diff --git a/src/rose/program_runtime.c b/src/rose/program_runtime.c
index 97101e64..7f5150e0 100644
--- a/src/rose/program_runtime.c
+++ b/src/rose/program_runtime.c
@@ -2813,6 +2813,10 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
     const char *pc_base = getByOffset(t, programOffset);
     const char *pc = pc_base;
 
+    // If this program has an effect, work_done will be set to one (which may
+    // allow the program to squash groups).
+    int work_done = 0;
+
     struct RoseContext *tctxt = &scratch->tctxt;
 
     assert(*(const u8 *)pc != ROSE_INSTR_END);
@@ -2893,6 +2897,7 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
                                INVALID_EKEY) == HWLM_TERMINATE_MATCHING) {
                     return HWLM_TERMINATE_MATCHING;
                 }
+                work_done = 1;
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
@@ -2902,6 +2907,7 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
                                ri->ekey) == HWLM_TERMINATE_MATCHING) {
                     return HWLM_TERMINATE_MATCHING;
                 }
+                work_done = 1;
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
@@ -2912,6 +2918,7 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
                                   INVALID_EKEY) == HWLM_TERMINATE_MATCHING) {
                     return HWLM_TERMINATE_MATCHING;
                 }
+                work_done = 1;
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
@@ -2939,6 +2946,7 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
                                ekey) == HWLM_TERMINATE_MATCHING) {
                     return HWLM_TERMINATE_MATCHING;
                 }
+                work_done = 1;
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
@@ -2969,6 +2977,16 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
+            L_PROGRAM_CASE(SQUASH_GROUPS) {
+                assert(popcount64(ri->groups) == 63); // Squash only one group.
+                if (work_done) {
+                    tctxt->groups &= ri->groups;
+                    DEBUG_PRINTF("squash groups 0x%llx -> 0x%llx\n", ri->groups,
+                                 tctxt->groups);
+                }
+            }
+            L_PROGRAM_NEXT_INSTRUCTION
+
             L_PROGRAM_CASE(CHECK_LONG_LIT) {
                 const char nocase = 0;
                 if (!roseCheckLongLiteral(t, scratch, end, ri->lit_offset,
@@ -3017,6 +3035,12 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
             }
             L_PROGRAM_NEXT_INSTRUCTION
 
+            L_PROGRAM_CASE(CLEAR_WORK_DONE) {
+                DEBUG_PRINTF("clear work_done flag\n");
+                work_done = 0;
+            }
+            L_PROGRAM_NEXT_INSTRUCTION
+
             L_PROGRAM_CASE(SET_LOGICAL) {
                 DEBUG_PRINTF("set logical value of lkey %u, offset_adjust=%d\n",
                              ri->lkey, ri->offset_adjust);
@@ -3054,6 +3078,7 @@ hwlmcb_rv_t roseRunProgram_l(const struct RoseEngine *t,
                         == HWLM_TERMINATE_MATCHING) {
                     return HWLM_TERMINATE_MATCHING;
                 }
+                work_done = 1;
             }
             L_PROGRAM_NEXT_INSTRUCTION