diff --git a/.github/workflows/perf.yml b/.github/workflows/perf.yml
index d34bf6a2333a..348959d32167 100644
--- a/.github/workflows/perf.yml
+++ b/.github/workflows/perf.yml
@@ -107,6 +107,8 @@ jobs:
           dub run -- \
             --base-dmd "$RUNNER_TEMP/base/dmd/$built" \
             --head-dmd "$RUNNER_TEMP/head/dmd/$built" \
+            --base-phobos "$RUNNER_TEMP/base/phobos" \
+            --head-phobos "$RUNNER_TEMP/head/phobos" \
             --base-sha "${{ steps.refs.outputs.base }}" \
             --head-sha "${{ steps.refs.outputs.head }}" \
             --pr "${{ github.event.pull_request.number || 0 }}" \
diff --git a/compiler/src/dmd/root/aav.d b/compiler/src/dmd/root/aav.d
index 381a97988b91..277bc128ca71 100644
--- a/compiler/src/dmd/root/aav.d
+++ b/compiler/src/dmd/root/aav.d
@@ -18,8 +18,28 @@ nothrow:
 
 private size_t hash(size_t a) pure nothrow @nogc @safe
 {
-    a ^= (a >> 20) ^ (a >> 12);
-    return a ^ (a >> 7) ^ (a >> 4);
+    // Keys are pointers: 16-byte aligned (low bits zero) and roughly sequential
+    // from the bump allocator, so they are highly correlated. The bucket index
+    // is `hash & (b_length - 1)`, i.e. the low bits, so the mix must spread
+    // entropy all the way down. This is the murmur3 fmix full-avalanche
+    // finalizer (fmix64 on 64-bit hosts, fmix32 on 32-bit).
+    static if (size_t.sizeof == 8)
+    {
+        a ^= a >> 33;
+        a *= 0xff51afd7ed558ccd;
+        a ^= a >> 33;
+        a *= 0xc4ceb9fe1a85ec53;
+        a ^= a >> 33;
+    }
+    else
+    {
+        a ^= a >> 16;
+        a *= 0x85ebca6b;
+        a ^= a >> 13;
+        a *= 0xc2b2ae35;
+        a ^= a >> 16;
+    }
+    return a;
 }
 
 private struct KeyValueTemplate(K,V)
diff --git a/tools/perfrunner/source/app.d b/tools/perfrunner/source/app.d
index 617995eb6a80..f9fa8e047639 100644
--- a/tools/perfrunner/source/app.d
+++ b/tools/perfrunner/source/app.d
@@ -15,6 +15,7 @@ version (unittest) {} else
 int main(string[] args)
 {
     string baseDmd, headDmd, baseSha, headSha, hostDmd;
+    string basePhobos, headPhobos;
     string os = "ubuntu-latest";
     string outPath = "results.json";
     long pr;
@@ -22,6 +23,8 @@ int main(string[] args)
     auto help = getopt(args,
         "base-dmd", "path to the base (merge-base) dmd binary", &baseDmd,
         "head-dmd", "path to the head (PR) dmd binary", &headDmd,
+        "base-phobos", "path to the base phobos checkout (enables Phobos workload)", &basePhobos,
+        "head-phobos", "path to the head phobos checkout (enables Phobos workload)", &headPhobos,
         "base-sha", "base commit sha (metadata)", &baseSha,
         "head-sha", "head commit sha (metadata)", &headSha,
         "pr",       "pull request number (metadata)", &pr,
@@ -46,13 +49,14 @@ int main(string[] args)
     auto tmp = buildPath(tempDir, "perfrunner");
     mkdirRecurse(tmp);
 
-    auto base = measure(baseDmd, workload, tmp, "base");
-    auto head = measure(headDmd, workload, tmp, "head");
+    auto base = measure(baseDmd, workload, basePhobos, tmp, "base");
+    auto head = measure(headDmd, workload, headPhobos, tmp, "head");
 
     MetricResult[] metrics;
     foreach (def; initials)
-        metrics ~= MetricResult(def.id, def.label, def.unit, def.method,
-            base[def.id], head[def.id]);
+        if (def.id in base && def.id in head)
+            metrics ~= MetricResult(def.id, def.label, def.unit, def.method,
+                base[def.id], head[def.id]);
 
     auto rep = Report(baseSha, "merge-base", headSha, pr, os, hostDmd, metrics);
     write(outPath, render(rep));
diff --git a/tools/perfrunner/source/cachegrind.d b/tools/perfrunner/source/cachegrind.d
index c0f997bcff10..969e46b7a477 100644
--- a/tools/perfrunner/source/cachegrind.d
+++ b/tools/perfrunner/source/cachegrind.d
@@ -5,7 +5,7 @@ import std.array : split;
 import std.conv : to;
 import std.file : readText;
 import std.path : buildPath;
-import std.process : execute;
+import std.process : execute, Config;
 import std.string : lineSplitter, strip;
 
 long parseInstructions(string cgout)
@@ -16,19 +16,25 @@ long parseInstructions(string cgout)
     throw new Exception("could not find 'summary:' line in cachegrind output");
 }
 
