diff --git a/c/Makefile b/c/Makefile
index 395e136..bc609fa 100644
--- a/c/Makefile
+++ b/c/Makefile
@@ -320,6 +320,11 @@ tests/test_stops$(EXE): tests/test_stops.c glm.c st.h uring.h json.h tok.h tok_u
 tests/test_topp$(EXE): tests/test_topp.c glm.c st.h uring.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
 	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
 
+# bench_topp is a microbenchmark (old qsort vs new heap partial-select, #335), NOT a test
+# gate -- intentionally absent from TEST_BINS. Build on demand: make tests/bench_topp
+tests/bench_topp$(EXE): tests/bench_topp.c glm.c st.h uring.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
+	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+
 tests/test_kv_alloc$(EXE): tests/test_kv_alloc.c glm.c st.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
 	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
 
diff --git a/c/tests/bench_topp.c b/c/tests/bench_topp.c
new file mode 100644
index 0000000..9c0682f
--- /dev/null
+++ b/c/tests/bench_topp.c
@@ -0,0 +1,131 @@
+/* Microbenchmark: old (full-vocab qsort) vs new (heap partial-select) top-p truncation.
+ *
+ * This is NOT a unit test -- test_topp.c proves correctness. This measures the headline
+ * claim of #335: that replacing the O(V log V) qsort over all 151936 vocab entries with
+ * an O(V) heapify + k*log-V pops is materially faster per call, which is the win the issue
+ * was opened for.
+ *
+ * It re-implements the OLD dist_build inline (qsort + scan) on a private buffer so the A/B
+ * runs in one process, same inputs, same warm caches -- a controlled comparison. It calls
+ * the REAL (new) dist_build via the include-glm.c pattern on the global g_pbuf.
+ *
+ * Methodology (chosen to be honest, not to flatter the change):
+ *   - V = 151936 (the actual GLM-5.2 vocab), g_nuc swept across the values that matter
+ *     for serving: 0.5 / 0.9 (serve default) / 0.95 / 0.99.
+ *   - Three logit shapes: (a) realistic peaked -- one hot token, long exponential tail,
+ *     the shape real language-model logits take; (b) uniform -- worst case for the heap,
+ *     maximum pop count; (c) a plateau of ties, to exercise the tie path.
+ *   - Each (shape, nuc) is timed over N_REPEAT=2000 iterations, with the RNG/logits frozen
+ *     so both algorithms do IDENTICAL work. We report median ns/call and the new/old ratio.
+ *   - A warmup pass primes caches before timing.
+ *
+ * Run:  make tests/bench_topp && ./tests/bench_topp   (not in TEST_BINS -- not a gate)
+ */
+#define main coli_glm_main_unused
+#include "../glm.c"
+#undef main
+
+#include <math.h>
+#include <stdint.h>
+
+/* ---- the OLD algorithm, verbatim from dev before #354, on a private buffer ---- */
+static float *s_pbuf; static int *s_pidx; static double *s_ref;
+static int cmp_pdesc_old(const void *a, const void *b){
+    double pa = s_ref[*(const int*)a], pb = s_ref[*(const int*)b];
+    return pa < pb ? 1 : pa > pb ? -1 : 0; }
+static void dist_build_old(const float *lo, int V, double temp, double nuc){
+    double mx = lo[0]; for (int i = 1; i < V; i++) if (lo[i] > mx) mx = lo[i];
+    double s = 0, invt = 1.0 / (temp > 1e-4 ? temp : 1e-4);
+    for (int i = 0; i < V; i++){ s_ref[i] = exp((lo[i]-mx)*invt); s += s_ref[i]; }
+    for (int i = 0; i < V; i++) s_ref[i] /= s;
+    if (nuc > 0 && nuc < 1.0){
+        for (int i = 0; i < V; i++) s_pidx[i] = i;
+        qsort(s_pidx, V, sizeof(int), cmp_pdesc_old);
+        double cum = 0; int keep = V;
+        for (int i = 0; i < V; i++){ cum += s_ref[s_pidx[i]]; if (cum >= nuc){ keep = i+1; break; } }
+        double s2 = 0;
+        for (int i = keep; i < V; i++) s_ref[s_pidx[i]] = 0;
+        for (int i = 0; i < keep; i++) s2 += s_ref[s_pidx[i]];
+        for (int i = 0; i < keep; i++) s_ref[s_pidx[i]] /= s2;
+    }
+    (void)s_pbuf;
+}
+
+/* ---- timing: median of N_REPEAT runs in ns/call, sorted ascending ---- */
+#define N_REPEAT 2000
+static double bench_ns(void (*fn)(const float*,int,double,double),
+                       const float *lo, int V, double temp, double nuc){
+    static double ts[N_REPEAT];
+    for (int r = 0; r < N_REPEAT; r++){
+        double t0 = now_s();
+        fn(lo, V, temp, nuc);
+        ts[r] = (now_s() - t0) * 1e9;
+    }
+    /* insertion sort the N_REPEAT samples (small), take median */
+    for (int a = 1; a < N_REPEAT; a++){ double k = ts[a]; int b = a-1;
+        while (b >= 0 && ts[b] > k){ ts[b+1] = ts[b]; b--; } ts[b+1] = k; }
+    return ts[N_REPEAT/2];
+}
+
+/* the NEW algorithm is the real dist_build, but it writes g_pbuf (not a private buf).
