2 files changed, 48 insertions, 13 deletions

diff --git a/lib/std/Build/Fuzz/WebServer.zig b/lib/std/Build/Fuzz/WebServer.zig
index 7b0c24c3e2..17bc2c918f 100644
--- a/lib/std/Build/Fuzz/WebServer.zig
+++ b/lib/std/Build/Fuzz/WebServer.zig
@@ -7,6 +7,7 @@ const Step = std.Build.Step;
 const Coverage = std.debug.Coverage;
 const abi = std.Build.Fuzz.abi;
 const log = std.log;
+const assert = std.debug.assert;
 
 const WebServer = @This();
 
@@ -383,7 +384,7 @@ fn sendCoverageContext(
     // TODO: make each events URL correspond to one coverage map
     const coverage_map = &coverage_maps[0];
     const cov_header: *const abi.SeenPcsHeader = @ptrCast(coverage_map.mapped_memory[0..@sizeOf(abi.SeenPcsHeader)]);
-    const seen_pcs = coverage_map.mapped_memory[@sizeOf(abi.SeenPcsHeader) + coverage_map.source_locations.len * @sizeOf(usize) ..];
+    const seen_pcs = cov_header.seenBits();
     const n_runs = @atomicLoad(usize, &cov_header.n_runs, .monotonic);
     const unique_runs = @atomicLoad(usize, &cov_header.unique_runs, .monotonic);
     const lowest_stack = @atomicLoad(usize, &cov_header.lowest_stack, .monotonic);
@@ -415,7 +416,7 @@ fn sendCoverageContext(
         };
         const iovecs: [2]std.posix.iovec_const = .{
             makeIov(std.mem.asBytes(&header)),
-            makeIov(seen_pcs),
+            makeIov(std.mem.sliceAsBytes(seen_pcs)),
         };
         try web_socket.writeMessagev(&iovecs, .binary);
 
@@ -630,8 +631,7 @@ fn prepareTables(
     gop.value_ptr.mapped_memory = mapped_memory;
 
     const header: *const abi.SeenPcsHeader = @ptrCast(mapped_memory[0..@sizeOf(abi.SeenPcsHeader)]);
-    const pcs_bytes = mapped_memory[@sizeOf(abi.SeenPcsHeader)..][0 .. header.pcs_len * @sizeOf(usize)];
-    const pcs = std.mem.bytesAsSlice(usize, pcs_bytes);
+    const pcs = header.pcAddrs();
     const source_locations = try gpa.alloc(Coverage.SourceLocation, pcs.len);
     errdefer gpa.free(source_locations);
     debug_info.resolveAddresses(gpa, pcs, source_locations) catch |err| {
@@ -648,9 +648,8 @@ fn addEntryPoint(ws: *WebServer, coverage_id: u64, addr: u64) error{ AlreadyRepo
     defer ws.coverage_mutex.unlock();
 
     const coverage_map = ws.coverage_files.getPtr(coverage_id).?;
-    const ptr = coverage_map.mapped_memory;
-    const pcs_bytes = ptr[@sizeOf(abi.SeenPcsHeader)..][0 .. coverage_map.source_locations.len * @sizeOf(usize)];
-    const pcs: []const usize = @alignCast(std.mem.bytesAsSlice(usize, pcs_bytes));
+    const header: *const abi.SeenPcsHeader = @ptrCast(coverage_map.mapped_memory[0..@sizeOf(abi.SeenPcsHeader)]);
+    const pcs = header.pcAddrs();
     const index = std.sort.upperBound(usize, pcs, addr, struct {
         fn order(context: usize, item: usize) std.math.Order {
             return std.math.order(item, context);
diff --git a/lib/std/Build/Fuzz/abi.zig b/lib/std/Build/Fuzz/abi.zig
index f385f9b08a..0e16f0d5fa 100644
--- a/lib/std/Build/Fuzz/abi.zig
+++ b/lib/std/Build/Fuzz/abi.zig
@@ -7,13 +7,40 @@
 /// make the ints be the size of the target used with libfuzzer.
 ///
 /// Trailing:
-/// * pc_addr: usize for each pcs_len
 /// * 1 bit per pc_addr, usize elements
+/// * pc_addr: usize for each pcs_len
 pub const SeenPcsHeader = extern struct {
     n_runs: usize,
     unique_runs: usize,
     pcs_len: usize,
     lowest_stack: usize,
+
+    /// Used for comptime assertions. Provides a mechanism for strategically
+    /// causing compile errors.
+    pub const trailing = .{
+        .pc_bits_usize,
+        .pc_addr,
+    };
+
+    pub fn headerEnd(header: *const SeenPcsHeader) []const usize {
+        const ptr: [*]align(@alignOf(usize)) const u8 = @ptrCast(header);
+        const header_end_ptr: [*]const usize = @ptrCast(ptr + @sizeOf(SeenPcsHeader));
+        const pcs_len = header.pcs_len;
+        return header_end_ptr[0 .. pcs_len + seenElemsLen(pcs_len)];
+    }
+
+    pub fn seenBits(header: *const SeenPcsHeader) []const usize {
+        return header.headerEnd()[0..seenElemsLen(header.pcs_len)];
+    }
+
+    pub fn seenElemsLen(pcs_len: usize) usize {
+        return (pcs_len + @bitSizeOf(usize) - 1) / @bitSizeOf(usize);
+    }
+
+    pub fn pcAddrs(header: *const SeenPcsHeader) []const usize {
+        const pcs_len = header.pcs_len;
+        return header.headerEnd()[seenElemsLen(pcs_len)..][0..pcs_len];
+    }
 };
 
 pub const ToClientTag = enum(u8) {
@@ -47,12 +74,21 @@ pub const SourceIndexHeader = extern struct {
 /// changes.
 ///
 /// Trailing:
-/// * one bit per source_locations_len, contained in u8 elements
+/// * one bit per source_locations_len, contained in u64 elements
 pub const CoverageUpdateHeader = extern struct {
-    tag: ToClientTag = .coverage_update,
-    n_runs: u64 align(1),
-    unique_runs: u64 align(1),
-    lowest_stack: u64 align(1),
+    flags: Flags = .{},
+    n_runs: u64,
+    unique_runs: u64,
+    lowest_stack: u64,
+
+    pub const Flags = packed struct(u64) {
+        tag: ToClientTag = .coverage_update,
+        _: u56 = 0,
+    };
+
+    pub const trailing = .{
+        .pc_bits_usize,
+    };
 };
 
 /// Sent to the fuzzer web client when the set of entry points is updated.