remove unused jump table entries

lib/expr.c

@@ -175,6 +175,28 @@ read_expr_common(const uint8_t **pp, const uint8_t *ep, struct expr *expr,
                                 i, j->pc, j->targetpc);
         }
 #endif
+        if (vctx->can_shrink_jump_table) {
+                assert(ei->jumps != NULL);
+                assert(ei->njumps > 0);
+                const struct jump *from = ei->jumps;
+                struct jump *to = ei->jumps;
+                const struct jump *ep = ei->jumps + ei->njumps;
+                while (from < ep) {
+                        if (from->targetpc != JUMP_TABLE_INVALID_PC) {
+                                *to++ = *from;
+                        }
+                        from++;
+                }
+                assert(to < from);
+                xlog_trace("jump table shrinked from %" PRIu32 " to %zu",
+                           ei->njumps, to - ei->jumps);
+                ei->njumps = to - ei->jumps;
+                ret = resize_array((void **)&ei->jumps, sizeof(*ei->jumps),
+                                   ei->njumps);
+                if (ret != 0) {
+                        xlog_error("ignoring jump table resize failure");
+                }
+        }
         *pp = p;
 #if defined(TOYWASM_ENABLE_WRITER)
         expr->end = p;

lib/type.h

@@ -17,9 +17,11 @@
 #define WASM_PAGE_SIZE 65536
 #define WASM_MAX_PAGES 65536
 
+#define JUMP_TABLE_INVALID_PC 0xffffffff
+
 struct jump {
-        uint32_t pc;
-        uint32_t targetpc;
+        uint32_t pc; /* the address of block instruction (eg. block, loop) */
+        uint32_t targetpc; /* the jump target addreass */
 };
 
 struct type_annotation {

lib/validation.c

@@ -190,6 +190,7 @@ push_ctrlframe(uint32_t pc, enum ctrlframe_op op, uint32_t jumpslot,
         cframe->start_types = start_types;
         cframe->end_types = end_types;
         cframe->unreachable = false;
+        cframe->seen = false;
         cframe->height = ctx->valtypes.lsize;
         cframe->height_cell = ctx->ncells;
         assert(nsize == ctx->cframes.lsize);
@@ -224,8 +225,12 @@ pop_ctrlframe(uint32_t pc, bool is_else, struct ctrlframe *cframep,
                 assert(jump->targetpc == 0);
                 if (cframe->op == FRAME_OP_LOOP) {
                         jump->targetpc = jump->pc;
-                } else {
+                } else if (cframe->seen || (cframe->op == FRAME_OP_IF ||
+                                            cframe->op == FRAME_OP_ELSE)) {
                         jump->targetpc = pc;
+                } else {
+                        ctx->can_shrink_jump_table = true;
+                        jump->targetpc = JUMP_TABLE_INVALID_PC;
                 }
         }
         ret = pop_valtypes(cframe->end_types, ctx);
@@ -296,6 +301,7 @@ validation_context_reuse(struct validation_context *ctx)
         ctx->valtypes.lsize = 0;
         ctx->ncells = 0;
         ctx->locals.lsize = 0;
+        ctx->can_shrink_jump_table = false;
 }
 
 void
@@ -324,6 +330,7 @@ target_label_types(struct validation_context *ctx, uint32_t labelidx,
         struct ctrlframe *cframe =
                 &VEC_ELEM(ctx->cframes, ctx->cframes.lsize - labelidx - 1);
         const struct resulttype *rt = label_types(cframe);
+        cframe->seen = true;
         *rtp = rt;
         return 0;
 }

lib/validation.h

@@ -11,6 +11,7 @@ struct ctrlframe {
         uint32_t height;
         uint32_t height_cell;
         bool unreachable;
+        bool seen;
 };
 
 struct validation_context {
@@ -28,6 +29,7 @@ struct validation_context {
         VEC(, enum valtype) locals;
 
         bool const_expr;
+        bool can_shrink_jump_table;
 
         bool has_datacount;
         uint32_t ndatas_in_datacount;