wip mem

hsivonen · Mar 13, 2024 · 966fc0a · 966fc0a
1 parent 2407220
commit 966fc0a
Showing 1 changed file with 14 additions and 0 deletions.
diff --git a/src/mem.rs b/src/mem.rs
@@ -116,6 +116,11 @@ macro_rules! by_unit_check_alu {
                     }
                     let len_minus_stride = len - ALU_ALIGNMENT / unit_size;
                     if offset + (4 * (ALU_ALIGNMENT / unit_size)) <= len {
+                        // Safety: the above check lets us perform 4 consecutive reads of
+                        // length ALU_ALIGNMENT / unit_size. ALU_ALIGNMENT is the size of usize, and unit_size
+                        // is the size of the `src` pointer, so this is equal to performing four usize reads.
+                        //
+                        // This invariant is upheld on all loop iterations
                         let len_minus_unroll = len - (4 * (ALU_ALIGNMENT / unit_size));
                         loop {
                             let unroll_accu = unsafe { *(src.add(offset) as *const usize) }
@@ -134,12 +139,14 @@ macro_rules! by_unit_check_alu {
                                 return false;
                             }
                             offset += 4 * (ALU_ALIGNMENT / unit_size);
+                            // Safety: this check lets us continue to perform the 4 reads earlier
                             if offset > len_minus_unroll {
                                 break;
                             }
                         }
                     }
                     while offset <= len_minus_stride {
+                        // Safety: the above check lets us perform one usize read.
                         accu |= unsafe { *(src.add(offset) as *const usize) };
                         offset += ALU_ALIGNMENT / unit_size;
                     }
@@ -189,6 +196,11 @@ macro_rules! by_unit_check_simd {
                     }
                     let len_minus_stride = len - SIMD_STRIDE_SIZE / unit_size;
                     if offset + (4 * (SIMD_STRIDE_SIZE / unit_size)) <= len {
+                        // Safety: the above check lets us perform 4 consecutive reads of
+                        // length SIMD_STRIDE_SIZE / unit_size. SIMD_STRIDE_SIZE is the size of $simd_ty, and unit_size
+                        // is the size of the `src` pointer, so this is equal to performing four $simd_ty reads.
+                        //
+                        // This invariant is upheld on all loop iterations
                         let len_minus_unroll = len - (4 * (SIMD_STRIDE_SIZE / unit_size));
                         loop {
                             let unroll_accu = unsafe { *(src.add(offset) as *const $simd_ty) }
@@ -208,13 +220,15 @@ macro_rules! by_unit_check_simd {
                                 return false;
                             }
                             offset += 4 * (SIMD_STRIDE_SIZE / unit_size);
+                            // Safety: this check lets us continue to perform the 4 reads earlier
                             if offset > len_minus_unroll {
                                 break;
                             }
                         }
                     }
                     let mut simd_accu = $splat;
                     while offset <= len_minus_stride {
+                        // Safety: the above check lets us perform one $simd_ty read.
                         simd_accu = simd_accu | unsafe { *(src.add(offset) as *const $simd_ty) };
                         offset += SIMD_STRIDE_SIZE / unit_size;
                     }