5 files changed, 35 insertions, 22 deletions

diff --git a/lib/std/array_list.zig b/lib/std/array_list.zig
index 77ba0646cf..8dbd9f8dae 100644
--- a/lib/std/array_list.zig
+++ b/lib/std/array_list.zig
@@ -56,19 +56,11 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
         }
 
         /// Initialize with capacity to hold at least `num` elements.
+        /// The resulting capacity is likely to be equal to `num`.
         /// Deinitialize with `deinit` or use `toOwnedSlice`.
         pub fn initCapacity(allocator: *Allocator, num: usize) !Self {
             var self = Self.init(allocator);
-
-            if (@sizeOf(T) > 0) {
-                const new_memory = try self.allocator.allocAdvanced(T, alignment, num, .at_least);
-                self.items.ptr = new_memory.ptr;
-                self.capacity = new_memory.len;
-            } else {
-                // If `T` is a zero-sized type, then we do not need to allocate memory.
-                self.capacity = std.math.maxInt(usize);
-            }
-
+            try self.ensureTotalCapacityPrecise(num);
             return self;
         }
 
@@ -330,8 +322,22 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
                     if (better_capacity >= new_capacity) break;
                 }
 
+                return self.ensureTotalCapacityPrecise(better_capacity);
+            } else {
+                self.capacity = std.math.maxInt(usize);
+            }
+        }
+
+        /// Modify the array so that it can hold at least `new_capacity` items.
+        /// Like `ensureTotalCapacity`, but the resulting capacity is much more likely
+        /// (but not guaranteed) to be equal to `new_capacity`.
+        /// Invalidates pointers if additional memory is needed.
+        pub fn ensureTotalCapacityPrecise(self: *Self, new_capacity: usize) !void {
+            if (@sizeOf(T) > 0) {
+                if (self.capacity >= new_capacity) return;
+
                 // TODO This can be optimized to avoid needlessly copying undefined memory.
-                const new_memory = try self.allocator.reallocAtLeast(self.allocatedSlice(), better_capacity);
+                const new_memory = try self.allocator.reallocAtLeast(self.allocatedSlice(), new_capacity);
                 self.items.ptr = new_memory.ptr;
                 self.capacity = new_memory.len;
             } else {
@@ -464,14 +470,11 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
         pub const Slice = if (alignment) |a| ([]align(a) T) else []T;
 
         /// Initialize with capacity to hold at least num elements.
+        /// The resulting capacity is likely to be equal to `num`.
         /// Deinitialize with `deinit` or use `toOwnedSlice`.
         pub fn initCapacity(allocator: *Allocator, num: usize) !Self {
             var self = Self{};
-
-            const new_memory = try allocator.allocAdvanced(T, alignment, num, .at_least);
-            self.items.ptr = new_memory.ptr;
-            self.capacity = new_memory.len;
-
+            try self.ensureTotalCapacityPrecise(allocator, num);
             return self;
         }
 
@@ -685,7 +688,17 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
                 if (better_capacity >= new_capacity) break;
             }
 
-            const new_memory = try allocator.reallocAtLeast(self.allocatedSlice(), better_capacity);
+            return self.ensureTotalCapacityPrecise(allocator, better_capacity);
+        }
+
+        /// Modify the array so that it can hold at least `new_capacity` items.
+        /// Like `ensureTotalCapacity`, but the resulting capacity is much more likely
+        /// (but not guaranteed) to be equal to `new_capacity`.
+        /// Invalidates pointers if additional memory is needed.
+        pub fn ensureTotalCapacityPrecise(self: *Self, allocator: *Allocator, new_capacity: usize) !void {
+            if (self.capacity >= new_capacity) return;
+
+            const new_memory = try allocator.reallocAtLeast(self.allocatedSlice(), new_capacity);
             self.items.ptr = new_memory.ptr;
             self.capacity = new_memory.len;
         }
diff --git a/lib/std/coff.zig b/lib/std/coff.zig
index c3823f6c53..961cd8ade6 100644
--- a/lib/std/coff.zig
+++ b/lib/std/coff.zig
@@ -276,7 +276,7 @@ pub const Coff = struct {
         if (self.sections.items.len == self.coff_header.number_of_sections)
             return;
 
-        try self.sections.ensureTotalCapacity(self.coff_header.number_of_sections);
+        try self.sections.ensureTotalCapacityPrecise(self.coff_header.number_of_sections);
 
         const in = self.in_file.reader();
 
@@ -297,7 +297,7 @@ pub const Coff = struct {
                 std.mem.set(u8, name[8..], 0);
             }
 
-            try self.sections.append(Section{
+            self.sections.appendAssumeCapacity(Section{
                 .header = SectionHeader{
                     .name = name,
                     .misc = SectionHeader.Misc{ .virtual_size = try in.readIntLittle(u32) },
diff --git a/src/Module.zig b/src/Module.zig
index 1f394a0833..c975fc1c36 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -4146,7 +4146,7 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: *Allocator) Se
     // for the runtime ones.
     const fn_ty = decl.ty;
     const runtime_params_len = @intCast(u32, fn_ty.fnParamLen());
-    try inner_block.instructions.ensureTotalCapacity(gpa, runtime_params_len);
+    try inner_block.instructions.ensureTotalCapacityPrecise(gpa, runtime_params_len);
     try sema.air_instructions.ensureUnusedCapacity(gpa, fn_info.total_params_len * 2); // * 2 for the `addType`
     try sema.inst_map.ensureUnusedCapacity(gpa, fn_info.total_params_len);
 
diff --git a/src/link/MachO/CodeSignature.zig b/src/link/MachO/CodeSignature.zig
index 845122f5e3..18c85eb8f7 100644
--- a/src/link/MachO/CodeSignature.zig
+++ b/src/link/MachO/CodeSignature.zig
@@ -102,7 +102,7 @@ pub fn calcAdhocSignature(
     var buffer = try allocator.alloc(u8, page_size);
     defer allocator.free(buffer);
 
-    try cdir.data.ensureTotalCapacity(allocator, total_pages * hash_size + id.len + 1);
+    try cdir.data.ensureTotalCapacityPrecise(allocator, total_pages * hash_size + id.len + 1);
 
     // 1. Save the identifier and update offsets
     cdir.inner.identOffset = cdir.inner.length;
diff --git a/src/link/MachO/commands.zig b/src/link/MachO/commands.zig
index 35512886d4..b3a96c8047 100644
--- a/src/link/MachO/commands.zig
+++ b/src/link/MachO/commands.zig
@@ -223,7 +223,7 @@ pub const SegmentCommand = struct {
         var segment = SegmentCommand{
             .inner = inner,
         };
-        try segment.sections.ensureTotalCapacity(alloc, inner.nsects);
+        try segment.sections.ensureTotalCapacityPrecise(alloc, inner.nsects);
 
         var i: usize = 0;
         while (i < inner.nsects) : (i += 1) {