[0-9]+}} 32, i32 1, i1 false)
+    // check that calls whose destination is a field of a packed struct
+    // go through an alloca rather than calling the function with an
+    // unaligned destination.
+    BigPacked { dealign: 0, data: f() }
+}
+
+#[repr(packed)]
+#[derive(Copy, Clone)]
+pub struct PackedPair(u8, u32);
+
+// CHECK-LABEL: @pkd_pair
+#[no_mangle]
+pub fn pkd_pair(pair1: &mut PackedPair, pair2: &mut PackedPair) {
+    // CHECK: [[V1:%[a-z0-9]+]] = load i8, i8* %{{.*}}, align 1
+    // CHECK: [[V2:%[a-z0-9]+]] = load i32, i32* %{{.*}}, align 1
+    // CHECK: store i8 [[V1]], i8* {{.*}}, align 1
+    // CHECK: store i32 [[V2]], i32* {{.*}}, align 1
+    *pair2 = *pair1;
+}