compiler: split vector_align into cabi and llvmlike

Introduce a pair of functions that actually describe what they do,
because it wasn't clear the alignment is sometimes a forgery.

compiler/rustc_abi/src/callconv/reg.rs

@@ -57,7 +57,7 @@ impl Reg {
                 128 => dl.f128_align.abi,
                 _ => panic!("unsupported float: {self:?}"),
             },
-            RegKind::Vector => dl.vector_align(self.size).abi,
+            RegKind::Vector => dl.llvmlike_vector_align(self.size).abi,
         }
     }
 }

compiler/rustc_abi/src/lib.rs

@@ -408,16 +408,21 @@ impl TargetDataLayout {
         }
     }
 
+    /// psABI-mandated alignment for a vector type, if any
     #[inline]
-    pub fn vector_align(&self, vec_size: Size) -> AbiAndPrefAlign {
+    fn cabi_vector_align(&self, vec_size: Size) -> Option<AbiAndPrefAlign> {
-        for &(size, align) in &self.vector_align {
+        self.vector_align
-            if size == vec_size {
+            .iter()
-                return align;
+            .find(|(size, _align)| *size == vec_size)
+            .map(|(_size, align)| *align)
     }
-        }
+
-        // Default to natural alignment, which is what LLVM does.
+    /// an alignment resembling the one LLVM would pick for a vector
-        // That is, use the size, rounded up to a power of 2.
+    #[inline]
-        AbiAndPrefAlign::new(Align::from_bytes(vec_size.bytes().next_power_of_two()).unwrap())
+    pub fn llvmlike_vector_align(&self, vec_size: Size) -> AbiAndPrefAlign {
+        self.cabi_vector_align(vec_size).unwrap_or(AbiAndPrefAlign::new(
+            Align::from_bytes(vec_size.bytes().next_power_of_two()).unwrap(),
+        ))
     }
 }
 

compiler/rustc_ty_utils/src/layout.rs

@@ -535,11 +535,14 @@ fn layout_of_uncached<'tcx>(
                     BackendRepr::Memory { sized: true },
                     AbiAndPrefAlign {
                         abi: Align::max_aligned_factor(size),
-                        pref: dl.vector_align(size).pref,
+                        pref: dl.llvmlike_vector_align(size).pref,
                     },
                 )
             } else {
-                (BackendRepr::Vector { element: e_abi, count: e_len }, dl.vector_align(size))
+                (
+                    BackendRepr::Vector { element: e_abi, count: e_len },
+                    dl.llvmlike_vector_align(size),
+                )
             };
             let size = size.align_to(align.abi);