diff --git a/src/compiler/nir/nir_lower_subgroups.c b/src/compiler/nir/nir_lower_subgroups.c
index e46bf0d00fa..1f2ac17dda0 100644
--- a/src/compiler/nir/nir_lower_subgroups.c
+++ b/src/compiler/nir/nir_lower_subgroups.c
@@ -701,18 +701,32 @@ build_identity(nir_builder *b, unsigned bit_size, nir_op op)
 /* Implementation of scan/reduce that assumes a full subgroup */
 static nir_def *
 build_scan_full(nir_builder *b, nir_intrinsic_op op, nir_op red_op,
-                nir_def *data, unsigned cluster_size)
+                nir_def *data, unsigned cluster_size,
+                const nir_lower_subgroups_options *options)
 {
+   bool unknown_size = !options->subgroup_size;
+   nir_def *subgroup_size = unknown_size ? nir_load_subgroup_size(b) : NULL;
+
    switch (op) {
    case nir_intrinsic_exclusive_scan:
    case nir_intrinsic_inclusive_scan: {
       for (unsigned i = 1; i < cluster_size; i *= 2) {
+         nir_def *old_data = data;
+
+         if (unknown_size)
+            nir_push_if(b, nir_ugt_imm(b, subgroup_size, i));
+
          nir_def *idx = nir_load_subgroup_invocation(b);
          nir_def *has_buddy = nir_ige_imm(b, idx, i);
 
          nir_def *buddy_data = nir_shuffle_up(b, data, nir_imm_int(b, i));
          nir_def *accum = nir_build_alu2(b, red_op, data, buddy_data);
          data = nir_bcsel(b, has_buddy, accum, data);
+
+         if (unknown_size) {
+            nir_pop_if(b, NULL);
+            data = nir_if_phi(b, data, old_data);
+         }
       }
 
       if (op == nir_intrinsic_exclusive_scan) {
@@ -732,8 +746,18 @@ build_scan_full(nir_builder *b, nir_intrinsic_op op, nir_op red_op,
 
    case nir_intrinsic_reduce: {
       for (unsigned i = 1; i < cluster_size; i *= 2) {
+         nir_def *old_data = data;
+
+         if (unknown_size)
+            nir_push_if(b, nir_ugt_imm(b, subgroup_size, i));
+
          nir_def *buddy_data = nir_shuffle_xor(b, data, nir_imm_int(b, i));
          data = nir_build_alu2(b, red_op, data, buddy_data);
+
+         if (unknown_size) {
+            nir_pop_if(b, NULL);
+            data = nir_if_phi(b, data, old_data);
+         }
       }
       return data;
    }
@@ -749,6 +773,9 @@ build_scan_reduce(nir_builder *b, nir_intrinsic_op op, nir_op red_op,
                   nir_def *data, nir_def *mask, unsigned max_mask_bits,
                   const nir_lower_subgroups_options *options)
 {
+   bool unknown_size = !options->subgroup_size;
+   nir_def *subgroup_size = unknown_size ? nir_load_subgroup_size(b) : NULL;
+
    nir_def *lt_mask = nir_load_subgroup_lt_mask(b, options->ballot_components,
                                                 options->ballot_bit_size);
 
@@ -759,6 +786,12 @@ build_scan_reduce(nir_builder *b, nir_intrinsic_op op, nir_op red_op,
    nir_def *remaining = nir_iand(b, mask, lt_mask);
 
    for (unsigned i = 1; i < max_mask_bits; i *= 2) {
+      nir_def *old_data = data;
+      nir_def *old_remaining = remaining;
+
+      if (unknown_size)
+         nir_push_if(b, nir_ugt_imm(b, subgroup_size, i));
+
       /* At each step, our buddy channel is the first channel we have yet to
        * take into account in the accumulator.
        */
@@ -776,6 +809,12 @@ build_scan_reduce(nir_builder *b, nir_intrinsic_op op, nir_op red_op,
        */
       nir_def *buddy_remaining = nir_shuffle(b, remaining, buddy);
       remaining = nir_bcsel(b, has_buddy, buddy_remaining, nir_imm_int(b, 0));
+
+      if (unknown_size) {
+         nir_pop_if(b, NULL);
+         data = nir_if_phi(b, data, old_data);
+         remaining = nir_if_phi(b, remaining, old_remaining);
+      }
    }
 
    switch (op) {
@@ -868,7 +907,7 @@ lower_scan_reduce(nir_builder *b, nir_intrinsic_instr *intrin,
    nir_push_if(b, nir_ball_iequal(b, mask, build_subgroup_mask(b, options)));
    {
       full = build_scan_full(b, intrin->intrinsic, red_op,
-                             intrin->src[0].ssa, cluster_size);
+                             intrin->src[0].ssa, cluster_size, options);
    }
    nir_push_else(b, NULL);
    {