Rollup merge of rust-lang#152929 - scottmcm:better-alignment-ranges, r=mati865

Tighten the `!range` bounds on alignments in vtables

Right now we're only telling LLVM that they're non-zero, but alignments must be powers of two so can't be more than `isize::MAX+1`.  And we actually never emit anything beyond LLVM's limit of 2²⁹, so outside of 16-bit targets the limit is that.

(Pulled out from rust-lang#152867 which is starting to have too much in it.)

Author: matthiaskrgr

compiler/rustc_abi/src/lib.rs

@@ -1036,6 +1036,19 @@ impl Align {
     // LLVM has a maximal supported alignment of 2^29, we inherit that.
     pub const MAX: Align = Align { pow2: 29 };
 
+    /// Either `1 << (pointer_bits - 1)` or [`Align::MAX`], whichever is smaller.
+    #[inline]
+    pub fn max_for_target(tdl: &TargetDataLayout) -> Align {
+        let pointer_bits = tdl.pointer_size().bits();
+        if let Ok(pointer_bits) = u8::try_from(pointer_bits)
+            && pointer_bits <= Align::MAX.pow2
+        {
+            Align { pow2: pointer_bits - 1 }
+        } else {
+            Align::MAX
+        }
+    }
+
     #[inline]
     pub fn from_bits(bits: u64) -> Result<Align, AlignFromBytesError> {
         Align::from_bytes(Size::from_bits(bits).bytes())

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -1,4 +1,4 @@
-use rustc_abi::WrappingRange;
+use rustc_abi::{Align, WrappingRange};
 use rustc_middle::mir::SourceInfo;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
@@ -179,9 +179,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         let size_bound = bx.data_layout().ptr_sized_integer().signed_max() as u128;
                         bx.range_metadata(value, WrappingRange { start: 0, end: size_bound });
                     }
-                    // Alignment is always nonzero.
+                    // Alignment is always a power of two, thus 1..=0x800…000,
+                    // but also bounded by the maximum we support in type layout.
                     sym::vtable_align => {
-                        bx.range_metadata(value, WrappingRange { start: 1, end: !0 })
+                        let align_bound = Align::max_for_target(bx.data_layout()).bytes().into();
+                        bx.range_metadata(value, WrappingRange { start: 1, end: align_bound })
                     }
                     _ => {}
                 }

compiler/rustc_codegen_ssa/src/size_of_val.rs

@@ -1,6 +1,6 @@
 //! Computing the size and alignment of a value.
 
-use rustc_abi::WrappingRange;
+use rustc_abi::{Align, WrappingRange};
 use rustc_hir::LangItem;
 use rustc_middle::bug;
 use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
@@ -36,8 +36,10 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             // Size is always <= isize::MAX.
             let size_bound = bx.data_layout().ptr_sized_integer().signed_max() as u128;
             bx.range_metadata(size, WrappingRange { start: 0, end: size_bound });
-            // Alignment is always nonzero.
-            bx.range_metadata(align, WrappingRange { start: 1, end: !0 });
+            // Alignment is always a power of two, thus 1..=0x800…000,
+            // but also bounded by the maximum we support in type layout.
+            let align_bound = Align::max_for_target(bx.data_layout()).bytes().into();
+            bx.range_metadata(align, WrappingRange { start: 1, end: align_bound });
 
             (size, align)
         }

tests/codegen-llvm/dst-vtable-align-nonzero.rs

@@ -64,4 +64,4 @@ pub unsafe fn align_load_from_vtable_align_intrinsic(x: &dyn Trait) -> usize {
     core::intrinsics::vtable_align(vtable)
 }
 
-// CHECK: [[RANGE_META]] = !{[[USIZE]] 1, [[USIZE]] 0}
+// CHECK: [[RANGE_META]] = !{[[USIZE]] 1, [[USIZE]] [[#0x20000001]]