-// Compile the workload under cachegrind
-long instructions(string dmd, string[] dflags, string workload, string tmp, string tag)
+// Run `dmd args...` under cachegrind (optionally from `workDir`) and return
+// the retired-instruction count.
+long instructionsAt(string dmd, string[] args, string tmp, string tag, string workDir = null)
 {
-    auto obj = buildPath(tmp, tag ~ ".o");
     auto cgOut = buildPath(tmp, tag ~ ".cgout");
-    auto cmd = ["valgrind", "--tool=cachegrind", "--cachegrind-out-file=" ~ cgOut,
-        dmd, "-c"] ~ dflags ~ [workload, "-of=" ~ obj];
-    auto r = execute(cmd);
+    auto cmd = ["valgrind", "--tool=cachegrind", "--cachegrind-out-file=" ~ cgOut, dmd] ~ args;
+    auto r = execute(cmd, null, Config.none, size_t.max, workDir);
     if (r.status != 0)
         throw new Exception("cachegrind failed:\n" ~ r.output);
     return parseInstructions(readText(cgOut));
 }
 
+// Compile the workload under cachegrind
+long instructions(string dmd, string[] dflags, string workload, string tmp, string tag)
+{
+    auto obj = buildPath(tmp, tag ~ ".o");
+    return instructionsAt(dmd, ["-c"] ~ dflags ~ [workload, "-of=" ~ obj], tmp, tag);
+}
+
 unittest
 {
     auto sample = "events: Ir\nfn=(1) main\n5 100\nsummary: 1234500000\n";
diff --git a/tools/perfrunner/source/metrics.d b/tools/perfrunner/source/metrics.d
index ff9e5a9acf42..d84e06b90a97 100644
--- a/tools/perfrunner/source/metrics.d
+++ b/tools/perfrunner/source/metrics.d
@@ -5,9 +5,9 @@ import std.file : copy, exists, getSize, remove;
 import std.path : buildPath;
 import std.regex : ctRegex, matchFirst;
 
-import std.process : execute;
+import std.process : execute, Config;
 
-import cachegrind : instructions;
+import cachegrind : instructions, instructionsAt;
 
 struct MetricDef
 {
@@ -24,19 +24,35 @@ immutable MetricDef[] initials = [
     MetricDef("dmd_binary_size",             "dmd binary size (stripped)", "bytes", "stat"),
     MetricDef("hello_binary_size",           "hello binary size",          "bytes", "stat"),
     MetricDef("hello_max_rss",               "peak RSS (compile hello.d)", "kb",    "time -v"),
+    // Symbol-heavy front-end workload: analyse all of Phobos without codegen.
+    MetricDef("compile_phobos_instr",        "compile Phobos (instr)",     "count", "cachegrind"),
+    MetricDef("phobos_max_rss",              "peak RSS (Phobos)",          "kb",    "time -v"),
 ];
 
-// Measure every metric for one dmd binary. `tag` ("base"/"head")
-// keeps the two runs' temp files apart
-long[string] measure(string dmd, string workload, string tmp, string tag)
+// dmd arguments for the Phobos workload, run with the phobos dir as CWD so the
+// compiler's own dmd.conf resolves std/core imports. `-o-` keeps it front-end
+// only (no backend/codegen), `-i=std` forces full semantic of every std module.
+private immutable string[] phobosArgs = ["-o-", "-i=std", "-preview=dip1000", "std/package.d"];
+
+// Measure every metric for one dmd binary. `tag` ("base"/"head") keeps the two
+// runs' temp files apart. The Phobos metrics are only measured when `phobosDir`
+// is given (so local hello-only runs still work).
+long[string] measure(string dmd, string workload, string phobosDir, string tmp, string tag)
 {
-    return [
+    long[string] m = [
         "compile_hello_debug_instr":   instructions(dmd, [], workload, tmp, tag ~ "-dbg"),
         "compile_hello_release_instr": instructions(dmd, ["-O", "-release"], workload, tmp, tag ~ "-rel"),
         "dmd_binary_size":             strippedSize(dmd, buildPath(tmp, tag ~ "-dmd")),
         "hello_binary_size":           helloSize(dmd, workload, tmp, tag),
         "hello_max_rss":               maxRss(dmd, workload, tmp, tag),
     ];
+
+    if (phobosDir.length)
+    {
+        m["compile_phobos_instr"] = instructionsAt(dmd, phobosArgs.dup, tmp, tag ~ "-phobos", phobosDir);
+        m["phobos_max_rss"]       = maxRssCmd([dmd] ~ phobosArgs.dup, phobosDir);
+    }
+    return m;
 }
 
 // Byte size of `binary`
@@ -71,7 +87,13 @@ private void strip(string path)
 private long maxRss(string dmd, string workload, string tmp, string tag)
 {
     auto obj = buildPath(tmp, tag ~ "-rss.o");
-    auto r = execute(["/usr/bin/time", "-v", dmd, "-c", workload, "-of=" ~ obj]);
+    return maxRssCmd([dmd, "-c", workload, "-of=" ~ obj]);
+}
+
+// Peak RSS (KiB) of running `cmd` under /usr/bin/time, optionally from `workDir`.
+private long maxRssCmd(string[] cmd, string workDir = null)
+{
+    auto r = execute(["/usr/bin/time", "-v"] ~ cmd, null, Config.none, size_t.max, workDir);
     if (r.status != 0)
         throw new Exception("/usr/bin/time failed:\n" ~ r.output);
     return parseMaxRss(r.output);