+ * Wrap it so the bench signature matches, and set the globals it reads. */
+static void dist_build_new(const float *lo, int V, double temp, double nuc){
+    g_temp = (float)temp; g_nuc = (float)nuc;
+    dist_build(lo, V);
+}
+
+/* deterministic logit fill for three shapes */
+static uint32_t brng = 0xA5A5A5A5u;
+static double brand(void){ brng ^= brng << 13; brng ^= brng >> 17; brng ^= brng << 5;
+    return (double)(brng >> 8) * (1.0 / 16777216.0); }
+static void fill_realistic(float *lo, int V){     /* one hot, exponential tail -- like real logits */
+    for (int i = 0; i < V; i++) lo[i] = (float)(-4.0 * brand() - (double)i * 0.0001);
+    lo[0] = 6.f; lo[V/50] = 4.f; lo[V/200] = 3.f;
+}
+static void fill_uniform(float *lo, int V){       /* worst case for the heap: max pop count */
+    for (int i = 0; i < V; i++) lo[i] = 0.f;
+}
+static void fill_plateau(float *lo, int V){       /* ties: blocks of equal value */
+    for (int i = 0; i < V; i++) lo[i] = (float)(-(double)(i / 50));
+}
+
+int main(void){
+    int V = 151936;
+    float *lo = malloc((size_t)V * sizeof(float));
+    s_ref  = malloc((size_t)V * sizeof(double));
+    s_pidx = malloc((size_t)V * sizeof(int));
+    /* force the new dist_build to allocate g_pbuf/g_pidx at full V once */
+    g_temp = 0.7f; g_nuc = 0.9f; dist_build(lo, V);
+
+    double temp = 0.7;
+    struct { const char *name; void (*fill)(float*,int); } shapes[] = {
+        { "realistic", fill_realistic },
+        { "uniform",   fill_uniform   },
+        { "plateau",   fill_plateau   },
+    };
+    double nucs[] = { 0.5, 0.9, 0.95, 0.99 };
+
+    printf("bench_topp: top-p truncation, old (qsort) vs new (heap)  V=%d  temp=%.2f\n", V, temp);
+    printf("%-12s %6s %14s %14s %9s %9s\n", "shape", "nuc", "old ns/call", "new ns/call", "speedup", "keep");
+    printf("-----------------------------------------------------------------------------\n");
+
+    for (size_t sh = 0; sh < sizeof(shapes)/sizeof(shapes[0]); sh++){
+        shapes[sh].fill(lo, V);
+        for (size_t ni = 0; ni < sizeof(nucs)/sizeof(nucs[0]); ni++){
+            double nuc = nucs[ni];
+            /* warmup both paths so caches/branch predictors are primed */
+            for (int w = 0; w < 50; w++){ dist_build_old(lo, V, temp, nuc); dist_build_new(lo, V, temp, nuc); }
+            double t_old = bench_ns(dist_build_old, lo, V, temp, nuc);
+            double t_new = bench_ns(dist_build_new, lo, V, temp, nuc);
+            /* keep count = non-zero entries the new path leaves (== old's keep) */
+            int keep = 0; for (int i = 0; i < V; i++) if (g_pbuf[i] != 0.f) keep++;
+            printf("%-12s %6.2f %14.0f %14.0f %8.2fx %9d\n",
+                   shapes[sh].name, nuc, t_old, t_new, t_old / t_new, keep);
+        }
+        printf("\n");
+    }
+
+    printf("bench_topp: done (lower ns is better; speedup = old/new)\n");
+    free(lo); free(s_ref); free(s_pidx);
+    return 0;
+}