Rollup merge of #57053 - nikic:fix-gep-align, r=nagisa

Fix alignment for array indexing

We need to reduce the alignment with the used offset. If the offset isn't known, use the element size, as this will yield the minimum possible alignment.

This handles both direct array indexing, and array repeat expressions.

Fixes #56927.

r? @nagisa

4 files changed, 93 insertions, 3 deletions

diff --git a/src/librustc_codegen_ssa/mir/place.rs b/src/librustc_codegen_ssa/mir/place.rs
index 90aa9f6cbc7..5fad4a24b26 100644
--- a/src/librustc_codegen_ssa/mir/place.rs
+++ b/src/librustc_codegen_ssa/mir/place.rs
@@ -335,11 +335,20 @@ impl<'a, 'tcx: 'a, V: CodegenObject> PlaceRef<'tcx, V> {
         bx: &mut Bx,
         llindex: V
     ) -> Self {
+        // Statically compute the offset if we can, otherwise just use the element size,
+        // as this will yield the lowest alignment.
+        let layout = self.layout.field(bx, 0);
+        let offset = if bx.is_const_integral(llindex) {
+            layout.size.checked_mul(bx.const_to_uint(llindex), bx).unwrap_or(layout.size)
+        } else {
+            layout.size
+        };
+
         PlaceRef {
             llval: bx.inbounds_gep(self.llval, &[bx.cx().const_usize(0), llindex]),
             llextra: None,
-            layout: self.layout.field(bx.cx(), 0),
-            align: self.align
+            layout,
+            align: self.align.restrict_for_offset(offset),
         }
     }
 
diff --git a/src/librustc_codegen_ssa/mir/rvalue.rs b/src/librustc_codegen_ssa/mir/rvalue.rs
index c932ffd1c1b..052342dd759 100644
--- a/src/librustc_codegen_ssa/mir/rvalue.rs
+++ b/src/librustc_codegen_ssa/mir/rvalue.rs
@@ -131,8 +131,9 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let keep_going = header_bx.icmp(IntPredicate::IntNE, current, end);
                 header_bx.cond_br(keep_going, body_bx.llbb(), next_bx.llbb());
 
+                let align = dest.align.restrict_for_offset(dest.layout.field(bx.cx(), 0).size);
                 cg_elem.val.store(&mut body_bx,
-                    PlaceRef::new_sized(current, cg_elem.layout, dest.align));
+                    PlaceRef::new_sized(current, cg_elem.layout, align));
 
                 let next = body_bx.inbounds_gep(current, &[bx.cx().const_usize(1)]);
                 body_bx.br(header_bx.llbb());
diff --git a/src/test/codegen/issue-56927.rs b/src/test/codegen/issue-56927.rs
new file mode 100644
index 00000000000..0544ff86aac
--- /dev/null
+++ b/src/test/codegen/issue-56927.rs
@@ -0,0 +1,44 @@
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type="rlib"]
+use std::usize;
+
+#[repr(align(16))]
+pub struct S {
+    arr: [u32; 4],
+}
+
+// CHECK-LABEL: @test1
+// CHECK: store i32 0, i32* %{{.+}}, align 16
+// CHECK: store i32 1, i32* %{{.+}}, align 4
+// CHECK: store i32 2, i32* %{{.+}}, align 8
+// CHECK: store i32 3, i32* %{{.+}}, align 4
+#[no_mangle]
+pub fn test1(s: &mut S) {
+    s.arr[0] = 0;
+    s.arr[1] = 1;
+    s.arr[2] = 2;
+    s.arr[3] = 3;
+}
+
+// CHECK-LABEL: @test2
+// CHECK: store i32 4, i32* %{{.+}}, align 4
+#[allow(const_err)]
+#[no_mangle]
+pub fn test2(s: &mut S) {
+    s.arr[usize::MAX / 4 + 1] = 4;
+}
+
+// CHECK-LABEL: @test3
+// CHECK: store i32 5, i32* %{{.+}}, align 4
+#[no_mangle]
+pub fn test3(s: &mut S, i: usize) {
+    s.arr[i] = 5;
+}
+
+// CHECK-LABEL: @test4
+// CHECK: store i32 6, i32* %{{.+}}, align 4
+#[no_mangle]
+pub fn test4(s: &mut S) {
+    s.arr = [6; 4];
+}
diff --git a/src/test/codegen/packed.rs b/src/test/codegen/packed.rs
index fb6507fd22a..3b4e89f12e0 100644
--- a/src/test/codegen/packed.rs
+++ b/src/test/codegen/packed.rs
@@ -83,6 +83,42 @@ pub fn call_pkd2(f: fn() -> Array) -> BigPacked2 {
     BigPacked2 { dealign: 0, data: f() }
 }
 
+// CHECK-LABEL: @write_packed_array1
+// CHECK: store i32 0, i32* %{{.+}}, align 1
+// CHECK: store i32 1, i32* %{{.+}}, align 1
+// CHECK: store i32 2, i32* %{{.+}}, align 1
+#[no_mangle]
+pub fn write_packed_array1(p: &mut BigPacked1) {
+    p.data.0[0] = 0;
+    p.data.0[1] = 1;
+    p.data.0[2] = 2;
+}
+
+// CHECK-LABEL: @write_packed_array2
+// CHECK: store i32 0, i32* %{{.+}}, align 2
+// CHECK: store i32 1, i32* %{{.+}}, align 2
+// CHECK: store i32 2, i32* %{{.+}}, align 2
+#[no_mangle]
+pub fn write_packed_array2(p: &mut BigPacked2) {
+    p.data.0[0] = 0;
+    p.data.0[1] = 1;
+    p.data.0[2] = 2;
+}
+
+// CHECK-LABEL: @repeat_packed_array1
+// CHECK: store i32 42, i32* %{{.+}}, align 1
+#[no_mangle]
+pub fn repeat_packed_array1(p: &mut BigPacked1) {
+    p.data.0 = [42; 8];
+}
+
+// CHECK-LABEL: @repeat_packed_array2
+// CHECK: store i32 42, i32* %{{.+}}, align 2
+#[no_mangle]
+pub fn repeat_packed_array2(p: &mut BigPacked2) {
+    p.data.0 = [42; 8];
+}
+
 #[repr(packed)]
 #[derive(Copy, Clone)]
 pub struct Packed1Pair(u8, u32);