Don’t assume MIN_ALIGN for small sizes

Same as rust-lang/rust#46117

See also discussion of jemalloc’s behavior at
jemalloc/jemalloc#1072

Author: SimonSapin

src/lib.rs

@@ -47,8 +47,8 @@ fn mallocx_align(a: usize) -> c_int {
     a.trailing_zeros() as c_int
 }
 
-fn align_to_flags(align: usize) -> c_int {
-    if align <= MIN_ALIGN {
+fn align_to_flags(align: usize, size: usize) -> c_int {
+    if align <= MIN_ALIGN && align <= size {
         0
     } else {
         mallocx_align(align)
@@ -131,7 +131,7 @@ unsafe impl Alloc for Jemalloc {
 unsafe impl<'a> Alloc for &'a Jemalloc {
     #[inline]
     unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr> {
-        let flags = align_to_flags(layout.align());
+        let flags = align_to_flags(layout.align(), layout.size());
         let ptr = ffi::mallocx(layout.size(), flags);
         if ptr.is_null() {
             Err(AllocErr::Exhausted { request: layout })
@@ -144,10 +144,10 @@ unsafe impl<'a> Alloc for &'a Jemalloc {
     unsafe fn alloc_zeroed(&mut self, layout: Layout)
         -> Result<*mut u8, AllocErr>
     {
-        let ptr = if layout.align() <= MIN_ALIGN {
+        let ptr = if layout.align() <= MIN_ALIGN && layout.align() <= layout.size() {
             ffi::calloc(1, layout.size())
         } else {
-            let flags = align_to_flags(layout.align()) | ffi::MALLOCX_ZERO;
+            let flags = align_to_flags(layout.align(), layout.size()) | ffi::MALLOCX_ZERO;
             ffi::mallocx(layout.size(), flags)
         };
         if ptr.is_null() {
@@ -159,7 +159,7 @@ unsafe impl<'a> Alloc for &'a Jemalloc {
 
     #[inline]
     unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout) {
-        let flags = align_to_flags(layout.align());
+        let flags = align_to_flags(layout.align(), layout.size());
         ffi::sdallocx(ptr as *mut c_void, layout.size(), flags)
     }
 
@@ -171,7 +171,7 @@ unsafe impl<'a> Alloc for &'a Jemalloc {
         if old_layout.align() != new_layout.align() {
             return Err(AllocErr::Unsupported { details: "cannot change align" })
         }
-        let flags = align_to_flags(new_layout.align());
+        let flags = align_to_flags(new_layout.align(), new_layout.size());
         let ptr = ffi::rallocx(ptr as *mut c_void, new_layout.size(), flags);
         if ptr.is_null() {
             Err(AllocErr::Exhausted { request: new_layout })
@@ -186,7 +186,7 @@ unsafe impl<'a> Alloc for &'a Jemalloc {
 
     #[inline]
     fn usable_size(&self, layout: &Layout) -> (usize, usize) {
-        let flags = align_to_flags(layout.align());
+        let flags = align_to_flags(layout.align(), layout.size());
         unsafe {
             let max = ffi::nallocx(layout.size(), flags);
             (layout.size(), max)
@@ -209,7 +209,7 @@ unsafe impl<'a> Alloc for &'a Jemalloc {
         if old_layout.align() != new_layout.align() {
             return Err(CannotReallocInPlace)
         }
-        let flags = align_to_flags(new_layout.align());
+        let flags = align_to_flags(new_layout.align(), new_layout.size());
         let size = ffi::xallocx(ptr as *mut c_void, new_layout.size(), 0, flags);
         if size >= new_layout.size() {
             Err(CannotReallocInPlace)

tests/smoke.rs

@@ -1,8 +1,9 @@
-#![feature(global_allocator)]
+#![feature(global_allocator, allocator_api)]
 
 extern crate jemallocator;
 
 use jemallocator::Jemalloc;
+use std::heap::{Alloc, Layout};
 
 #[global_allocator]
 static A: Jemalloc = Jemalloc;
@@ -12,3 +13,24 @@ fn smoke() {
     let mut a = Vec::new();
     a.push(3);
 }
+
+/// https://github.com/rust-lang/rust/issues/45955
+#[test]
+fn overaligned() {
+    let size = 8;
+    let align = 16; // greater than size
+    let iterations = 100;
+    unsafe {
+        let pointers: Vec<_> = (0..iterations).map(|_| {
+            Jemalloc.alloc(Layout::from_size_align(size, align).unwrap()).unwrap()
+        }).collect();
+        for &ptr in &pointers {
+            assert_eq!((ptr as usize) % align, 0, "Got a pointer less aligned than requested")
+        }
+
+        // Clean up
+        for &ptr in &pointers {
+            Jemalloc.dealloc(ptr, Layout::from_size_align(size, align).unwrap())
+        }
+    }
+}